23 Magnetism By A. HARRISON Department of Chemistry University of Edinburgh The King's Buildings West Mains Road Edinburgh EH9 3JJ UK S. J. CLARKE Inorganic Chemistry Laboratory University of Oxford South Parks Road Oxford OX1 3QR UK 1 Introduction This review has a similar form to that of previous years. It is primarily concerned with insulating magnets that show cooperative magnet properties through the influence of superexchange. It does not cover work on metallic elements and intermetallic alloys in any detail,'-5 despite the current focus on heavy-fermion materials6-* such as UM,Si (where M is a transition metal such as Ni9 or Ru") or UBe,,," and on the complex magnetic structure of lanthanide' 2-25 and actinide26 metals and compounds. It does D.Gignoux in 'Materials Science and Technology Vol. 3A Electronic and Magnetic Properties of Metals and Ceramics ed. K. Heinz and J. Buschow VCH Weinheim Germany. 1992 Vol. 3A p. 367. N. H. Duc T. D. Hien P. E. Brommer and J. J. M. Franse J. Magn. Magn. Mar. 1992 104107 1252. V. Sechovsky and L. Havela J. Magn. Magn. Mat. 1992 104-107 7. R. Mueller M. Kuckel and H. U. Schuster J. Alloys Compd. 1991 176 167. B. Johansson L. Nordstroem 0.Eriksson and M. S. S. Brooks Phys. Scr. 1991 T39. 100. ' S. B. Palmer 'Proceedings of the International Conference on Magnetism (Edinburgh) Symposium on Heavy Fermions' J. Magn. Magn. Mat. 1992 108 1. A. De Visser J.J. M. Franse and J. Flouquet J. Magn. Magn. Mat. 1992 108. 15. D.T. Adroja and S. K. Malik J. Magn. Magn.Mat. 1992 100 126. L. Rebelsky H. Lin M. W. Mcelfresh M. F. Collins J. D. Garrett. W. J. L. Buyers and M. S. Torikachvili Physica B 1992 180. 43. lo A. P. Ramirez P. Coleman P. Chandra E. Bruck A.A. Menovsky. Z. Fisk and E. Bucher Phys. Rev. Lett. 1992 68 2680. " A. Devisser N. H. Vandijk K. Bakker J. J. M. Franse A. Lacerda. J. Flouquet Z. Fisk and J. L. Smith Phys. Rev. 8 1992 45 2962. l2 R. M. Moon and R. M. Nicklow J. May. Map. Mar. 1992 100 139. l3 B. Lebech and J. Wolny J. Magn. Magti. Mat. 1992 104-107 1501. l4 E. M. Forgan S.L. Lee W.G. Marshall and D. Fort J. Magn. Mugn. Mat.. 1992 104107. 913. Is J. Jensen and A.R. Mackintosh J. Magn. Magn. Mat. 1992 104-107. 1481. l6 D. A. Jehan D.F. McMorrow. R.A. Cowley and G. J. Mclntyre Europhys.Left. 1992 17 553. I' D. F. McMorrow C. Patterson H. Godfrin. and D.A. Jehan Physica B 1992 180 165. '*M.O. Steinitz D. A. Tindall and M. Kahrizi J. Magn. Magn. Mat. 1992. 104107 1531. l9 H. Lin M. F. Collins T. M. Holden and W. Wei. Phys. Rev. B 1992 45. 2873. *' H. Lin M. F. Collins T. M. Holden and W. Wei J. Magn. Magn. Mat. 1992 104-107 151 1. 21 R.S. Eccleston and S. B. Palmer J. Phys. Cond. Matt.. 1992 4 37. 22 P. M. Gehring L. Rebelsky D. Gibbs and G. Shirane Phys. Rev. B 1992 45 243. 23 C. C. Tang W. G. Stirling D. L. Jones C. C. Wilson P. W. Haycock A. J. Rollason A. H. Thomas. and D. Fort J. Map. Magn. Mat. 1992. 103 86. 24 D. Gibbs J. Magn. Magn. Mat. 1992 104107 1489. 25 E.M. Forgan J. Magn. Magn. Mat. 1992 104107 1485. 26 G. H. Lander and G.Aeppli J. Magn. Magn. Mat. 1991 100 151. 425 426 A. Harrison and S.J. Clarke not consider in detail the nitrogen interstitial corn pound^^^.^^ of iron-lanthanide metal alloys of the form R,Fe ,N such materials and in particular those in which R = Sm show a dramatic increase in T as the nitrogen content increases leading to new permanent magnets whose energy product may exceed that of Nd-Fe-B magnets. The review also ignores much of the remarkable work on thin films that has recently provided model systems to enrich our understanding of exchange mechanisms in metals as well as new materials for recording media in particular those that show giant magnetore~istance.~~ These fields have recently been covered well in the proceedings of international conferences and recent reviews .6930-34 W e will not consider nuclear magnetism despite the fascinating ~ork~~,~~ at pK temperatures on the ordering of nuclear moments in Cu and Ag the antiferromagnetic structures that result from competition between dipolar and RKKY interactions change to ferromagnetic structures at negative-spin temperatures.Throughout the review the isotropic exchange constant J for the coupling between a spin Si and its neighbours Sj will be defined such that the Hamiltonian X for the interaction is x = -J c S;S <ij> Where the sum is taken over all exchange pairs <ij> . The magnitude of J will be expressed in Kelvin as J/k, where k is Boltzmann’s constant. In one- or two-dimensional magnets J denotes the exchange within chains or planes respectively and J’ the exchange between them.The layout of the review is as follows. First there is an account of important meetings reviews and books followed by outlines of advances in experimental and theoretical techniques. New compounds and experimental results will then be described organized according to the type of compound with a broad division into cooperative ionic materials and molecular materials. Within the former category compounds are divided into chain-like (lD) planar (2D),or isotropic (3D)materials and ranked in order of the atomic number of the principal magnetic ion within each subgroup. 2 Major Meetings Books and General Reviews The triennial International Conference on Magnetism (ICM)was held in Edinburgh in 1991 and its proceedings published in 1992.30 It is tempting to direct the reader towards those proceedings and finish the review here.However a large proportion of the contents now appear dated. Further the Conference publicized few new materials because it had a strong bias towards physical measurements and theory and catered primarily for physicists. We hope to redress the balance in sections 5 and 6 of this review. The ICM meeting was accompanied by a meeting on neutron scattering which 21 J.D.M. Coey Phys. Scr. 1991 T39 21. 28 J.M.D. Coey H. Sun and D. P. F. Hurley J. Magn. Magn. Mut. 1991 101 310. 29 R.L. White I.E.E.E. Trans. Magn. 1992 28 2482. 30 S. R. Palmer ‘International Conference on Magnetism (Edinburgh)’.J. Magn. Magn. Mut. 1992 104107 1. 31 A.J. Freeman ‘Magnetism in the Nineties’ J. Magn. Magn. Mat. 1991 100 1. 32 R. W. Chantrell and K. Ogrady J. Phys. D:Appl. Phys. 1992 25 1. 33 A. J. Freeman and R. Q. Wu J. Magn. Map. Mar. 1992 104 107 I. 34 F.J. Cadieu Phys. Thin Films 1992 16 145. 35 P. J. Hakonen K. K. Nummila R.T. Vuorinen and S. Yin J. Magn. Mugn. Mar. 1992 104107 903. 36 P. J. Hakonen K. K. Nurnrnila R. T. Vuorinen and 0.V. Lounasrnaa. Phys. Rec. Lerr. 1992 68,365. Magnetism 427 had a large contribution from workers on magnetism,37 as well as a meeting on heavy-fermionic materials.6 The Journal of Magnetism and Magnetic Materials celebrated its 100th issue with a series of reviews of events over the 17 years since the journal was launched and some speculation about the developments that might occur in the rest of this de~ade.~' Applied magnetism has been the subject of large European38 and Internati~nal~~ meetings.Books on magnetism this year include works on transition metal compounds40 and lanthanide~,~' and amorphous metals and alloys,43 and on on random rnagnet~"~ permanent magnetic materials44 and application^.^^ A further volume of the Landolt-Bornstein Tables has been issued covering inorganic compounds based on transition elements. It describes binary oxides that do not have the spinel garnet or perovskite structure trirutile and pyrochlore oxides hexagonal ferrites and RFe,04 compounds where R is In Sc Y or a lanthanide element.46 3 Experimental Methods The development of experimental techniques has probably been most marked in the area of thin-film and surface magnetism.This is partly due to the intense commercial pressures to produce new devices and partly due to the academic interest in using such systems to model fundamental magnetism. The most spectacular advances have been in the direct imaging of magnetic surfaces at an atomic level using scanning-tunnelling microscopic (STM) technique^.^^,^^ The conventional STM probe may be replaced by a ferromagnetic material such as CrO which sensitizes the electron tunnelling probability to the spin-polarization of the surface being scanned. Probe tips of Fe which are sharp at an atomic length scale have allowed the imaging of the (001) surface of Fe30 and demonstrated the presence of a Wigner glass.Advances in the application of neutron scattering to magnetic problems have been summarized in the proceedings of the triennial International Conference on Neutron S~attering.~~ Over the past few years the value of the technique has been demonstrated through the contribution it has made in unravelling the static and dynamic behaviour of moments in high-temperature superconducting (high- T,) and heavy fermionic 31 K. A. McEwan W. G. Stirling A. D. Taylor. and C. C. Wilson 'Proceedings of the International Conference on Neutron Scattering (Oxford 1991)' Physica E. 1992 18@181 1. 38 S. Kobe and S. Roth 'Proceedings of the 4th European Meeting on Magnetic Materials and Applications' J. May. Map. Mat. 1991 101 I.39 S. Suzuki 'Proceedings of the International Magnetics Conference (Intermag '92)' IEEE 7ran.s. Mayn. 1991 28 I. 40 A. Kotani and N. Suzuki 'Recent Advances in Magnetism of Transition Metal Compounds' World Scientific Singapore. 1992. 41 J. Jensen and A. Macintosh. 'Rare Earth Magnetism Structures and Excitations' International Series of Monographs on Physics OUP Oxford 1991 Vol. 81. 42 D. H. Ryan 'Recent Progress in Random Magnets' World Scientific Singapore 1992. 43 J. A. Fernandez-Baca and W. Y. Ching. 'Magnetism of Amorphous Metals and Alloys' World Scientific. Singapore 1992. 44 S. G. Sankar 'Permanent Magnetic Materials' World Scientific. Singapore 1992. 45 F. Leccdbue and J. L. Sanchez 'Magnetism Magnetic Materials and their Applications Proceedings of the International Workshop Havana'.IOP 1992. 4h 0. Madelung 'Magnetic Properties of Nonmetallic Inorganic Compounds Based on Transition Elements'. Landolt-Bornstein Numerical Data and Functional Relationships in Science and Technology. New Series Springer 1992. Vol. III/27 g. 47 R. Wiesendanger I. V. Shvets D. Burgler. G. Tarrach H. J. Guntherodt. J. M. D. Coey and S. Graser Science 1992 255 583. 4R R. Wiesendanger I. V. Shvets. D. Burgler G. Tarrach H.J. Guntherodt and J. M. D. Coey 2.Phys. B 1992 86. 1. 428 A. Harrison and S. J. Clarke materials in applications to thin-film magnetism and in the determination of the complex spin structures of actinide metals and compounds. A new time-of-flight neutron spectrometer installed at ISIS in the UK and called MART has started to produce spin-wave dispersion curves up to several hundred meV with good spatial and energy res~lution.~~ The analysis of neutron scattering data for disordered magnetic materials may be aided with a reverse Monte Carlo technique similar to those used to elucidate density correlations in structurally disordered material^.^' The study of actinide and lanthanide magnetism and also of thin films and magnetic multilayers is the major beneficiary of advances in magnetic X-ray scattering method^.^' The technique takes various forms.Diffraction experiments with or without the X-ray energy tuned to an atomic absorption edge provides information about the separate contributions of spin and orbital angular momentum to the magnetization about the magnetic character of core to valence electronic transitions and about relative orientations of moment^.^*.^^^^^ The circular dichroism of soft X-rays scattered from surfaces also provides a probe of the magnetic polarization of a surface.Recent experiments involving the Compton scattering of circularly polarized light from the ferromagnet HoF indicate that such measurements probe the spin magnetization which is at variance with the predictions of recent theoretical work.55 Muon spin rotation and relaxation measurements are becoming more common in the wake of the publicity given by prominent experiments on high-T superconduc- and the work by theorists who relate magnetic phenomena to observables in pSR experimen ts.60-6 Advances in various types of susceptibility and magnetization measurements have been reviewed by various authors; perhaps the most significant recent trend is the increased awareness of the low cost and versatility of AC technique^.^^-^* 49 D.Welz M. Arai M. Nishi M. Kohgi and Y. Endoh Physica B 1992 18&181 147. 50 D.A. Keen and R.L. McGreevy J. Phys. Cond. Mutt.. 1991 3 7383. 51 G. H. Lander and W. G. Stirling Phys. Scr. 1992 T45 15. 52 D.B. McWhan J. R. Hastings C. C. Kao and D. P. Siddons. Rev. Sci.Instrum. 1992 63 1404. 53 V. Nunez P. J. Brown J. B. Forsyth and F. Tasset Physica B 1991 174,60. 54 M. J. Cooper E. Zukowski S. P. Collins D. N. Timms F. Itoh and H. Sakurai J. Phys. Cond. Mutt. 1992 4,L399. 55 S. W. Lovesey Phys. Scr..1991 44. 51. 56 C.E. Gough Springer Proc. Phps. (Exot. At. Condens. Mutter) 1992 59.259. 57 H.Keller Springer Proc. Phys. (Exct. At. Condens. Mutter) 1992 59 191. 58 E. M. Forgan Springer Proc. Phys. (Exct. At. Condens. Matter) 1992. 59,285. 59 C. Bucci Sol. State Commun. 1992 84 191. ‘O Y. Aoyama and M. Tanaka Phys. Stat. Sol. B 1992 171,K107. 61 S.W. Lovesey. Hyperfine Int. 1992 72,389. 62 R. Saito H. Kamimura and K. Nagamine Physica C 1991 185,1217. 63 T. Yamazaki Kotai Butsuri. 1991 26,688. 64 Q.Y. Chen in ‘Magnetic Susceptibility of Superconductors and Other Spin Systems (Proc. Off. Nav. Res. Workshop’ ed. R. A. Hein T. L. Francavilla and D. H. Liebenberg Plenum New York 1991 p. 81. 65 A. M. Campbell in ‘Magnetic Susceptibility of Superconductors and Other Spin Systems (Proc.Off. Nav. Res. Workshop)’,ed. R. A. Hein T. L. Francavilla and D. H. Liebenberg Plenum New York 1991 p. 129. ‘‘ F. Gomory in ‘Magnetic Susceptibility of Superconductors and Other Spin Systems (Proc. Off. Nav. Res. Workshop)’ ed. R. A. Hein T. L. Francavilla and D. H. Liebenberg Plenum New York 1991 p. 289. ” J. H. Claasen in ‘Magnetic Susceptibility ofSuperconductors and Other Spin Systems (Proc. Off. Nav. Res. Workshop)’ ed. R. A. Hein T. L. Francavilla and D. H. Liebenberg Plenum New York 1991 p. 405. 68 G. Williams in ‘Magnetic Susceptibility of Superconductors and Other Spin Systems (Proc. Off. Nav. Res. Workshop)’ ed. R. A. Hein T. L. Francavilla. and D. H. Liebenberg Plenum New York 1991 p. 475. Magnetism 429 The sample environment is an important component of many magnetic experiments and tempera- the provision of high pressures in AC susceptibility rneas~rements~~”~ tures of the order of 10-9K for susceptibility and neutron scattering measure- ment~~’.~~ have been described.4 Theoretical Work For the past few years experimentalists and theorists have worked closely on low-dimensional low-moment magnets with various degrees of magnetic frustra- ti~n.~’ The initial stimulus for this work was provided by the magnetic properties of the high-T,ceramics which are S = 3square Heisenberg antiferromagnets with competing first second and third nearest-neighbour antiferromagnetic interactions J, J, and J,. It was initially believed that their magnetic ground state is a quantum spin fluid or a Resonating Valence Bond state but a classical ground state is now favoured albeit with a significantly reduced ordered moment recent work has consolidated this belief.72 These cuprates are also very sensitive towards dilution by holes which place frustrating ferromagnetic bonds in the lattice.The mobility of the holes has been shown to disrupt the ground state f~rther,~,,~~ softening and damping spin-waves and increasing nuclear spin relaxation rates for the constituent atoms. The spin fluctuations in low-moment frustrated magnets seem to have an energy scale that is set by the temperature rather than by microscopic factors such as the exchange energy. It has been shown that this behaviour is to be expected of a magnet that lies near the boundary between classical ordered and quantum disordered phases as it is cooled from the phase at higher temperatures which is termed the quantum critical state.75 Models for high-T materials have been made more elaborate by the addition of more complex four-spin cyclic exchange interactions around the square plaquettes.Such terms which are known to occur in the Hubbard model produce a variety of collinear canted and dimerized phases,7h depending on their values relative to J and J,. There has been a healthy debate for some years now about the role of low-energy magnetic fluctuations in the coupling of charge carriers in the high- T superconduc-tors and it had previously been believed that the strength of the coupling was too weak to account for the relatively high value of T,.Recent calculations indicate that it is very important to take full consideration of the frequency and momentum distribution of the quasiparticle interaction. When this is done the predicted value for T,rises to be of the same order as the experimental value. There is a prediction that if this mechanism is responsible for the coupling the paired state must have d, -yz symmetry.77 It has also been shown that the fluctuations must be about the commensurate wave vector rather than the incommensurate vector otherwise the coupling mechanism is s~ppressed.~~ h9 ‘Magnetic Susceptibility of Superconductors and Other Spin Systems (Proc. Off. Nav. Res. Workshop)’ ed. R. A. Hein T. L. Francavilla and D. H. Liebenberg.Plenum New York. 1991. ’” ‘Frontiers of High Pressure Research’. ed. S. Klotz. J. S. Schilling. and P. Mueller NATO AS1 Series B Plenum 1991 Vol. 286 p. 473. ” B.I. Halperin J. Magn. Muyn. Mut.. 1992 104-107. 761. 72 H.Q. Lin and D. K. Campbell Phys. Ret’.Letr. 1992 69 2415. ’’ J. M. F. Gunn M. J. Godfrey and B. D. Simons J. Map. Mayn. Mut. 1992. 104107. 465. 74 I. R. Pimentel and R. Orbach Phjs. Re?. B,1992 46 2920. 75 S. Sachdev and J. Ye Phys. Rec. Lett. 1992 69 2411. 76 A. Chubukov E. Gagliano and C. Balseiro Phys. Rev. €3 1992. 45 7889. 77 P. Monthoux and D. Pines. Phjs. Rrr. Lett. 1992 69 961. ” A.V. Chubukov Ph-vs. Rev. B 1992 46 5588. A. Harrison and S. J. Clarke The most likely candidate for a magnet with a non-classical ground state that is also realized in nature is the Kagome antiferromagnet whose lattice is depicted in Figure l(a).The classical case with a variety of first second and third neighbour exchange interactions has been studied th~roughly’~ and it looks as though one of two Nee1 states may form at low temperatures. For J2 > J the so-called q = 0 state depicted in Figure 1(b) is favoured while the J3 x 43phase depicted in Figure 1(c) is stabilized for Figure 1 (a) The Kagome latrice and two dtferent spin arrays (b)the q = 0 state and (c) the J3 x J3 state J < J,. Small amounts of diamagnetic impurity will rapidly disrupt the ordered ground state and produce spin-glass phases. Calculations for n-component vector spins (n = 1 2 and 3 correspond to Ising XY and Heisenberg magnets respectively) indicate that the Ising magnet is disordered at T = 0.80Monte Carlo simulations and spin-wave calculations for the classical case indicate that thermal fluctuations favour spin configurations that are coplanar and select a ‘hidden’ type of spin order in which the order parameter is a correlated twist in the spins and is termed a spin nematic.81 There is a different manifestation of ‘order from disorder’ when the effect of quantum fluctuations on the ground state of such a magnet is con~idered.~~.~~ Quantum fluctuations lift the degeneracy of the classical ground state and restore long-range order at T = 0 even when J = 0.A new energy scale for the spin-wave velocity is produced which is a factor S-1’3 smaller than that predicted by conventional spin-wave theory.The heat capacity is predicted to adopt the form C(T)-(T/JS)2-S213at low temperatures in agreement with experimental observation^.^^ When the spins are small there is a quantum disordered ground state and no magnetic long-range order.83** The triangular Heisenberg antiferromagnet with nearest-neighbour or next-nearest- neighbour exchange also continues to attract theoretical attention. The concensus appears to lie with an ordered ground state for the S = 4case,83,85-86 though the ordered moment is thought to be very low. Thermal fluctuations also provide an ordering influence in the classical Ising-Heisenberg case quadratic fluctuations 79 A. B. Harris C. Kallin and A. J. Berlinsky Phys. Rev. B 1992 45 2899.8o D.A. Huse and A. D. Rutenberg Phys. Rev. B 1992 45 7536. ” J.T. Chalker P.C. W. Holdsworth and E. F. Shender Phys. Rev. Lett. 1992 68 855. 82 A. Chubukov Phys. Rev. Lett. 1992 69 832. 83 S. Sachdev Phys. Rev. B 1992 45 12377. ‘4 A. P. Ramirez G. P. Espinosa and A. S. Cooper Phys. Rev. B 1992 45 2505. ‘5 R. R. P. Singh and D.A. Huse Phys. Rer. Lett. 1992 68 1766. 86 B. Bernu C. Lhuillier and L. Pierre Phys. Rev. Lett. 1992. 69 2590. Magnetism 431 preserve the continuous ground state degeneracy but higher-order terms particularly with long wavelengths lift this degenera~y.~~ The pyrochlore lattice (Figure 2) has been described as a three-dimensional form of the Kagome lattice and the frustration produces only short-range order.This has been observed experimentallys8 and predicted from mean-field calculations and Monte Carlo simulation^.^^ The form of the powder neutron diffraction profiles for the magnetic scattering for this class of magnet as well as Kagome and rhombohedra1 cases has been simulated for a variety of temperatures. The diffuse scattering profiles could be matched with experimental profiles for certain values of exchange constants providing a method for their determination.” Figure 2 The pyrochlore lattice represented by uertex-sharing tetrahedra with the magnetic atoms at eiiery vertex Theoretical work on 1D magnets continues to be dominated by Haldane’s conjecture7’ and the study of the dynamic and static properties of pure and dilute 1D S = 1 antiferromagnets.Theory for ESR measurements on diamagnetically dilute forms of the magnet treats the broken chain ends as species with S = $ and adequately treats measurements on NENP (see section 5 under nickel compo~ndsj.~~ The magnetic excitations at the Brillouin zone centre (Q = 0) as opposed to the antiferromagnetic wavevector (Q = n) where the singlet-triplet gap is usually measured are predicted to give rise to a rounded peak in the structure factor S(Q,w) at an energy just above the two-magnon threshold.92 Spin-wave dispersion in 1D antiferromagnets such as the hexagonal perovskites CsNiC1 and RbNiCI, in which there is also frustration between the magnetic chains has been recognized to conform badly to linear spin-wave theory. New treatments of spin-wave dispersion using a Lagrangian formalism predict an additional longitudinal mode parasitic to the transverse modes and this accounts for some of the anomalous beha~iour.~,.~~ ’’ Q.Sheng and C. L. Henley J. Phys. Cond. Matt.. 1992 4. 2937. 88 B. D. Gaulin J. N. Reimers T. E. Mason. J. E. Greedan and Z. Tun Phys. Rev. Lett. 1992 69 3244. 89 J.N. Reimers Phys. Rev. B 1992. 45 7287 90 J.N. Reimers Phys. Rep. B 1992 46 193. 9’ P. P. Mitra B.I. Halperin and I. Affleck. Phys. Rev. B 1992 45 5299. 92 I. Affleck and R.A. Weston Phys. Rec. B 1992,45 4667. y3 M. L. Plumer and A. Caille Phys. Rev. Left. 1992 68,1042. 94 I. Affleck and G.F. Wellman Phys. Rev. B 1992 46 8934. 432 A. Harrison and S.J. Clarke The theory of magnetic X-ray scattering" and of muon spin-rotation and relaxation in fluctuating magnetic systems has been as has the theory of inelastic neutron spectroscopy for PrI*l and Tm"' which provides a probe for the magnetic wavef~nction.'~ The role of double exchange9' in magnetic solids has been reviewed and attention has been drawn to a special case of the Dzyaloshinsky-Moriya intera~tion.~~ When frustration is present the weak ferromagnetic moment that would arise from antisymmetric exchange adopts a unique orientation.Symmetric and antisymmetric exchange are shown to be important in determining the complex spin structure of the monophosphides of Mn and Fe and the monoarsenides of Cr and Mn.99 Work on the theory of magnetic interactions in molecular species is mainly concerned with the search for better organic ferromagnets,' O0 and is divided between work on nitronyl nitroxides' O3 and charge-transfer salts containing metallo- cene~.'~~-'~~ Magnetic interactions in three-centre four-electron clusters,'07~'08 in several types of iron-sulphur clusters,'09 and in manganese cubane clusters' lo have also been considered.Theoretical work on spin-glasses and random-field magnets' '' is not as prominent as it used to be. Reviews of aspects of spin-glasses have been produced covering re-entrant systems,' '' stretched-exponential relaxation for long-range processes in disordered materials,' ' and applications to neural networks.' l4 Monte Carlo simulations continue to be added to the repertoire of theorists though some would argue that the technique could be classed as an experimental one.' '' Progress stems partly from the rapid improvement in computer hardware and installation of algorithms on vector processors,' ' though this may create new problems in the form of the failure of random number generators to maintain randomness in long simulations.* ' There have also been improvements to algorithms problems concerning non-ergodicity have afflicted simulations of spin-glasses for some 95 M. J. Cooper Acta Phys. Pol. A 1992 82 137. 96 E. Balcar and S. W. Lovesey J. Phys. Cond. Matt. 1992. 4. 2271. 97 G. Blondin. S. Borshch and J.J. Girerd. Comments Inorg. Chem. 1992 12 315. 98 L. Shekhtman 0.Entin-Wohlman and A. Aharony Phys. Re].. Lett. 1992 69. 836. ')'J. Sjoestrom J.Phys. Cond. Mutt. 1992 4 5723. loo T. P. Radhakrishnan Curr. Sci. 1992 62 669. I01 K. Yamaguchi M. Okumura. J. Maki T. Noro. H. Namimoto. M. Nakano T. Fueno and K. Nakasuji. Chem. Phys. Lett.. 1992 190 353. lo' K. Yamaguchi M. Okumura and M. Nakano Chem. Phys. Lett. 1992. 191 237. lo' R.N. Musin P.V. Schastnev and S.A. Malinovskaya Inorg. Chem.. 1992 31. 41 18. A. L. Tchougreeff and I. A. Misurkin. Phys. Rev. B 1992. 46 5357. Io5 A. L. Tchougreeff J. Chrm. Phys. 1992 96 6026. 106 K. Yamaguchi. M. Okumura T. Kawamura T. Noro. and K. Nakasuji Mol. Cryst. Liq. Crq'st. Sci. Techno/. A. 1992 218 229. J. R. Hart A. K. Rappe. S.M. Gorun and T. H. Upton J. Phys. Chrm. 1992. 96 6255. lo' J.R. Ilart. A. K. Rappe S. M. Gorun and T. H. Upton. J. Phys. Chem.1992 96 6264. 109 L. C. Noodleman and A. David Adc. Inorg. Chem.. 1992 38 423. E.A. Schmitt L. Noodleman. E. J. Baerends and D. N. Hendrickson J. Am. Chem. Soc. 1992 114,6109 'I1 D.P. Belanger and A.P. Young J. Map. Matn. Mat. 1991 100 272. 112 I. A. Campbell and S. Senoussi. Phil. Mug. R 1992. 65. 1267. 'I3 J. Souletie Phil. Mug. B. 1992. 65. 1311. D. Sherrington M. Wong and A. Rau Phil. Mq. B 1992 65 1303. 'The Monte Carlo Method in Condensed Matter Physics'. ed. K. Binder Topics in Applied Physics Springer Berlin 1992 Vol. 71. '" D.P. Landau in 'The Monte Carlo Method in Condensed Matter Physics'. ed. K. Binder. Topics in Applied Physics Springer Berlin. 1992. Vol. 71. p. 23. 'I7 A.M. Ferrenberg. D. P. Landau and Y. J. Wong. Phys. Rrr. Ixtt..1992. 69. 3382. Mag net ism 433 time and new work should facilitate extension of simulations to lower tempera- tures,' '* and reduce relaxation times.' ' Improvements have been made to techniques for randomly disordered solids allowing faster determination of transition tempera- tures and critical exponents.12' A new algorithm has been developed to tackle the problem of trying to perform simulations for incommensurate magnetic structures while applying reasonable boundary conditions.' 5 Cooperative Magnetism in Ionic Solids Onedimensional Magnets.-The majority of work in this area still concerns Haldane's Conjecture and is described under chain-like compounds of nickel.' 22 There are few new materials even among the organic chain compounds discussed in section 6.Vanadium. RbVBr has a slightly distorted CsNiC1 structure and shows two long-range magnetic-ordering transitions on cooling.' 23 The first appears to have a collinear ferrimagnetic alignment of moments parallel to the chain axis with a small perpendicular ferromagnetic component that is attributed to a Dzyaloshinsky-Moriya interaction; the second shows a large hysteresis and is believed to be to a triangular structure with no ferromagnetic component. A new polymorph of LiVOPO has been synthesized and shown to contain vertex-sharing chains of VO octahedra crosslinked through PO bridges.' 24 The magnetic susceptibility was treated with the Bonner- Fisher model for a 1D S = 4 Heisenberg antiferromagnet. Corner-sharing VO chains are also found' 25 in the 1D antiferromagnet VO(H,AsO,),.Chromium. The quasi 1D antiferromagnet tetramethylammonium chromium(1r) trichloride has a transition to a 3D ordered state at T = 5.8K.12 Manganese. Over the past few years the hexagonal perovskite CsMnBr has provided a model hexagonal antiferrornagnet for the study of the new type of ordered magnetic phase that is expected when frustration and continuous (i.e.not Ising) spin symmetry are combined. The transition between the paramagnetic and chiral phases is tetracritical because the application of a magnetic field to the hexagonal basal plane produces an intermediate spin-flop phase. Measurements of the spin dynamics at the tetracritical point indicate that the critical exponent z adopts a value close to that for a conventional antiferrornagnet.' 27 Other workers have considered the effect of tilting the applied field away from the basal plane.'28 The introduction of a small in-plane lattice distortion upsets the balance of exchange forces within the plane.Such an effect occurs in RbMnBr and leads to an incommensurate magnetic structure which has been studied through linear birefringence measurements. '29 'I8 B. A. Berg and T. Celik Phys. RcL..Lett.. 1992 69 2292. 'Iy S. Liang Phys. Rev. Lett. 1992. 69 2145 120 A. J. F. de Souza and F.G. Brady Moreira Europhys. Lett.. 1992 17 491. 12' W. M. Saslow M. Gabay and W. M. Zhang Phys. Rev. Lett.. 1992 68. 3627. L22 K. Kakurai. Physicu B. 1992. 180 153. lZ3 H. Tanaka T. Kato K. Iio and K. Nagata.J. Phys. Soc. Japan 1992 61 3292. 124 K. H. Lii C. H. Li C. Y. Cheng. and S.L. Wang J. Solid State Chem. 1991 95,352. 12s P. Amoros A. Beltranporter G. Villeneuve. and D. Beltranporter Eur. J. Solid State Inory. Chem.. 1992 29. 257. 12h C. Bellitto L. P. Regnault and J. P. Kenard J. Magn. Mayn. Mut. 1991 102 116. 127 T.E. Mason. Y.S. Yang M.F. Collins B.D. Gaulin K.N. Clausen and A. Harrison J. Mayn. Magn. Mat.. 1992. 104-107. 197. I28 S. I. Abarzhi. A. N. Bazhan L. A. Prozorova and I.A. Zaliznyak J. Phys. Cod. Matt. 1992 4. 3307. 120 T. Kato. K. Iio T. Hoshino. T. Mitsui and H. Tanaka. J. Phys. Soc. Japan 1992. 61 275. 434 A. Harrison and S.J. Clarke Chains of vertex-sharing MnF octahedra are found13* in the new 1D S = 2 antiferromagnet Tl,MnF,-H,O; below TN= 27 1 K,3D antiferromagnetic order sets in.TIMnF,-H,O is a new 1D antiferr~magnet.'~~ It contains zig-zag chains of alternating MnF octahedra and Mn(H,O),F sharing trans corners; magnetic susceptibility data may be fitted to an isotropic Heisenberg model with S = 2 and J = -3.9K. The small value of exchange is attributed to the relatively small Mn-F-Mn bridging angle of 138'. The isomorphous compounds MnXO,.D,O (X = P As) behave as Curie-Weiss paramagnets at high temperatures and order antiferromagnetically at 33 and 24K with ordered moments of 3.52(5) and 3.54(5)pB for the X = P and As compounds respectively.'32 Their structure may be regarded from a magnetic viewpoint as based on chains of Mn-0-Mn units. Axially-distorted MnO octahedra are linked through their vertices to form zig-zag chains which in turn are linked through the phosphate groups to form a 3D network.The compound MnMn(CDTA)-7H20 (CDTA is the tetra anion of trans-cyclo- hexane-1 ,2-diarnine-N,N,N',Nf-tetraacetic acid) contains chains of dimeric manganese molecules connected through carbolate bridges (J as depicted in Figure 3;'33 Mna--Mna--Mna--ana--J1 Figure 3 Exchange pathways in MnMn(CDTA).7H20 alternatively it may be regarded as a chain of exchange-coupled triangles which share corners. Magnetic susceptibility measurements were successfully interpreted with a frustrated chain model with sites a and b connected through three different exchange constants. MnCu(obze)(H,O) [obze = oxamido(N-benzoato-N'-ethanoato) and MnCu(pbaOH)(H,O) (pbaOH = 2-hydroxy-1,3-propylenebis(oxamato)]behave as ferrimagnets.34 Antiferromagnetic exchange propagates through 0-carboxylato bridges to produce values for T,of 4.6and 30K for the two materials the latter being a record for a molecular-based magnet. Iron. The hexagonal perovskites AFeX (A = Rb NH, Cs and T1; X = CI Br) provide good examples of induced moment magnets. The isolated Fe" ions have a singlet ground state but exchange with neighbouring ions may mix in a low-lying excited doublet to produce a moment if the ratio of exchange to singlet-doublet 130 P. Nunez A. Tressaud J. Darriet. P. Hagenmuller G. Hahn,G. Frenzen W. Massa D. Babel,A. Boireau. and J. L. Soubeyroux Inory. Chem. 1992 31. 170. 13' P.Nunez A. Tressaud F. Hahn W. Massa D. Babel A. Boireau and J. L. Soubeyroux Phys. Stuf.Solidi 1991 127. 505. 132 M.A.G. Aranda J. P. Attfield S. Bruque and F. Palacio. J. Muter. Chem. 1992 2. 501. 133 J. J. Borras-Almenar E. Coronado J. C. Gallart R. Georges and C. J. Gomez-Garcia J. Magn. Magn. Mut. 1992 104-107 835. 134 E. Codjovi. P. Bergerat K. Nakatani P. Yu and 0.Kahn J. Mugn. Mugn. Mat. 1992. 104107. 2103. Magnetism 435 splitting is sufficiently large or if an external magnetic field is applied. The relation between structure and magnetism has been reviewed135 and the dispersion of magnetic excitations in RbFeBr has been used to test theories of excitations in this class of magnet.' 36 A variety of effective Hamiltonians have been tested against quadrupole- splitting data taken from Mossbauer measurements for AFeX (A = Rb Cs; X = C1 Br).'37 CsFeBr remains a singlet ground-state material down to zero Kelvin'38 but long-range magnetic order may be induced by a magnetic field of 4.1 T.'39 The dispersion of spin-waves in the 1D Heisenberg antiferromagnet KFeS has been measured out to the Brillouin zone boundary energy of 221(4)meV using the new time-of-flight spectrometer MAR1 at ISIS.14' This value is far greater than that obtained from extrapolation of measurements taken with a conventional spectrometer at lower energies nearer the zone centre.TlFeS is a 1D Heisenberg antiferromagnet with a small degree of dimerization within the chain of low-spin Fe"' ions.141 Careful neutron and SQUID magnetometry measurements on single crystals indicate no evidence for singlet formation within the dimeric units and that the moments are greatly reduced through covalency.Cobalt. CsCoX (X = C1 Br) continues to provide a good model S = $ Ising antiferrornagnet.l4' The role of solitons in and between the different ordered phases of pure and diamagnetically-doped samples has been studied by neutron scattering 143 muon or ' spin rela~ation,'~~~~~~ or high-field differential magneti~ation,'~~ magneto-optical measurements. 47 The pseudo 1D Ising antiferromagnetic [(CH,),NH]Co -.Ni,CI shows a transition to 3D long-range canted antiferromag- netic order'48 with a critical exponent /3 = 0.306. Compounds doped with Ni" were also studied in the belief that a re-entrant spin-glass phase might be found at low temperatures and intermediate compositions but no clear evidence of such a phase could be found.Co2(EDTA).6H,O and CoCu(EDTA)-6H20 are two of a series of compounds with general formula MM'(EDTA).6H20 which contain zigzag chains of alternating hydrated and chelated -M(H,O),O,-M'(EDTA)-. EPR measurements of Co-doped compounds in which MM' = (Zn) and the bimetallic compound with MM' = CoCu provide estimates of the g-tensors of both metal ions and indicate that there is dimerization of Co-Cu exchange coupled pairs within the chains.' 50 135 D. Visser and A. Harrison J. Magn. Magn. Mat. 1992 116 80. 136 A. Harrison and D. Visser J. Phys. C 1992 4 6977. 13' V. H. McCann J.A. Laban and N. Sheen Hyperfine Int.1992 71 1363. 13' B. Schmid B. Dorner D. Visser and M. Steiner Z. Phys. B. 1992 86 257. 139 B. Schmid B. Dorner D. Visser and M. Steiner J. Mugn. Map. Mat. 1992 104107 771. I4O D. Web M. Kohgi Y. Endoh M. Nishi and M. Arai Phys. Rw. B 1992 45 2319. I4l D. Welz and M. Nishi. Phys. Reu. B 1992 45 9806. 14' J.A. Laban and V.H. McCann Hyperjine Int. 1992 71 1367. 143 K. Okuda S. Noguchi K. Konishi H. Deguchi and K. Takeda J. Map. Magn. Mat. 1992 104-107 817. '44 M. Mekata S.Onoe H. Kuriyama B. J. Sternleib Y. Uemura and K. Nagamine J. Magn. Map.Mat. 1992. 104-107 825. 145 T. Kohmoto,T. Goto S. Maegawa N. Fujiwara Y. Fukuda M. Kunitomo,and M. Mekata Phys. Lett.A 1992 167 493. 146 H. Hori H. Mikami M. Date Y. Ajiro and N. Mori J. Magn. Magn.Mat. 1992 104107 815. 14' H. Hori H. Mikami and M. Date Physica B 1992 177 363. 148 T. Bruckel W. Prandl and K. Hagdorn J. Magn. Map. Mat. 1992. 104107 1629. '49 E. Coronado J. Chrrn. Phys. 1991 88. 2167. J.J. Borras-Almenar E. Coronado. D. Gatteschi and C. Zanchini Inorg. Chem. 1992 31. 294. 436 A. Harrison and S. J. Clarke A new double-chain compound has been ~repared'~' in the form of CO(OH)(NO,).H,O.~~~ Susceptibility measurements indicate that the coupling between S = $ Ising spins may be described in terms of two nearest-neighbour in-chain antiferromagnetic exchange constants J = 22.8 K and J = 9.75 K; interchain exchange of the order of 0.2 K leads to 3D magnetic long-range order below about 2.5 K. Nickel. Chain-like compounds of Ni" continue to provide the most extensive source of model materials with which to test Haldane's Conjecture it has been proposed that 1D Heisenberg antiferromagnets composed of integral spins have a non-classical ground state and excitations.The effect becomes more pronounced as the moment decreases hence the interest in S = 1 1D antiferromagnets with small single-ion anisotropy. The majority of work concerns Ni(C2H,N,),N0,C104 (NENP) and the related materials Ni(C3HloN2),(C104) (NINO) Ni(C,HloN,),N,(C104) (NINAZ) and Me,NNi- (NO,) (TMNIN)." The application of a strong magnetic field (-10T) to single crystals of NENP and NINO closes the gap in energy between the ground state and the first excited state and provides a value for both the Haldane gap and crystal field parameters.' 53 Similar measurements on TMNIN and NINAZ provide estimates of their gap energy.'54 NENP has been the subject of inelastic neutron scattering measurements of the magnetic correlation length'" and its response to applied magnetic fields'56 as well as magnetic susceptibility,'" heat capacity,'58 ESR,'59,'60 and muon spin relaxation measurements of the fluctuations in the magnetic moments.'61*'62 These appear to confirm that the excitations are to a state with S = 1 and also that when the magnetic chains are broken the free ends behave as though the moments on Nil' have S = $.This last result which is also difficult to understand with a classical picture of localized moments coupled through exchange forces confirms the belief that the moment of S = 1 on a localized Ni" site is best regarded as the symmetric part of two species with S = 4.'63,164 The application of Haldane's theory to CsNiC1 and RbNiC1 runs into difficulties on account of their 1D character.16sp' 69 Detailed neutron scattering measurements of S.Angelov M. Drillon E. Zhecheva R. Stoyanova M. Belaiche A. Derory. and A. Herr Inorg. Chrm. 1992 31 1514. L52 K. Kindo T. Takeuchi T. Yosida and M. Date Physica B 1992 177 381. 153 T. Takeuchi H. Hori T. Yosida Y. Akio. K. Katsurnata. J. P. Renard. V. Gadet M. Verdaguer and M. Date J. Phys. SOC.Japan 1992 61 3251. 154 T. Takeuchi H. Hori T. Yosida. A. Yamagishi K. Katsurnata J. P. Renard V. Gadet M. Verdaguer and M. Date J. Phys. SOC.Japan 1992 61 3262. Is5 L.P. Regnault J. Rossat-Mignod and J. P. Renard J. Magn. Magn. Mar.. 1992. 104107 869. L. P. Regnault C. Vettier J. Rossat-Mignod and J. P. Renard Physica B. 1992 18Q-181 188. 15' 0.Golinelli T. Jolicoeur. and R. Lacaze Phys. Reu. B 1992. 45 9798. 158 T. Kobayashi Y. Tabuchi K. Amaya Y. Ajiro,T. Yosida and M. Date,J. Phys. Soc.Jupan 1992.61 1772. 159 L. C. Brunel T. M. Brill I. Zaliznyak J. P. Boucher and J. P. Renard Phys. Reu. Lett. 1992 69 1699. 160 M. Hagiwara and K. Katsurnata. J. Phys. Soc. Japan. 1992 61. 1481. 16' B. J. Sternlieb L. P. Le G.M. Luke W. D. Wu Y. J. Uernura T. M. Riseman J. H. Brewer Y. Ajiro and M. Mekata J. Map. Magn. Mat. 1992 104-107 801. 162 M. Motokawa H. Nojiri K. Nishiyama K. Nagamine and T. Yosida. Physica B 1992 177 389.I63 M. Hagiwara K. Katsumata J.P. Renard. I. Affleck and B. I. Halperin J. Magn. Map Mat. 1992 104107 839. 164 M. Hagiwara K. Katsumata. H. Hori T. Takeuchi M. Date A. Yamagishi J. P. Renard and I. Affleck Physica B 1992 177 386. 165 L. S. Carnpana A. Caramico D'Auria F. Esposito U. Esposito and G. Kamieniarx Phys. R~G. B.. 1992 45 5035. K. Nakajima,K. Kakurai H. Hiraka,H.Tanaka K. Ito,and Y. End0h.J. Phys. Soc.Japun,1992,61,3355. Magnetism 437 the excitations in CsNiC1 in an applied magnetic field have been used to deduce which components of the spins fluctuate in the various dispersion curves and although in agreement with Affleck's extension of Haldane's theory to coupled chains a more conventional interpretation cannot be ruled out.A comparison between these and the non-integral-spin antiferromagnet CsMnI has been made with aid of quasi-elastic neutron scattering measurements of the dependence on temperature of the magnetic correlation length. It indicates that there is a difference between the two types of magnets with the integral-spin materials showing a finite correlation length at T = 0.170 Hexagonal perovskites of the form ANiX (A = Rb Cs; X = F C1 Br) provide models for the study of magnetic phase transitions in frustrated magnets' 73 and of solitons.' 74 CsNiI shows a magnetic susceptibility that indicates itinerant electrons rather than localized spins.' 74 The lanthanide nickelates Ln,BaNiO (Ln = Y Nd-Gd and Dy-Tm) contain isolated chains of flattened NiO octahedra.Magnetic susceptibility measurements on compounds with Ln = Nd Eu Dy and Ho indicate 1D behaviour involving Ni" above room temperature and 3D ordering at temperatures between 10K and 44K depending on Ln. '75 Chain-like structures containing Ni-0-0-Ni exchange path- ways are found in NiSb,O and susceptibility data could be fitted to a 1D Heisenberg model with J -45 K.'75 Copper. The ID S = $ Heisenberg antiferromagnet KCuF has been studied by high-energy inelastic neutron scattering measurements.' 76 These show that even the gross features of the dispersion of magnetic excitations can only be described with a model that takes account of quantum effects. Magnetic susceptibility measurements on salts derived from CuX (X = C1 Br) indicate various types of low-dimensional magnetism CuCl,.DMSO appears to be a 1D S = 5 Heisenberg ferr~magnet,"~ (piperazinium),CuCl,.CH,OH may be a weak 1D antiferrornagnet as is (1,2- dimethylpyridinium),Cu,Br, which is composed of trimers of moments with ferromagnetic exchange within trimers and antiferromagnetic coupling between them.'78 Zig-zag chains of Cu" ions are found in the new 1D ferromagnet CuL(H,O) (H,L = 1,3-dimethyl-5-((2-carboxyphenyl)azo)barbituric acid).' 79 The high-ir superconducting ceramic YBa,Cu,O contains both layers and +X chains of coupled moments on Cu".The magnetic properties of Ca,CuO, which 16' M. Enderle K. Kakurai M. Steiner and B. Dorner Physica B 1992 18(t-181. 233. IhHM. Enderle K. Kakurai M. Steiner and H. Weinfurter. J. Mayn.Mayn. Mat. 1992 104-107 809. I69 W. J. L. Buyers Z. Tun A. Harrison J.A. Rayne and R. M. Nicklow Physicu B 1992 180 181 222. "" K. Kakurai K. Nakajima Y. Endoh K. Iio H. Tanaka and M. Steiner. J. Mayn. Mayn. Mar. 1992 104-107 857. 171 Y. Oohara. K. Lo H. Tanaka and K. Nagata J. Phys. Soc. Japan 1991. 60. 4280. "'S. Maegawa T. Kohrnoto T. Goto and N. Fujiwara. Phys. Rw. B 1992 44. 12617. T. Mitsui. K. Abe and K. Lo J. Magn. Map Mat. 1992. 104-107 819. T. Delica W.J. M. DeJonge K. Kopinga H. Leschke and H. Mikeska J. Magn. Mayn. Mat. 1992. 104107 795. R. Saez-Puche. J.M. Coronado. J. M. Martin-Llorente and I. Rasines. Mat. Chrm. Phys. 1992,31 151. I76 S. E. Nagler D.A.Tennant R.A. Cowley T.G. Perring and S. K. Satija. Phys. Reu. B 1991.44. 12361. li' K.Ravindran and J. E. Drumheller J. Mugn. Magn. Mat. 1992 104-107. 833. T. E. Grigereit Y. Liu P. Zhou J. E. Drumheller. A. Bonomartini-Corradi M. R. Bond H. Place and R. D. Willett J. Map. Magn. Mat. 1992 104-107 831. ''' E. Colacio J. M. Dominguez-Vera J. P. Costes R. Kivekas J. P. Laurent. J. Ruiz. and M. Sundberp. Inory. Chem. 1992 31 774. A. Harrison and S. J. Clarke contains a similar type of chain of coupled moments have been interpreted using a 1D Heisenberg antiferromagnetic model which indicates no magnetic moments; the same scientists studied Li2Cu04 which has Cu-0,-Cu chains with 90" exchange angles and show that it contains ferromagnetically coupled chains of moments which freeze to 3D antiferromagnetic order below 9.3 K.'" Two-dimensional Magnets.-Much of the interest in 2D systems in recent years has centred around the layered cuprate ceramic superconductors and associated with this the possibility of attaining a quantum ground state in a frustrated low-moment system such as a triangular or Kagome lattice as discussed in section 4.Titanium. NaTiO remains the most promising candidate for an S = triangular Heisenberg antiferromagnet but is dogged by difficulties in sample preparation. Recent work suggests that the pure material undergoes a magnetic phase transition at 260 K but there is no evidence for long-range antiferromagnetic order. la' Chromium.The garnet SrCr,Ga -,O remains the only Kagome system that has been investigated in any detail,a4 but has the disadvantage of incomplete occupancy of the Kagome planes.Heat capacity measurements indicate an unusual dependence on temperature ofthe form T2,which corresponds to the predictions of theoretical work.82 Rb,Cr,Mn -,Cl is a mixed easy-axis-easy-plane and mixed ferro-antiferromag- netic material which has been studied for several years as a spin-glass. Raman182 and FIR' 83 measurements confirm ferromagnetic long-range order for chromium-rich compositions; antiferromagnetic Mn-Cr exchange and long-range easy-plane antifer- romagnetism occur only at very low chromium concentrations (x < 0.05).Competi-tion between 2D and 3D ordering is observed near the ferro- to paramagnetic transition for a sample of composition x = 0.8; as expected 2D order changes to 3D order on cooling followed by a reversion to a 2D ordered phase on further cooling.' 84 A new example of an insulating ferromagnet (PhCH2NH3)2CrBr3~,T,,7 has been prepared:18' it crystallizes in the K2NiF structure and has a Curie temperature of 51 K.Manganese. Antiferromagnetic resonance measurements on the 2D easy-plane anti- ferromagnet Rb2MnCI4 indicate that there is a phase transition to an orthorhombic structure accompanied by a transition to a multi rather than single domain antiferromagnetic structure.Ia6 Mn(HCO0),.2((H2N),CO) is one of a series of isomorphous compounds of the form M(HCOO),-2((H2N)CO) which provide good model square antiferromagnets with a large ratio (JIJ').Work on samples with M = Mn Fe Co and Ni have been reported to have Nee1 temperatures of 3.77 K (Mn) 7.9 K (Fe) 6.34 K (Co) and 15 K (Ni).la7 lB0 K.Okuda S. Noguchi K. Konishi H. Deguchi and K. Takeda J. Magn. Magn. Mat. 1992 104107 817. l'' K. Takeda K. Miyake and K. Hirakawa J. Phys. SOC.Japan 1992,61. 2156. lB2 A. T. Abdalian C. Dugautier P. Moch and B. Briat Phase Trans. 1991 33 177. T. Grieb T. Pabst A. Kieslich J. Linder H. Rauschrnann K. Strobel W. Treutmann and R. Geick J. Phys. Cond. Matt. 1991 3 9751. D. Sieger W. Schmidt H. Tietza-Jaensch R. Geick P. Schweiss W. Treutmann and H. Godfrin J. Magn. Magn. Mat. 1992 104107 895. lB5G. Staulo and C. Bellitto J. Mater. Chem. 1991 1 915. '" H. Greb T. Pabst M. Rothaler. A. A. Mukhin A. Y. Pronin K. Strobel and R. Geick J. Phys. Cond. Matt. 1992 4 2281. K. Yamagata T.Abe Y. Higuchi H. Deguchi K. Takeda K. Kaneko H. Nojiri and M. Motokawa J. Magn. Magn. Mat. 1992 104-107 803. Mag net ism 439 Layered titanates containing manganese and other transition ions are among many such materials prepared in anticipation of spin-glass properties. Ni,Mn -,TiO combines NiTiO and MnTiO in which the transition ions have different directions of single-ion anisotropy that combined with exchange frustration leads to re-entrant spin-glass phases and modified antiferromagnetic phases.' 88 In Fe -,Mn,TiO, transition ions in the parent compounds have similar single-ion anisotropies both having Ising moments but the details of the exchange differ producing spin-glass behaviour in mixed compounds.' 89*190 TlMnF is a 2D antiferromagnet which undergoes 3D ordering at 4.2K.19' New work on KMnF and RbMnF using powder neutron diffra~tion'~ indicates that these compounds have antiferromagnetic ordering temperatures of 6.6 K and 3.9 K respectively.Iron. In Lu,Fe,O, there appears to be two types of antiferromagnetic ordering at 60 and 230K corresponding to the ordering of different layers in the struct~re.'~~ The mixed-valence compound a-Fe,(PO,)O possesses ferromagnetic sheets which are coupled antiferromagnetically . 94 Cobalt. Single-crystal neutron diffraction from the helical antiferrornagnet CoI indicates that there is a first-order magnetic phase transition at 9.4K below the established ordering transition of 11.O K.'95There was however no detectable change in the spin structure below this transition.Nickel. There has been much interest in nickel analogues of the high T superconduc-tors such as La,NiO, Nd,NiO,. and Pr,NiO, both in relation to the unusual magnetic and electronic properties of the cuprates and in their own right as antiferromagnets with a rich variety of spin structures. La,NiO orders antiferromagnetically at 325 K and undergoes a structural phase transition at 185K which is accompanied by the appearance of a ferromagnetic moment due to canting of the spins out of the antiferromagnetic layers.' 96,197 Both J and J' are much smaller than in the corresponding cuprates and the nickelate is more three-dimensional (J'IJ)is 5 x compared with 2 x for the corresponding cuprates. When La is progressively replaced by Sr the material is transformed from an antiferromagnetic insulator to a metallic state as may be demonstrated by the steady reduction in internal field in '39La NMR and NQR measurement^.'^^ This change is accompanied by the replacement of commensurate antiferromagnetic order with the wavevector (n,n)by incommensurate correlations centred at Q = (ni-Sn,n 2 Sn) and (n& dn n T dn) with 6 -0.16 for x = 0.2 in La2-xSrxNi0,+,.The length- '*' A. Ito H. Kawano H. Yoshizawa and K. Motoya J. Magn. Magn. Mar. 1992. 104-107 1637. A. Ito S. Ebii H.A. Katori and T. Goto J. Mayn. Mayn. Mat. 1992. 104-107 1635. 190 H.A. Katori T. Goto S. Ebii and A. 110. J. Magn. Magn. Mat. 1992. 104107. 1639. 19' P. Nunez A. Tressaud J. Grannec P.Hagenmuller W. Massa D. Babe1,A.Boireau and J.L. Soubeyroux 2.Anorg. Allg. Chern. 1992 609 71. 19' M.C. Moron F. Palacio and J. Rodriguez-Carvajal Physica B 1992 180 125. 193 J. Iida M. Tanaka and S. Funahashi J. Magn. Magn. Mat. 1992 104-107 827. 194 M. Ijjaali G. Malaman C. Gleitzer G.J Long and F. Grandjean J. Phys. Cond. Matt. 1991 3,9597. 19' M. Mekata H. Kuriyama Y. Ajiro. S. Mitsuda and H. Yoshizawa. J. Magn. Mugn. Mat. 1992,104-107 859. 196 K. Yamada T. Omata K. Nakajima S. Hosoya. T. Sumida and Y. Endoh. Physica C 1992 191 15. 19' X. Batlle X. Obradors. M. J. Sayagues M. Vallet and J. Gonzalez-Calbet J. Phys.:Cond. Matt. 1992.4. 487. 198 Y. Furukawa and S. Wada. J. Phys. Soc. Japan 1992. 61 1182. 440 A. Harrison and S. J. Clarke scale of the correlations is equal to the average separation of the Sr atoms.’99 This should be contrasted with the cuprates in which the incommensurate correlations are centred at Q = (n n +6n)and (n& 6n,n).The microscopic influence of the dopant is still unclear.DC and AC susceptibility measurements show the existence of five different magnetic phase transitions in Nd,NiO in an applied magnetic field.200 Pr,NiO shows a structural phase transition at 117K which signals the emergence of a ferromagnetic moment as with the La compound.201 Copper.Work on layered copper compounds continues to be dominated by the high-Tc superconducting ceramics their parent compounds and related cuprates. As was discussed in section 4,much of the inspiration for this work lies in the belief that there may still be some causal link between the magnetic fluctuations and the superconduct- ing behaviour of these materials.The field is not quite as active as in past years but its greater maturity has lead to a steady flow of significant results. We wish to draw the reader’s attention to a small collection of findings that are primarily concerned with the nature of the magnetic fluctuations in materials derived from La,CuO and YBa2Cu,0,. It is now well established that the magnetic fluctuations in the normal state of La,-,Sr,CuO are centred at incommensurate wave-vectors Q = (n n _+ 6n) and (n+ dn n) with 6 -x; inelastic neutron scattering measurements of samples with x = 0.14 and 0.15 indicate that the structure factor S(Q,w) for paramagnetic fluctuations peak in Q as expe~ted,’~’,~~~ but differ in their observations of the energy and temperature dependence.The first set of workers observed a peak in S(Q,w) at T when the sample is cooled for energy transfer down to 4meV; they argued that the pairing energy is very much less than 3.5kTCand comparable to the energy of the incommensurate fluctuations. The onset of superconductivity seems to suppress the fluctuations though the correlation length does not change with temperature. The second set of workers observed no maximum in S(Q,o)at T, but rather a rise on cooling to T followed by a levelling-off. In both cases there is a clear discrepancy with NMR measurements that probe the antiferromagnetic fluctuations at zero energy. Recent improvements to the theory of the coupling between the various nuclear moments and the fluctuations allow an extension to be made to finite frequencies but the data still require a temperature-dependent antiferromagnetic correlation length that is appreciably longer than that measured with neutrons.204 Spin-wave dispersion measurements on superconducting samples of YBa,Cu,O + show an energy gap E which rises relative to T as x increases the ratio EG/T is very small for x -0.1 and rises to the asymptotic value 3.5 as x increases to 0.92 though it 199 S.M.Hayden G. H. Lander J. Zarestky P. J. Brown C. Stassis. P. Metcalf and J. M. Honig Phys. Rec. Lett.. 1992 68 1061. X. Batlle B. Martinez X. Obradors M. Pernet. M. Vallet. J. Gonzalez-Calvet and J. Alonso. J. Mugn.Magn. Mat. 1992 104-107 918. 20 1 M. T. Fernandez-Diaz J. L. Martinez J. Rodriguez-Carvajal P. Odier. G. Fillion J. Beille B. Barbara and M. Cyrot Physica C 1991 185189 1225. 202 T. E. Mason G. Aeppli and H.A. Mook Phys. Rev. Lett. 1992 68 1414. ’03 T. R. Thurston P. M. Gehring G. Shirane R. J. Birgeneau K. A. Kastner Y. Endoh M. Matsuda K. Yamada H. Kojima and I. Tanaka Phys. Rev. B 1992. 46 9128. ’04 A. J. Millis and H. Monien Phys. Re[>.E. 1992 45 3059. Magnet ism 441 remains considerably smaller than the ratio of the superconducting charge gap 2A to kTc.205 The layered salt Cu(HC002),.4H20 has been recognized for some years to be a S = '2 square Heisenberg antiferromagnet. Recently attention was drawn to its suitability to test advances in the theory of this class of magnet as applied to high-T superconducting cuprates.It provides certain advantages over the cuprates as a model magnet it is relatively easy to grow large pure single crystals that when deuterated are suitable for inelastic neutron scattering measurements; the in-plane exchange is much smaller than in the cuprates so certain magnetic properties such as the spin-wave dispersion may be measured more easily.206 A neutron scattering study of the critical exponents for the long-range ordering transition at 16.5 K showed a cross-over in b from a value of 0.23(1) to 0.32(2) at the reduced temperature of 0.06K.207The dependence of the magnetic correlation length on temperature in the paramagnetic phase was fitted to the expression derived by Chakravarty Halperin and Nelson for this class of magnet and the exchange constant shown to be -89(3)K which compares with a value of -72 K derived from susceptibility measurements.On doping with a concentration x of the diamagnetic ions Mg2 or Zn2 +,T was seen to fall off + with x in a similar manner to La2Cul -xZnx04 and other non-Ising 2D antiferromag- nets.208 Three-dimensional Magnets.-Once again the technological importance of ferrites and garnets drives research in magnetism to such an extent that articles on these materials greatly outnumber those on the remainder of materials that cannot sensibly be regarded as low-dimensional. There are no comprehensive reviews this year and we have incorporated work on the fundamental properties of such materials throughout this section.The magnetic properties of diluted magnetic semiconductors also receive considerable attention in the form of and individual papers cited below. Vanadium. VNbF is built from corner-sharing VF and NbF octahedra in a similar manner to LiSbF,. Superexchange between Nb" and V" leads to a ferrimagnetic transition at T = 3 K." Chromium. The magnetic structure of Cr2F has been determined by powder neutron The magnetic space group C2/c is the same as the crystal space group and at variance with previous predictions leading to a revision of the superexchange mechanism. The first-order paramagnetic to antiferromagnetic transition in Cr,O has been studied by linear birefringence213 and the critical exponent shown to be 0.355.The role of magnetostriction and the response of the magnetic properties to pressure changes has been deduced for the NiAs-structure semiconductors CrTe -xSex (x = 0.2-0.4).214 Exchange interactions in a wide range of chromium spinels and their 205 J. Rossat-Mignod L. P. Regnault P. Bourges C. Vettier P. Burlet. and J. Y. Henry Phys. Scr. 1992 T45. 74. 206 A. Harrison S.J. Clarke T. E. Mason. and D. Visser J. Magn. Magn. Mat. 1992 104-107 557. '"'S.J. Clarke A. Harrison T. E. Mason G.J. Mcintyre and D. Visser. J. Phys.:Cond. Matt. 1992.4. L71. 'Ox S. J. Clarke and A. Harrison J. Phys. Cond. Mutt. 1992 4. 6217. 'OY W.J. M. Dejonge and H. J. M. Swagten J. Magn. Magn. Mat. 1992 100 322. 210 A. Twardowski Phys. Scr. 1991 T39 124. T.Lemercier J. Chassaing D. Bizot and M. Quarton Mat. Res. Bull. 1992 27 259. '" P. Lacorre G. Ferey and J. Pannetier J. Solid State Chem. 1992 96.227. *I3 R. V. Pisarev B. B. Krichevtsov and V. V. Pavlov Phase Trans. 1991 37 63. 214 H. Yoshida T. Kaneko M. Yuzuri Y. Adachi T. Kanomata and T. Suzuki J. Magn. Magn. Mat.. 1992 104-107 1983. 442 A. Harrison and S.J. Clarke sulfur and selenium analogues have been reviewed. l5This type of compound provides a wide range of helimagnet canted and glassy spin structures thiospinels related to CuCr,S and doped with Ge Sb and Gd,210*216*217 based and selenium spinels218-220 on ZnCr,Se or CdCr,Se have produced such magnetic phases. The thiospinel Zn,Cd -xCr2S4 shows a transition from ferromagnetic221 to spin-glass behaviour as x is increased from 0 and substituent ions alter the ratio of nearest and next-nearest exchange interactions.222 Finally the ,'Te Mossbauer spectrum of Cr -,Fe,TeO (x = 0.4-1.5) indicates that the replacement of chromium by iron produces an imbalance in the exchange interaction through Te which in turn leads to a magnetic hyperfine field there.,' Manganese.MnF has been the subject of magneto-optical studies of exciton-magnon sidebands for some years and the tradition continues with work on the dephasing of large wave-vector magnons through intersublattice transitions,, and the manner in which magnons diffuse through the solid.225 The diamagnetically doped compound Mn,,,Zn,,,F still excites interest because it is a random-field Ising magnet.226 The bimetallic salt [Cr(H,0)(NH3),][FeC16] is composed of a network of ordered coordination polyhedra centred on high-spin Fe"' (S= 3)and Cr"' (S = $) and linked by ionic forces and H-b~nds.,,~ Antiferromagnetic exchange produces several ferrimagnetic phases at low temperatures.CuMnO has been prepared by heating a mixture of CuO and MnO under pressures of up to 50 kbar and shown to be a ferrimagnet containing Mn" and Mn'" with a Nee1 temperature of 235 5 2K.,,* Magnetic semiconductors containing manganese have been prepared as thin MnTe is stabilized in a zinc blende rather than a NiAs structure when grown epitaxially on GaAs or in strained MnTe/ZnTe superlattices and has been the subject of a study of the influence of strain on the magnetic order.23 Spin-glass transitions in the diluted magnetic semiconductor (Zn -,Mnx),As2 have been observed for samples with x = 0.10 and 0.13 at the unusually high temperature of 200 One method of producing frustrated magnetic 2'5 S.Juszczyk. J. Map. Magn. Mat. 1992. 112. 449. 216 R. K. Gubaidullin R.A. Sadykov T.G. Aminov and E. V. Amerikova Neorg. Mat. 1992 28 1377. '" K. P. Belov L. I. Koroleva N. P. Pislyakova E.A. Amerikova G.G. Shabunina and T.G. Aminov Inorg. Mat. 1991 27 1873. 218 H. Rej A. Bombik J. Kusz A. Oles M. Pinot and J. Warczewski J. Magn. Map. Mat. 1992. 111.47. 219 J. Krok-Kowalski J. Warczewski T. Mydlarz A. Pacyna A. Bombik J. Kopyczok and I. Okonska-Kozlowska J. Magn. Map. Mat. 1992 111 50. 220 Y. H. Kim S. M. Bhagat M.A. Manheimer S. Tyagi L. Maksymowicz and M. Lubecka IEEE Trans. Magn. 1992 28 3195. 221 S. Pouget M. Alba N. Fanjat and M. Nogues Physicu B 1992 180-181 244. 222 M. Nogues D. Fiorani J. Tejada. J. L. Dormann. S. Sayouri A.M. Testa and E. Agostinelli J. Magn. Magn. Mat. 1992. 10k107 1641. "' F. Berry and C.D. Gibbs Hyperfine Int. 1991 67 513. 224 M. L. J. Hollman A. F. M. Arts and H. W. De Wijn J. Magn. Magn. Mat. 1992 104-107 1063. 225 L. D. Rotter W. M. Dennis and W.M. Yen Phys. Rev. B 1991 44,11 806. 226 F.C. Montenegro J.C.O. De Jesus F. L.A. Machado E. Montarroyos and S.M. Rezende J. Magn. Magn. Mar. 1992 104107 277. 227 M.C. Moron F. Palacio J. Pons and J. Casabo J. Map. Magn. Mat. 1992 114. 243. "' I. 0.Troyanchuk A. A. Shemyakov and V. K. Prokopenko Fiz.Tverd. Tela 1991. 33 964. 22y K. Ando K. Takahashi and T. Okuda J. Map. Mayn. Mat. 1992. 104-107 993. 230 T. M. Gielbultowicz P. Klosowski J.J. Rhyne N. Samarth L. Hong and J. K. Furdyna Physicu B 1992 18@ 181 485. 23' P. Klosowski T.M. Giebultowicz N. Samarth H. Luo J.K. Furdyna and J.J. Rhyne J. Magn. Magn. Mat. 1992 104-107 1795. 232 A.V. Lashkul E. Lahderanta. R. Laiho and V.S. Zachvalinskiy Phys. Rev. B 1992 46 6251. Magnetism 443 structures that has not been applied to inorganic solids very much is the use of a glassy host material :measurements on manganese aluminosilicates clearly indicate spin-glass transition^.^^^'^^^ When Ising spins are added as in Mn,Ho,Al -.T(Si04)3 (x = 0-0.05),a second ordering transition is observed in low-field measurements of the imaginary part of the magnetic s~sceptibility.~~~ The two transitions are believed to correspond to separate ordering of the Ising and the Heisenberg moments with the possibility of an intermediate semi-spin-glass phase.Iron.One polymorph of FeF adopts a pyrochlore structure which may be described as a network of edge-sharing FeF tetrahedra (Figure 2). The topological frustration is believed to place the material in a new magnetic universality class leading to an unusual value for the critical exponent fi of 0.18(2) as measured by powder neutron difli-action. Complementary Monte Carlo simulations confirm this value and produce estimates for other critical exponents v = 0.38(2) y = l.l(l) and a = 0.6(1).236 Compounds of iron and fluorine feature prominently in a review on fluorinated bronzes of divalent and trivalent 3d transition metal ions and new frustrated iron compounds.237 The nature of the ferrimagnetic transition at T -88 K in the Weberite material Na2NiFeF7 has been studied further by Mossbauer spectroscopy measure- ments which indicate that the net moment points along the a axis.238 Similar measurements on (NH,),FeCI ,.H,O suggest that previous work on the relation between structure and magnetism in this material is wrong and indicates that the a axis is an easy axis.239 Neutron diffraction measurements have revealed the magnetic structures of a number of iron compounds.Fe,Na,(PO,) is a fast-iron conductor which is a member of the NASICON family and has a structure based on vertex-sharing FeO and PO polyhedra.When Fe-0-P-0-Fe superexchange is combined with the lattice topology a frustrated structure with a weak ferromagnetic component is The orthoferrite BiFeO has been shown to have a cycloidal spiral structure241 with a pitch of 620A and a short-range canted antiferromagnetic structure is found in a series of Li-Zn ferrites.,, The Mg-Zn ferrites Zn,Mg _,Fe2O4 show a transition from ferri- (x < 0.5) to antiferromagnetic behaviour (x = 1) uia a randomly canted phase and re-entrant spin-glass beha~iour.~, Work on other spinels and thiospinels includes GdFe,O, Fe,,,Cu,~,Rh,S, and the Verway transition in Fe,0,.244-248 A high (S = 2) to low (S = 1) spin transition may be produced in CaFeO when 233 M.U.Rana T. Abbas and M.A. Chaudhry. Mod. Phys. Lett. B 1991 5 1669. 234 C. Bellouard M. Hennion I. Mirebeau and B. Hennion J. Magn. Magn. Mat. 1992 104-107. 1627. 235 K. Hinrichs K. Herz K. Knorr H. J. May J. Pohl and W. Prandl. J. Magn. Magn. Mat. 1992 104-107 1676. 236 J.N. Reimers. J. E. Greedan and M. Bjorgvinsson Phys. Rev. B 1992. 45 7295. 237 G. Ferey ‘Mixed Valency Systems Applications to Chemistry Physics and Biology’. NATO ASI SeriesC. 1991 Vol. 343. 238 G. R. Thompson. Q.A. Pankhurst and C. E. Johnson J. Magn. Magn. Mat. 1992 104-107 893. 239 S.R. Brown and I. Hall J. Magn. Map. Mat. 1992 104-107. 921. 240 N. Fanjat and J. L. Soubeyroux J. Mayn. Magn. Mat. 1992 10.5107 933. 24 1 I. Sosnowska M. Loewenhaupt W. I. F. David and R. M. Ibberson.Physica B 1992 18&181 117. 242 Y. Chen and R.Y. He J. Magn. Mayn. Mat. 1992. 116 231. 243 M. Nogues J. L. Dormann J. Teillet and G. Villers J. Magn. Magn. Mat.. 1992 104-107 415. 244 A.N. Thakur K. Gaur and H. B. Lal J. Mat. Sci. Lert. 1992 11 496. 245 R. Plumier M. Sougi. and J. L. Soubeyroux J. Alloys Compd. 1992 178 51. 246 R. Aragon Phys. Rev. B 1992 46,5328 24’ R. Aragon Phys. Rev. B 1992 46,5334 24H 2. Inglot K. P. Lieb M. Uhrmacher. T. Wenzel and D. Wiarda Z. Phys. B. 1992 87 323. 444 A. Harrison and S.J. Clarke 30 GPa pressure is applied and is related to the transition from localized to itinerant electron behavio~r.~~~ Magnetic susceptibility studies of Fe,V,O 3 FeVMoO, and F~,V,MO,O~~ between 76 K and room temperature indicate that even well above the Nee1 temperature there is appreciable antiferromagnetic ordering due to the strong Fe"'-O-Fe"' e~change.~"A new low-temperature Morin transition is observed in r-Fe,O, in addition to the normal transition after irradiation with neutrons in a reactor.The result is interpreted in terms of a change in single-ion anisotropy through radiation damage.251 Neutron and susceptibility studies of Cr,Fe -xVO solid solutions (x = 0.25,0.5,0.75)reveal some ordering of the Cr"' and Fe"' ions over two types of octahedral sites with more Cr"' in the more regular site.252 Frustrated antiferromagnetic exchange produces a magnetic spiral structure that propagates along the monoclinic axis for low concentrations of Cr"'; at higher concentration increased frustration suppresses long-range order.The application of Mossbauer spectroscopy to the study of magnetic order in mixed metal oxides has been reviewed with an emphasis on spin-glass behaviour in compounds of iron.253 The technique has been applied to the cubic perovskite Sr,FeTiO -,and revealed a spin-glass freezing transition at about 20 K which is believed to be a consequence of the structural disorder of the B-site cations and the mixed (III)-(IV) oxidation states of iron.254 Magnetic order with both spin-glass and long-range antiferromagnetic character is observed255 through l1 9Sn and 57Fe Mossbauer studies of the perovskite SrLaFeSnO,. Cobalt. Spin-glass behaviour resulting from competition between ferro- and anti- ferromagnetic exchange has been observed in CoCI,.H,O below 7 K.256 A spin-glass transition has been detected in BaCo,Ti,O, through the observation of a sharp anomaly in the DC susceptibility at Tf= 13.6K and a divergence of the field-cooled and zero-field-cooled s~sceptibility.~ 57 A detailed analysis of the imagin- ary part of the AC susceptibility is best interpreted when it is assumed that a true phase transition occurs at T,.Cobalt iodine boracite Co,B,O 3I has been shown by neutron diffraction to have a canted antiferromagnetic structure.258 FTTR measurements of the magnetic excitations in Co,Si04 and Fe2Si0 were combined with a knowledge of the canted antiferromagnet spin structure to produce a model for the exchange interactions and anisotropy constants and thence an explanation259 for the high-field behaviour of Co2Si0,.Co,TiO shows a para- to ferrimagnetic transition at 55 K and an anomaly in the specific heat at 49 K. The latter transition has been shown to correspond to a random anisotropy phase arising from the random lattice distortions that occur 249 M. Takano S. Nasu. T. Abe K. Yamamoto S. Endo Y.Takeda and J. B. Goodenough. Phvs. Rer:.Lett. 1991. 67 3267. M. Kurzawa. J. Mat. Sci.. 1992 27. 1361. 251 0.F. Bakkaloglu 0.Nikolov and M. F. Thomas J. Phys. Cond. Muff.,1992 4. 7839. 252 J. P. Attfield. A. K. Cheetham. D.C. Johnson and T. Novet. J. Muter. Chem.. 1991 I. 867. 253 F.J. Berry J. F. Marco. M. I. Sarson. and M. R. Smith Hyperfine Int. 1991 66 25. 2s4 T. C. Gibb P. D. Battle S. K. Bollen and R.J. Whitehead J. Muter. Chem.. 1992 2 11 1. 255 T . c . G'ibb J. Muter. Chem. 1992 2 415. 256 G.C. Defotis. R.V. Chamberlain W. R.A. Jdrvis. and D.J. Krovich. J. Magn. Magn. Mat. 1992 104-107. 1603. "' A. Labarta. X. Batlle B. Martinez. and X. Obradors Phys. Rer. B. 1992. 46 8994. "' M. Clin H. Schmid P. Schobinger and P. Fischer. Phuse Trans.. 1991 33. 149. 25y C. Brotzeller H. Jaitner. B. Hock 0.Neumann R. Geick. W. Treutmann. S.Hosoya and H. Kato. J. Muyn. A4ugn. Mat. 1992. 104-107 949. Magnet ism 445 through the large charge difference between Co" and Measurements on Co,O indicate that an anomaly in the thermal expansion of the lattice at 600K is due to a low-high spin transition.261 A comparison has been made between the antiferromagnetic Co-Co exchange in Zn -,Co,S and Zn _,Co,Se (x = 0.0055-0.063) and found to be stronger in the selenide than the sulfide and three times stronger than in the Mn analogues.262 Heat capacity measurements on Zn,-,Co,S were interpreted with the aid of a model in which magnetic exchange beyond nearest neighbours fell off with the separation Y in A as -30r-6.3 K 263 Nickel.NiO attracts attention as a simple fcc Heisenberg antiferr~magnet.~~~.~~' Measurements on the diamagnetically dilute compounds Ni _,Mg,O indicate a similar reduction in TN with x as observed in Co,-,Mg,O for (1 -x)> 0.32. Measurement of the superparamagnetism of fine particles of NiO ( 5 100nm in size) indicate that it is due to lattice imperfections at the surface rather than to non-stoichiometry and provides a method of determining particle size in this Spin-waves in NiO have been measured through Raman spectroscopy and sharp low-frequency excitations attributed to surface and bulk one-magnon excita- tion~.~~' The orthorhombic perovskites PrNiO and NdNiO display a metal ~ insulator transition that is accompanied by an abrupt 3D magnetic ordering transition of moments on Ni with an ordered moment 11 -0.9pp and an unprecedented commensurate spin-density wave with Q = (112 0 1,'2).268 Molybdenum.The pyrochlores Ln Mo207,where R is a lanthanide ion have attracted attention over the past few years on account of the high frustration involving both Mo" and Ln"' moments. Work on the Y Sm and Gd compounds indicate spin-glass transitions at 18 68 and 55 K respectively.269 Ruthenium.Spin-glass behaviour is believed to occur in Sr,FeRuO and Sr,FeRuO below 23 and 11 K respectively and is attributed to the competition that arises between Ru"' and Fe"' in nearest- and next-nearest-neighbour sites."' Lanthanides. The spin dynamics of the fcc type-I1 antiferromagnet CeAs with S = 4 have been studied by elastic inelastic and diffuse critical neutron scattering.'? Successful interpretation of the data requires the presence of significant anisotropic nearest-neighbour exchange as well as next-nearest-neighbour exchange. BaPrO is a cubic perovskite with an orthorhombic distortion. Below 1 1.7(2) K there is a continuous and reversible antiferromagnetic ordering of the moments with a 260 G.Gavoille. J. Hunsch and S. Koutani. J. Mayn. Muin. Mur.. 1991 102 283. Zh' V.A. M. Brabers and A. D. D. Broemme. J. Magn. Mayn. .Mar. 1992 104-107.405. 262 C. Chen W. Gao Z. Qin W. Hu M. Qu and W. Giriat J. Appl. Ph~..s.,1992. 70. 6277. 2h3 H.J.M. Swagten A. Twardowski E.W. Janse. P.J.T. Eggenkamp. and W.J.M. Dejonge J. Mugn. Magn. Mat.. 1992. 104-107. 989. Z64 M. M. Ibrahim Z. Feng J.C. Dean anti M.S. Seehra J. Phys. Cotid. ,Matt.. 1992. 4 7127. "' Z. Feng and M.S. Seehra Ph~s.Rer. B. 1992. 45 2184. 266 J.T. Richardson D. I. Yiagas. B. Turk. K. Forster and M. Twigg. J. .4ppl. Phjs.. 1991 70. 6977. Zh7 D.J. Lockwood M.G. Cottam and J. H. Baskey. J. Mayn. Magpi. Mat.. 1992. 104 107. 1053. 26n J. L. Garcia-Munoz J. Rodriguez-Carvajal.and P. Lacorre. Europhys. Lrrt.. 1992 20. 241. "') N. P. Raju E. Gmelin. and R.K. Kremer Pky.s. Rw. €3 1999. 46. 5405. 270 P. 9.Rattle. S. K. Bollen and A. V. Powell. J. Solid Srurc. Chm. 1992. 99. 267. 271 A. Donni A. Furrer. P. Fischer. F. Hulliger. and S. M. Hayden. J. Muyri. hfuyn. Mur.. 1992. 104- 107. 1 204. 446 A. Harrison and S.J. Clarke low-temperature average moment of 0.35(5)& and a small ferromagnetic compo- nent .2 72.27 3 The magnetic ordering in Nd(OH) has been measured down to 40mK with a SQUID magnetometer but there is no sign of long-range magnetic order;274 frustration in the structure is believed to restrict magnetic correlations to a short range. Europium oxide and sulfide continue to provide simple model ferromagnets to test theoretical work concerning the nature of the magnetic fluctuations near the Curie temperature of a ferr~magnet~~~ and the diamagnetically-dilute derivative Eu -$r,S still provides one of the best model insulating spin-gla~ses.~~~ Heavier chalcogenides are antiferromagnetic careful specific heat measurements on EuTe show a crossover in the critical exponent o! from a very small value (0.0085(30)) to a larger value (0.38(11)) as T is approached from above.277 The crossover is attributed to the influence of weak dipolar forces.GdVO appears to be a tetragonal paramagnet which orders at 2.5 K to produce a simple two-sublattice antiferromagnetic array :278 antiferromagnetic resonance measurements are in accord with the predictions of mean-field theory.CdGd2Se4 shows antiferromagnetic order below 9.5 K driven by both dipolar and exchange force^.^ 79 Magnetic order whose origin is presumed to lie in dipolar interactions has been observed in the trifluoromethane sulfonate salts of Nd Gd Er and Yb and with the exception of the Yb salt appear very similar to the corresponding isostructural lanthanide ethyl salts.280 TbAsO has been shown to possess an incommensurate magnetic structure at low temperatures and the ordering vector has been determined by powder neutron diffraction.281 The magnetic properties of the two-sublattice antiferromagnet DyPO have been probed through the EPR signal of Yb3+ or Er3+ doped into the lattice.282 The exchange field experienced by these ions may be accurately determined and is found not to agree well with the predictions of a model in which there is isotropic exchange between real electron spins on the lanthanide ions.Er203 has been shown to order antiferromagnetically at 3.3 K. BaHo2F8 shows antiferromagnetic order below 1.76K and considerable magnetic 272 N. Rosov J. W. Lynn Q. Lin,G. Cao J. W. O'Reilly P. Pernambuco-Wise and J. E. Crow Phys. Rev. B. 1992 45 982. 273 I. Felner Y. Yeshurun G. Hilscher T. Holubar G. Schaudy U. Yaron 0.Cohen Y. Wolfus E.R. Yacoby. L. Klein F.H. Potter C.S.Rastomjee and R.G. Egdell Phys. Rev. B 1992. 46 9132. 274 G. Helgesen A.T. Skjeltorp and H. Bratsberg 1. Magn. Magn. Mat. 1992 111 5. 275 D. Gorlitz J. Kotzler and T. Lange J. Magn. Magn. Mat. 1992 104-107 339.'16 9. Ozcelik K. Kiymac J. C. Verstelle A. J. Van Duyneveldt and J. A. Mydosh J. Phys. Cond. Matt. 1992 4 6639. 27' E. Scheer J. Wosnitza. H. V. Loehneysen R. Kuersch M. Lang and F. Steglich J. Magn. Magn. Mat. 1992 104-107 175. 278 M. M. Abraham J. M. Baker B. Bleaney J. Z. Pfeffer and M. R. Wells J. Phys. Cond. Matt. 1992 4 5443. 27q E. Mochowska S. Pokrzywnicki and M. Duczmal J. Phys. Cond. Matt. 1992 4 5339. M. R. Roser J.C. Xu and L. R. Corruccini J. Low Temp. Phys. 1991 85 255. 281 W. Kockelmann W. Schafer and G. Will J. Phys. Chem. Solids 1992 53 913. 282 M. M. Abraham J. M. Baker 9.Bleaney A. A.Jenkins P. M. Martineau and J. Z. Pfeffer,Proc. Roy. SOC. London Series A 1991 435 605. 283 Y. J. Tang X. W. Cao J. C. Ho and H. C.Ku Phys. Rev.B 1992 46,1213. Magnetism 447 short-range order at 4.2K.284The magnitude of TNis not consistent with purely dipolar ordering forces and it is deduced that exchange is dominant. Actinides. The magnetic properties of monochalcogenides and monopnictides have been reviewed with emphasis on the nature of the actinide ion which is generally trivalent the role of crystal field and the form of the magnetic exchange.285 One of the success stories of magnetic X-ray scattering has been the elucidation of the magnetic properties of many actinide compounds. Attention has recently turned to Uranium compounds with preliminary measurements286 of the enhanced resonance scattering near the M edges of UO, USb and U,,,,Th,,,,Sb; no enhancement was seen for the scattering from Th.The magnetic susceptibilities of the solid solutions MgyU,-,,O +x indicate an increase in Nee1 temperature and effective moment on U as the oxidation state is raised from (IV) to (v),but not to (vI),on doping.287 Doping with La rather than Mg led to a decrease in effective moment as U was oxidized further to Uv'.288LiUO has an anomalously large magnetic susceptibility relative to the predictions of mean-field and crystal-field calculations which is attributed to ferromagnetic character.289 Inelastic neutron scattering measurements on NpO demonstrate the relation between the crystal field levels of Np" and the ferromagnetic phase transition at 25 K.290 The transition is accompanied by a collective Jahn-Teller distortion of the oxygen sub-lattice.Neptunium chalcogenides crystallize in the rock salt structure. Recent neutron scattering measurements indicate that the sulfide and selenide order at 23 and 38 K respectively to a type-I1 antiferromagnetic structure with inequivalent Np sites.291 This behaviour is considerably more complex than that shown by the uranium or plutonium counterparts which behave as ferromagnets or temperature- independent paramagnets respectively. NpSe shows an antiferromagnetic transition at 38K but the ordered structure has not been determined while NaAs shows antiferromagnetic order below TN= 175 K and a ferrimagnetic phase transition in a magnetic field greater than 6 T. The mixed compounds NaAs-NpSe were made to see if new magnetic phases could be produced.The addition of NpSe leads to an increase in ferromagnetic interactions increased frustration and a reduction in ordered moments. The critical exponents of the incommensurate-commensurate transition in NpAs have been measured by critical scattering experiment^.^'^ 6 Molecular Solids Magnetic Clusters.-Our trawl through this year's literature has netted relatively few examples of work on dimers as compared with previous years. What there is is M. M. Abraham B. Bleaney R. W. Hill M. J. M. Leask R.C. C. Ward and M. R. Wells Proc. Roy. SOC. London Series A 1991 435 159. K. Mattenberger and 0.Vogt Phys. Scr. 1992 T45,103. 2R6 C. C. Tang W.G. Stirling G.H. Lander D. Gibbs W. Herzog P. Carra B. T. Thole K. Mattenberger and 0.Vogt Phys. Rev. B 1992.46,5287. 2M7 Y. Hinatsu J. Solid State Chem. 1991 95 300. Y. Hinatsu J. Solid State Chem. 1991 95 430. Y. Hinatsu T. Fujino and N. Edelstein. J. Solid State Chem. 1992 99,182. 290 G. Amoretti A. Blaise R. Caciuffo D. Di Cola J. M. Forunier M. T. Hutchings G. H. Lander R. Osborn and A. Severing J. Phys. Cond. Mutt. 1992 4 3459. 2y1 A. Blaise M. N. Bouillet F. Bourdarot P. Burlet J. Rebizant J. Rossat-Mignod J. P. Sanchez J.C. Spirlet and 0.Vogt J. Magn. Magn. Mat. 1992 104-107 33. 2y2 D. L. Jones S. Langridge W. G. Stirling G. H. Lander J. Rebizant J. C. Spirlet M. Alba and 0.Vogt Physica B 1992 1W181,88. 448 A. Harrison and S.J. Clarke primarily concerned with dimers of Cu" both horn~nuclear~~~~~~~ and heteronuclear with CoII 304.305 Nil1 305 307 or Gd"1 308,309 Work has also been performed on homo- nuclear dimers of V with both V1v310and a mixed V1ll-V1v dimer,31' Cr312 314 and ~~312,315,316 319 ~i11 ~~~ ~~,317 320 W,3l4 and A u as well as a variety of heteronuclear dimers of first-row transtion-series elements.322 Several of these contribute to our understanding of the relation between sign and strength of exchange and the length and geometry of the exchange bridges.306.312,318,322 One or both of the species in a dimer may also be a radical.Ferromagnetic coupling has been observed between a radical and Gd"' in nitronyl complexes323 and between spins on biradicals coordinated to There is a wide variety of trimeric species that have been the subject of magnetochemical study.Some of these have biological significance as in homonuclear Cu" complexes related to multicopper o~idases,~~~ and the heteronuclear complexes Fe"'-Ni"-Fe"' with M = Zn Cu Ni Co Fe and Mn which may provide models for electron transport in biological species."' The p,-oxide mixed-valance Mn complex 2v3 H. Uekusa S. Ohba. T. Tokii. Y. Muto K. Michinobu S. Husebye 0.W. Steward S.C. Chang and J. P. Rose Acta Cryst. B 1992 48. 650. 294 T.E. Grigereit. J. E. Drumheller. R. Scott. G. Pon and R. D. Willett. J. Mtzgn. Magn. Mat. 1992 104-107. 1981. 205 J. P. Chyn K. J. Shieh J. L. Chou Y. Wang G. H. Lee. and C. P. Chen. J. Chin. Chem. Soc. 1991,38,549. 296 M.T. Garland J. Y. Saillard and E. Spodine J. Cryst. Sprctr. Res.. 1992 22. 467. 297 D.Lelievre L. Bosio J. Simon J. J. Andre. and F. Bensebaa J. Am. Chem. Soc.. 1992 114 4475. 298 A. Tosik. W. Maniukiewicz M. Bukowskastrzyzewska J. Mrozinski. M. P. Sigalas and C. A. Tsipis. Inory. Chim. Acta 1991 190 193. 299 I. Castro J. Sletten. J. Faus. M. Julve Y. Journaux. F. Lloret and S. Alvarez Inory. Chrm.. 1992,31 1889. 300 J. P. Costes. F. Dahan. and J. P. Laurent Inorg. Chrm. 1992. 31 284. 301 L. P. Battaglia. A. B. Corradi. S. Ianelli M. A. Zoroddu. and G. Sanna J. Chrm.Soc. Faraday Trans.. 1991. 87 3863. 302 S.S. Tandon L. K. Thompson. and R. C. Hynes Inorg. Chem.. 1992. 31 2210. 303 S. Wang S. Trepanier. J. C. Zheng. Z. Pang and M. J. Wagner Inory. Chern. 1992 31 21 18. 304 D. Z. Liao S. Juan Z.H. Jiang. S. P. Yan P. Cheng and G.L. Wang Polyhedron 1992 11 2621. 305 A. Escuer. R. Vicente and J. Ribas. Polyhedron 1992. 11 453. 306 A. Escuer. R. Vicente J. Ribas R. Costa and X. Solans. Inory. Chem.. 1992. 31 2627. 307 B. Srinivas N. Arulsamy. and P.S. Zacharias. Polyhedron 1992. 11 21 I. 30x M. Sakamoto. M. Hashimura K. Matsuki. N. Matsumoto K. Inoue. and H. Okawa Bull. Chrm. Soc. Jupun 1991 64 3639. 3nv 0.Guillou. P. Bergerat. 0.Kahn E. Bakalbassis K. Roubekeur P. Batail. and M. Guillot Inory. Chrm. 1992 31. 110. 310 C. W. Hahn P.G. Rasmussen. and J. C. Bayon Inory. Chem. 1992. 31 1963. 31 1 D.B. Sable and W.H. Armstrong. Inorg. Chrm. 1992. 31. 161. 312 A. Niemann U. Bossek K. Wieghardt C. Butzlaff. A. X. Trautwein and B. Nuber Angrw Chrm. 1992.104 345. 313 M. Nakahanada T. Fujihara A. Fuyuhiro. and S. Kaizaki. Inor(/. Chrm.. 1992 31 1315. 314 F.A. Cotton and T. Ren J. Am. Chrm. Soc.. 1992 114 2237. 315 M. Suzuki Y. Hayashi K. Munezawa M. Suenaga. H. Senda. and A. Uehara. Chem. Letr. 1991 1929. 316 S. Pal J. W. Gohdes. W. C.A. Wilisch and W. H. Armstrong Inorg. Chrm. 1992 31. 713. 317 Z. Ding S. Bhattacharya J. K. McCusker P. M. Hagen. D. N. Hendrickson. and C.G. Pierpont. Inory. Chrm. 1992 31,870. 318 M. Mikuriya. Y. Yamato and T. Tokii Chrm. Lett.. 1992. 1571. 314 K. J. Oberhausen J. F. Richardson. R. J. Obrien R. M. Buchanan. J. K. McCusker. R. J. Webb and D. N. Hendrickson Inorg. Chrm.. 1992. 31 1123. 320 A. Escuer R. Vicente. and J. Ribas J. Mayn. Magn. &fur.. 1992 110.181. 321 A. P. Koley S. Purohit. L.S. Prasad S. Ghosh. and P.T. Manoharan. lrrorg. Chem. 1992. 31 305. 3 22 R. Flotzelmann. K. Wieghardt. U. Florke H. J. Haupt. D. C. Weatherburn J. Bonvoisin. G. Blondin and J. J. Girerd J. Am. Chon. Soc.. 1992. 114 I68 1. 323 C. Benelli. A. Caneschi. D. Gatteschi. and L. Pardi. Inory. Chrm.. 1992. 31 741. 324 A. Caneschi A. Dei. and D. Gatteschi. J. Chrm. Soc.. Chem. Commun. 1992. 630. 72s P. Chaudhuri. I. Karpenstein. M. Winter,C. Butzlaff E.Bill. A. X. Trautwein U. Florke. and H. J. Haupt J. Chrrn. Soc. Chem. Commun. 1992 321. 726 E. Bill. C. Butzlaff. A. X. Trautwein H. Winkler M. Winter and P. Chaudhuri Hjlpwfine lnr.. 1991. 68 229. Magnetism 449 [Mn"Mn~10(0,CCH3),(py)3]py has a ground state with a spin rather than the expected value of i,which is believed to arise from spin frustration.327 A trimer of Ni" also exists,328 connected to a polytungstate in [Ni,(H,O),PW lo03,H,0]7-.328 Fer-romagnetic exchange within the trimer produces an overall spin of 3 which is the largest spin ground state yet observed in a heteropolyoxometalate.Finally inelastic neutron scattering measurements have detected magnetic intercluster interactions in the iron trimer compound [Fe"',Fe"O(0,CCD,),(CsD,N)3](CsDsN).329 Biological interest and polyoxometalates feature in higher-nuclearity clusters too. The cubane complex [Mn'VMn~'03Cl,(0,CCH3)3]3-may be used to model the behaviour of the active core of the water-oxidation photosystem II.330 LCAO X calculations predict the correct magnetic ground state and indicate that the spin polarization is important in determining the spin state of the complex.Exchange energies vary between 5and & of those in similar Fe-S clusters. A cubane core of four nickel atoms linked through hydroxy bridges to produce different Ni -0-Ni exchange pathways has been synthesized in the complex [Ni4(OH),(tzdt),(py),].2py (where tzdt = 1,3-thia~olidine-2-thionate).~~ The exchange may be ferro- or antiferromag- netic depending on the geometry of the exchange path. Different types of exchange pathways are also produced in another small group of metal ions in the tetranuclear complex [Cu,(TNLR)(p,-OH),(H20)8](CF3S03), (TNLR = 1,4,6,9-tetrakis((R-2-pyridyl)amino(benzodipyridazine));332 the central rectangle of Cu" ions is linked along the different edgzs and diagonals by a selection of antiferromagnetic exchange constants.The tetranuclear Co" or Cu" complexes [M,(H,O),(PW,O,,),] -and [M,(H,0),(P,W,,0,,)2]'6- contain a rhombus of edge-sharing MO units which are coupled antiferromagneticallj for Cu and ferromagnetically for CO.,,~The exchange in K o[Co,(H,0),(PW,03,),] has been determined by inelastic neutron scattering to be 3 meV.334 Bonding schemes within larger polynuclear transition metal clusters have been elucidated through magnetic susceptibility and ESR measurements on the high nuclearity V cluster compounds K,[Vl ,As,0,,(H20)]~8H,0 and (NH,)[V,,AS~O,~(SO~)].~~~ The data have been interpreted by breaking down the clusters into fragments identifying different exchange pathways and estimating exchange energies.The first cooperative magnets containing C,, i.e. the complex TDAE-C, (where TDAE is tetrakis(dimethylamino)ethylene,C,N,(CH,),) were reported last year. Further work confirms the presence of approximately one unpaired spin per molecule and indicates a transition to a ferromagnetic ordered state at T = 16.7-17.5 K.336.337 327 J. K. McCusker H.G. Jang. S.Wang G. Christou and D. N. Hendrickson. Inory. Chem.. 1993.31. 1874. 328 C. J. Gomez-Garcia E. Coronado and L. Ouahab Anyew. Chrm. Inr. Ed. En<//..1992 31 649. 329 U. A. Jayasooriya R. D. Cannon. C. E. Anson S. K. Arapkoske. R. P. Whitc and G. J. Kearley. J. Chmi. Soc. Chem. Commun. 1992 319. 33" E. A.Schmitt L. Noodleman E. J. Baerends. and D. N. Hendrickson. J. Am. Chem. Soc.. 1992. 114,6109. 33' L. Ballester. E. Coronado A. Gutierrez. A. Monge M. F. Perpinan E. Pinilia. and T. Rico Inory. Chem. 1992 31 2053. 332 S.S. Tandon S.K. Mandal. L. K. Thompson and R.C. Hynes. Inory. Chem. 1992 31. 2215. 333 C.J. GomeT-Garcia E. Coronado and I. J. Rorras-Almenar. Inorcq. Chem.. 1992. 31. 1667. 334 C.J. Gomez-Garcia E. Coronado J.J. Borras-Almenar. M. Aebersold. H. U. Gudel. and H. Mutka. Physicu B 1992 186181 238. 335 A. L. Barra D. Gatteschi L. Pardi A. Muller and J. Doring .I.Am. Chem. Soc. 1992 114 8509. 33b K. Tanaka. A. A. Zakhidov K. Yoshizawa K. Okahara,T. Yamabe. K. Yakushi K. Kikuchi. S. Suzuki. I. Ikemoto. and Y. Achiba. Phys. Lett. A. 1992 164. 221.337 G. Sparn J. D. Thompson P. M. Allemand Q. Li. F. Wudl. K. Holuer. and P. W. Stephens Solid Slurp Commun. 1992 82 719. A. Harrison and S. J. Clarke The application of pressure produces a rapid depression of T,. The analogous C, compound shows no ferromagnetism down to 4.5 K. More exotic clusters may be produced in a molecular beam and an estimate of the magnetization made by measuring their Stern-Gerlach deflection^.^ 38 In this manner the magnetism of Fe,, and Ni,, was studied as a function of rotational and vibrational temperature. Chains Layers and Three-dimensional Magnets.-Most of the work on 1D molecular magnetic materials this year concerns nitroxide radicals such as p-NPNN (p- nitrophenyl nitronyl nitroxide) in its various crystal forms labelled a,p ph,and y.The so-called p form shows bulk ferromagnetism below 0.60K with an entropy change for the transition corresponding to one unpaired spin per radical entity.339 The y form appears to behave as ferromagnetic chains of Heisenberg moments (J -4.3 K)coupled weakly through both ferro- and antiferromagnetic and is metastable relative to the y form.Table 1 contains an entry for p-NPPP as well as several Table 1 Molecular magnets. The structures of some of these materials are given in Figure 4. The acronyms we use are the ones defined by the various authors and are given either in the text in section 6.2 or are as follows NITR = 2-(4-R)-4,4,5,5-tetramethyl-4,5-dihydro-l H-imidazo-line-1-0xyl3 oxide; DEAPNN = 2-(4’-diethylaminophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-N-oxide; m-MPYNN = 2-(3-N-methylpyridinium)-4,4,5,5-tetra-methyl-4,5-dihydro-1H-imidazol-1-oxyl 3-N-oxide; MOTMP = 4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl; ET = bisethylenedithiotetrathiafulvalene; Pc = diphthalo-cyanine; TCNE = tetracyanoethene; TPTA = 2,4,6-triphenoxy- 1,3,5-triazine.FM and AFM stand for ferro- and antiferromagnet(ic) respectively. Material Figure Comments Ref. P-NPNN B form is FM below 0.60 K; ;’ form is 1 D o.h y-NPNN FM with AFM interchain exchange and TN= 0.65 K Cu(hfac),NIT-Me Spin density study by polarized neutron c CuCl,(NIT-+) diffraction shows Cu-NIT-R coupling p-pyridyl nitronyl FM Jlk = 0.27 K d nitroxide DEAPNN 1D AFM with J -~ 2.45K e m-MPYNN(BF,) 2 2D triangular AFM lattice of FM- f I0.28(H20)0.17 coupled dimers of m-MPYNN MOTMP Galvinoxyl radicals 4 (c) 4 (d) ID FM T -0.14K Short-range FM (0 = 4.4K) if prepared by freeze-drying producing different P h ET,Cu[(N(CN),]Cl molecular stacking in crystal structure TN= 45K; weak FM below 22K with moment -(8.10-4)p,/formula unit i ET,KHg(SCN) FM and AFM phases at low tempera- J tures [YPc,]CH,Cl Chains of Pc macrocycles sign of ex- k change depends on relative orienta- tions to produce 1D FM or AFM 338 J.A.Becker and W. A. Deheer Ber. Bunsen-Ger. Phys. Chem. 1992,% 1237. 339 Y. Nakazawa M. Tamura N. Shirakawa D. Shiomi M. Takahashi M. Kinoshita and M. Ishikawa Phys. Rev. B 1992 46 8906. 340 M. Takahashi M. Kinoshita and M. Ishikawa J. Phys.SOC.Japan 1992 61. 3745. Magnetism 451 Material Figure Comments Ref' CFeCPJCTCNEI Product of pyrolysis of T falls as % spinless defects decreases Composed of C and H with -3% N; 1.2-diaminopropane random network of sp2 and sp3 carbon with high-spin areas coupled AFM to give -0.022 p$C Pyrolytic carbon from Samples superparamagnetic and poss- " adamantane ibly FM at low temperature not due to FM oxide "Y. Nakazawa M. Tamura N. Shirakawa I>. Shiomi M. Takahashi M. Kinoshita and M. Ishikawa Phys. Rev. B 1992,468904. 'M.X. Wan H. L. Wang and J.G. Zhao J. Mugn. Mugn. Mat.. 1992. 104-107,2096. 'J. X. Boucherle B. Gillon E. Ressouche P. Rey and J. Schweizer. Physica B 1992,180-181. 135. dK. Awaga T. Inabe and Y.Maruyama Chem. Phys.Lett. 1992.190,349. 'T. Sugano T. Goto and M. Kinoshita Sdid State Commun.. 1991,80 1021. IK. Awaga,T. Inabe Y. Maruyama T. Nakamura,and M.. Matsumoto Chem. Phys. Letr. 1992 195 21. 9H. Sugimoto H. Aota A. Harada Y. Morishima M. Kamachi W. Mori M. Kishita N. Ohmae M. Nakano and M. Sorai Chem. Letr. 1991. 2095. hL.Y.Chiang R. B. Upasani H. S. Sheu D. P. Goshorn and C. H. Lee J. Chem. Soc. Chem. Commun. 1992 959. 'U.Welp S. Fleshler W. K. Kwok G.W. Crabtree K. D. Carlson H. H. Wang U. Geiser J. M. Williams. and V. M. Hitsman Phys. Rev. Letr.,1992,69.840.'5. S. Brooks C.C. Agosta S.J. Klepper M. Tokumoto. N. Kinoshita H. Anzai S. Uji H. Aoki A. S. Perel G.J. Athas and D. A. Howe Phys. Rev. Lett. 1992,69. 156. 'J. L. Paillaud M. Drillon A. Decian J. Fischer R. Weiss R.Poinsot and A. Herr Physicu B. 1991 175 337. 'K. S. Narayan B.G. Morin. J.S. Miller and A.J. Epstein Phys. Rev. B 1992. 46 6195. "K. Murata H. Ushijima H. Ueda and K. Kawaguchi J. Chem. SOC.,Chem. Commun.. 1992 567. "K. Tanaka M. Kobashi H. Sanekata A. Takata T. Yamabe. S. Mizogami K. Kawabata and J. Yamauchi J. Appl. Phys.. 1992 71. 836. derivatives in which p-nitrophenyl is replaced by another organic moiety R. There is little consistency in the way in which different authors devise acronyms for these compounds we use such acronyms in the table and give the full names below and specify R in the caption to Figure 4. (c) (d) Figure 4 Organic radicals in some of the ID molecular magnets referred to in the second column ?f Table 1. The abbreviations are dejned in the text (section 6)or the caption to Table 1.(a) NPNN (h) NIT-K with K = (i) methyl or phenyl (ii) p-pyridyl (iii) 4-diethylaminophenyl (iz:) 3N-methylpyridinium (c) MOTMP (d) galuinovyl There is relatively little work on higher-dimensional molecular magnets if only because it is much harder to devise efficient ferromagnetic exchange pathways in more than one dimension in a molecular crystal. We intend to provide an account of A. Harrison and S. J. Clarke intercalation compounds next year. The amorphous material V(TCNE);y(CH,CI,) (x -2 y -i),which created considerable excitement last year on account of the observation of a spontaneous moment at room temperature has not appeared in the literature for 1992.The amorphous structure and extreme sensitivity towards chemical attack has hindered progress.