摘要:

We have measured the resistivity of a series of Ni&sngbnd;Cu alloys as a function of temperature between 1.5° and 300°K, and as a function of field from 0 to 50 kG at 4.2°K. Experiments have been completed on nine paramagnetic alloys ranging in composition from 30 to 44 at.% Ni. A minimum occurs at low temperatures1and the depth of the minimum increases with Ni content. At higher temperatures, the slope of the resistivity‐temperature curve decreases with increasing temperature, and in fact, becomes negative for high nickel content. The high‐field magnetoresistance is negative in all cases and is proportional to the concentration of giant polarization clouds2as a function of Ni&sngbnd;Cu composition. These properties may be attributed to the scattering of conduction electrons from polarization clouds acting as large superparamagnetic moments. Results will also be presented for alloys beyond the critical composition for ferromagnetism.

ISSN:0021-8979    

DOI:10.1063/1.1658994

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The existence of antiferromagnetism is investigated in the single‐band Hubbard Hamiltonian in the limit of bandwidth much less than intra‐atomic Coulomb interaction of electrons. We make use of the canonical transformation and ``spectral decomposition'' of the electron creation operators proposed by Harris and Lange, to write down a Green's function which describes electrons in the lower of the split bands of Hubbard's solution. The equation of motion is solved using the moment‐conserving decoupling approximation of Tahir‐Kheli and Jarrett [R. Tahir‐Kheli and H. S. Jarrett, Phys. Rev.180, 544 (1968)]. We find within our approximation that it is impossible to have an antiferromagnetic state for other than one electron per site. To remedy this defect of the single‐band model we investigate a simplified two‐band model in the limit of intra‐atomic Coulomb and exchange interaction much greater than the bandwidth, and find that antiferromagnetism is possible for the two nearly half‐filled bands. We also discuss effects of the antiferromagnetic ordering on the conductivity in our simplified model, and discuss applicability of the theory to real transition metals and transition‐metal oxides.

ISSN:0021-8979    

DOI:10.1063/1.1658995

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

We have considered the question of whether the Hubbard Hamiltonian can lead to properties characteristic of two types of semiconductors, depending on the value of the ratio &Dgr;/Iof bandwidth to intra‐atomic Coulomb integral. In one type there is a transformation, with increasingT, from a magnetic insulating state to a paramagnetic insulating state, and in the other the system goes from a magnetic insulating state to a paramagnetic metallic state. We have applied a new variational single‐determinantal approximation which, in contrast to the standard thermal Hartree‐Fock approximation, duplicates the exact behavior of the model both in the atomic limit, &Dgr;/I=0, and the band limit &Dgr;/I=∞. Limiting ourselves to well‐known types of one‐electron states, we have obtained stability boundaries (as determined by the free energy) between various phases. While the boundaries obtained to date have intrinsic interest, we find that further calculations are necessary to completely answer the question raised above.

ISSN:0021-8979    

DOI:10.1063/1.1658996

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Semiconductor‐to‐metal‐type transitions were studied by magnetic susceptibility measurements on pure single phases of Ti3O5, Ti4O7, Ti5O9, and Ti6O11. This work represents a more detailed investigation than those reported previously. From this and related studies the following model is proposed to explain the observed phenomena. Below the transition, groups of cations are distributed periodically throughout the lattice. Within these groups thedelectrons are delocalized; however, ``constrained‐type'' antiferromagnetism sets in between specific neighboringdelectrons through homopolar bonding of cations. Neighboring groups interact via thermal excitation of electrons. Above the transition this type of magnetic ordering is modified by changes in crystal structure; here 3doverlap is large enough to bring about a nearly complete delocalization of electrons over the entire lattice. The behavior is described by the free‐electron gas model. In addition, in all but Ti3O4, a Curie‐Weiss law phenomenon is seen to be superimposed on this transition, which may result from relatively small (<11%) interactions between the unpaired spins. In proposing this model, calculations of the effective electronic mass and Goodenough's ideas have been used.

ISSN:0021-8979    

DOI:10.1063/1.1658997

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The Verwey transition in magnetite is described on a band model, with the electrons self‐consistently breaking the symmetry by ordering in the self‐induced Coulomb potential. The order parameter is the difference in occupation numbers on alternate sites. There is a gap in the single‐particle excitation spectrum, and the gap and order parameter undergo a second‐order phase transition. The theory is BCS‐like, with the Verwey temperature being given in the weak‐coupling limit by an exponential in the reciprocal of the density of states at the Fermi level.

ISSN:0021-8979    

DOI:10.1063/1.1658998

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Vanadium ``monoxide'' VOxexists over a wide homogeneity range (VO0.79–VO1.30) in the cubic NaCl structure. With increasing oxygen content or V‐vacancy content, its properties progress from weak temperature‐independent paramagnetism to a stronger temperature‐dependent paramagnetism, and from metallic to semiconducting conductivity. In the present work we have observed the nuclear resonance of51V in small single‐crystal pieces of pressure‐annealed VO0.86, VO1.02, and VO1.23, using cw and spin‐echo techniques in fields of 9–50 kOe at temperatures from 1.4°–300°K. In contrast with previously reported measurements, in no compound was there evidence of a sharp metal‐to‐insulator transition, and nuclear resonance was observable at all temperatures in all the compounds. A Knight shift of 0.4%, independent of composition and temperature, was observed, while the resonance linewidths increased with increasing susceptibility to a value of 5% of the applied field at 1.4°K in VO1.23. These results indicate that the bulk of the temperature‐dependent magnetization comes from local moments on a minority of sites whose nuclear resonances are unobservable. These moments, however, cause the broadening of the observed majority‐site resonance. A maximum in the transverse relaxation rates observed between 1.4° and 77°K in VO1.23in a 47‐kOe field apparently results from the onset of spin‐spin correlations in this range.

ISSN:0021-8979    

DOI:10.1063/1.1658999

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Neutron diffraction measurements on powder and single‐crystal samples of V2O3have confirmed the existence of long‐range magnetic order in the low temperature (monoclinic) phase. Paoletti and Pickart1discovered an extra diffraction peak at low temperature which they attributed to nuclear scattering. A polarization analysis experiment on a powder sample showed that this peak was magnetic in origin. In addition, several much weaker magnetic peaks were identified. These peaks index in the monoclinic cell given by McWahn,2with indices satisfying the relationh+k+l=2n+1. The magnetic peaks show little temperature dependence from 77°K up to the crystallographic transition at 170°K, where they vanish abruptly. On cooling, they suddenly reappear at about 154°K. The data agree with a model in which the V moments are ferromagnetically aligned in (010)mlayers (perpendicular to hexagonalaaxis) with a reversal between adjacent layers. The ordered moment of 1.2 &mgr;Bper V atom is oriented perpendicular to the hexagonalaaxis at an angle of about 71° from the hexagonalcaxis.

ISSN:0021-8979    

DOI:10.1063/1.1659000

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Far‐infrared transmission measurements have been made on pure and Fe2+‐doped MnF2at 4.2°K using magnetic fields up to 105 kOe. By means of Fourier‐transform spectroscopy we have investigated the behavior of the spin‐wave modes near the spin‐flop transition. Both the AFMR and the two‐magnon absorption with theEfield perpendicular to thecaxis have been measured. In Fe2+‐doped MnF2the spin flop occurs at higher fields depending on the Fe2+‐concentration. The localized mode which splits linearly in low fields no longer shows a linear dependence in the spin‐flop region.

ISSN:0021-8979    

DOI:10.1063/1.1659001

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The temperature dependence of magnon‐pair excitations has been obtained in the simple antiferromagnets RbMnF3and FeF2, by the technique of inelastic light scattering. At low temperatures the pair‐mode frequency is nearly twice that of a Brillouin‐zone boundary magnon, decreased slightly by magnon‐magnon interaction effects. As the temperature is increased, the magnon‐pair modes broaden and decrease in frequency. The excitation evolves smoothly asTis raised aboveTNinto the paramagnetic phase. In both materials, light scattering from the paramagnetic vestiges of the magnon‐pair modes could be observed up to 300°K. Together with earlier results on NiF2, these data indicate the trends in magnon‐pair temperature behavior for materials with different spins and different magnon‐magnon interaction effects. These trends may be useful in guiding theories for magnon‐pair excitations.

ISSN:0021-8979    

DOI:10.1063/1.1659002

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Fluorescence from the lowest spin orbital component of the3T2manifold of Ni ions in MnF2is observed to terminate on the ground state3A2of the Ni with excitation of 0, 1, and 2 spin deviations centered on the nickel ion and corresponding, respectively, to the crystal ground state (G), and excitation of thes0ands02impurity magnon modes. Application of a magnetic field lifts the degeneracy of otherwise equivalent states of ions on different sublattices resulting in splittings of the ground and excited states.gfactors and frequencies of these states were obtained by taking to second order the exchange off‐diagonal to the Ising‐Hamiltonian.gs02‐gs0andgG‐gs0are predicted and found to have essentially the ionic Ni2+gfactor of 2.33 in agreement with that measured by ESR in ZnF2:Ni; magnetic field broadening of the irs0fpair mode observed at 168 cm−1is also as predicted. Frequencies of the three pair modes observed in Raman scattering and far ir absorption together with thes0ands02frequencies were computed to an accuracy of 0.5% or better by adjusting the impurity exchange integrals,J1′ = −1.1 cm−1andJ2′ = −3.41 cm−1.

ISSN:0021-8979    

DOI:10.1063/1.1659003

出版商:AIP    

年代:1970

数据来源: AIP