摘要:

The anomalously low magnetic moments forMnxFe3−xO4spinels with1<x≤3are qualitatively explainedby a Yafet-Kittel triangular moment arrangement. Values of&bgr;,n, and&agr;are calculated from measurementsof magnetic moment, high field susceptibility, and Curie temperature. The measurements onMn3O4yielda moment of 1.73 Bohr magnetons and a susceptibility of3.5×10−4 gauss/oersted. An extrapolated anisotropyfield of 150 kilo-oersted is found. Transitions from the triangular state to a Ne´el-type arrangementare found to occur forx = 1.25,x = 1.55, andx = 1.80. ForMn3O4,&agr;&bgr;values suggest that the vanishing of thespontaneous magnetization at 42°K represents a transition to a paramagnetic-antiferromagnetic state.

ISSN:0021-8979    

DOI:10.1063/1.2000499

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Observations of spin-flopping, or decoupling between the direction of antiferromagnetism and the easyaxis of the crystal, are presented forMnF2in single crystal and powder form at low temperatures. Magnetizationmeasurements in pulsed fields to 140 koe are employed to explore the critical interaction energy inthis antiferromagnet, as an alternative tool to microwave resonance. The orientation dependence predictedby Ne´el in 1936 is confirmed. The critical field for spin-flopping given by the combination of anisotropy andexchange,(2HEHA)12is found to be 93±2 koe in excellent agreement with resonance results of S. Foner[(a) Phys. Rev.107, 683 (1957); (b) J. phys. radium20, 336 (1959)], and F. M. Johnson and A. H.Nethercot [Phys. Rev.104, 847 (1956);114, 705 (1959)].

ISSN:0021-8979    

DOI:10.1063/1.2000500

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Results of pulsed field antiferromagnetic resonance (AFMR) experiments are briefly summarized for themixed crystal system(Cr2O3)1−x·(Al2O3)xas a function of temperature, andX, whereX = 0.003, 0.006,0.008, 0.06, 0.1, 0.2, and 0.9. The temperature dependence of the critical field,(2&lgr;K)12, for pureCr2O3andX = 0.1are compared in detail. The increase of(2&lgr;K)12at 4.2°K from about 59 kilogauss forCr2O3to 79kilogauss forX = 0.1is attributed to a reversal in sign and rapid increase in magnitude of the effectivecrystalline field contribution to the anisotropy energy asXis increased. AFMR linewidths from 800 to2000 gauss were observed, but forX≤0.2the linewidth was almost independent of temperature for eachcomposition. The decrease of Ne´el temperature with increase ofXis also summarized.

ISSN:0021-8979    

DOI:10.1063/1.2000501

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Antiferromagnetic resonance has been observed in powdered MnO at 364 &mgr; at 1.5°K. The resonancefrequency as a function of temperature falls roughly as the square root of the Brillouin function of spin52.Fields of the order of104oe have no observable effect on the line at 1.5°K. All of these results give quantitativeconfirmation of a model in which the sublattice magnetizations are constrained to a (111) plane bymagnetic dipolar interactions and are held to a preferred direction within the plane by a much weakeranisotropy.

ISSN:0021-8979    

DOI:10.1063/1.2000502

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The magnetic susceptibility of powderedFeCl2·4H2Ohas been measured between 1.3° and 4.2°K andfrom 14° to 21°K by an ac mutual-inductance method. A maximum, apparently associated with an anti-ferromagnetic-paramagnetictransition, is found near 1.6°K. In the hydrogen region, a Curie-Weiss law isobeyed, the Curie constant yielding a splitting factorg = 2.19for theFe++ion if its effective spin is assumedto beS = 2.

ISSN:0021-8979    

DOI:10.1063/1.2000503

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Nickel chromite is a normal spinel whose structure has atetragonal distortion below about 310°K. The structure parametershave been determined by neutron diffraction at 77°K, at whichtemperature thec/aratio of a face-centered unit cell is 1.04. Thespace group isI41/amdwith four Ni ions in positions 4(a), eightCr ions in positions 8(d), and 16 O ions in positions 16(h) withy = 0.239andz = 0.392. The two independent Cr-O distances are1.98 A and 1.99 A, indicating that the octahedron is not greatlydistorted. The tetrahedron around the nickel is elongated bynearly 14% parallel to thecaxis. Below a Curie temperature ofabout 65°K, nickel chromite has a small ferromagnetic moment.The neutron diffraction pattern at 4.2°K shows extra peaks whichcan be indexed on the basis of a primitive cell having the samedimensions as the body-centered chemical cell. The intensities ofthese peaks cannot be accounted for by any simple model for themagnetic structure. In fact, a systematic search of possiblestructures belonging to magnetic space groups with symmetriesas low as classesm′m′2 andmm′2′ has failed to turn up anystructure which is consistent with the diffraction data.

ISSN:0021-8979    

DOI:10.1063/1.2000504

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The magnetic and optical anisotropies of evaporated iron-nickelfilms deposited at an angle to the substrate normal are found to besensitive to the rate of deposition and the orientation of themagnetizationMduring deposition. It is postulated that crystalliteelongation toward the vapor beam or in the direction ofMoccurs.Study of spatial inhomogeneities in the normal-incidenceM-inducedanisotropy fieldHkis an aid in understanding the origin ofthis anisotropy and the switching behavior of films. Small-angledispersion ofHkis attributed to local deviation from isotropy ofthe isotropic tension present in films; measurements support thismodel. Magnitude dispersion is estimated from rotational hysteresisexperiments. Large-angle dispersion of a special type has beenidentified wherein certain regions of a film have an easy axisperpendicular to the field present during deposition, i.e.,Hkisnegative in these regions. For alloy compositions within a fewpercent of 83% Ni and 17% Fe, the normalized wall coerciveforcehw = Hw/Hkhas significance beyond the defining concepts.Thus,hwis a measure of resonance linewidth and an internal fieldinferred from resonance data; forhw>1, a unique “locking”phenomenon occurs. Bitter-pattern studies of quasi-static switchingshow that negative-Hkregions are present and thathwis insome sense a measure of these regions. A tentative model forM-induced anisotropy which qualitatively explains both positiveand negativeHkis proposed which invokes directed pairs of ironatoms and strain anisotropy arising from elongated crystallites.In nonlocking films, quasi-static switching by coherent rotation isbroken up by labyrinth domains consisting of alternating regionsof switched and unswitchedM. Propagation of these domains ispostulated to be caused by dispersion in the magnitude and orientationofHk. If a pulsed reversing field exceeds the switchingthreshold of all regions of the film, a transition from labyrinthswitching to coherent switching should occur, as has been observedexperimentally.

ISSN:0021-8979    

DOI:10.1063/1.2000505

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A number of single-crystal films of iron, nickel, and Permalloy were made epitaxially by evaporation onto(001) cleavage surfaces of NcCl and MgO. Magnetic anisotropies for the rotation of magnetization withinthe plane as well as out of the plane of the film were measured. Values of the observed fourth-order anisotropyK1were anomalously large for Ni on NaCl and Ni on MgO, but it was normal for Fe on MgO. The effects ofmagnetic annealing were also investigated for single-crystal films of Permalloy and nickel.

ISSN:0021-8979    

DOI:10.1063/1.2000506

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The magnetic anisotropies in evaporated polycrystalline nickel films have been measured with a torquemagnetometer over a range of fields from 10 to 2000 oe. The films are deposited in a vacuum better than10−8 mmHg, and the measurements are carried out in the vacuum system at pressures below10−9 mm.The magnitude of the induced uniaxial anisotropy has been measured as a function of temperature up tothe Curie point of nickel (360°C), and the kinetics of the magnetic annealing process have been measuredfrom room temperature to 200°C. The anisotropyKudecreases roughly linearly with temperature aboveroom temperature, reaching zero near the Curie temperature. Iffis the fraction of the annealing processcompleted at a given temperature, we find that1−f = exp(−t/&tgr;)13, and1/&tgr; = 1/&tgr;0 exp(−9000/RT).

ISSN:0021-8979    

DOI:10.1063/1.2000507

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The anisotropy of evaporated Permalloy films (composition near zero magnetostriction) deposited on glassat 45° to the substrate normal and at a substrate temperature of 200°C has been studied after anneal forseveral hours at 300°C. After anneal the room temperature easy axis of films with positive magnetostrictionis in the original direction. However, films with negative magnetostriction develop a new room temperatureeasy axis 90° to the original one; at 300°C the easy axis is in the original direction. Application of a largemagnetic field in any direction during anneal does not influence the final magnetic anisotropy in either case.These effects are qualitatively explained by assuming the anneal to increase the tension along the long axisof oblique-incidence chains, a process which converts surface energy into strain energy. Anisotropy wasmeasured by resonance in a coaxial bridge over the frequency range 50 Mc to 2100 Mc. A rectangular coaxialcavity is used which can be heated to 500°C in a vacuum of10−6 mmHg; continuous angular orientation ofthe film is possible with a resolution of ±0.5°. Bridge balance is good enough to permit samples of less than100 A thickness to be measured.

ISSN:0021-8979    

DOI:10.1063/1.2000508

出版商:AIP    

年代:1961

数据来源: AIP