摘要:

The Stoner‐Wohlfarth model is used to calculate the remanence of a randomly‐oriented assembly of singledomain particles when the applied field changes in direction. The cases considered are (1) fields applied successively in directions differing by multiples of ½&pgr; and (2) fields rotated through multiples of ½&pgr;. Some consideration is also given to the anhysteretic remanence when the unidirectional and alternating fields are orthogonal. Experimental results obtained on disks of &ggr;Fe2O3recording tape are found to be in reasonably good qualitative agreement with the calculations.

ISSN:0021-8979    

DOI:10.1063/1.1728713

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Very fine particles reverse their magnetization spontaneously, passing the energy barrier by means of thermal activation. In the same way one could argue that small portions of larger particles could be spontaneously reversed, thereby serving as nucleii for reversed domains. These can then grow, because it is known that the energy favors subdivision into domains. In treating such a possibility, however, one should take into account the additional barrier for forming a wall around the ``nucleus,'' which is usually left out in standard domain theory calculations when the wall energy is used. This energy is rather small but the barrier for formation of the wall is large.The model studied is a small sphere, centrally located inside a spherical particle, the magnetization in which starts to rotate gradually from thezdirection in which the rest of the particle is magnetized. The angle between magnetization and thezaxis is assumed to vary linearily in the radial coordinate, from zero on the surface of the inner sphere, to its maximum value in the center. This value eventually reaches &pgr;, thus completing a wall around a reversed domain of radius zero, which should then start to grow by further supply of energy.Studying the energy barrier for such a process, it is seen that the main contribution is from the exchange energy, which practically forbids the process. The probability for a domain wall to nucleate at room temperature is thus shown to be negligibly small, which justifies the neglection of temperature affects in Brown's equations.

ISSN:0021-8979    

DOI:10.1063/1.1728714

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Some types of permanent magnets are compacts of very small particles. Their properties depend largely on mutual magnetic influence between particles or groups of them. To determine these, more should be known about packing arrangements, orientation, separating layers, and the homogeneity of the compact. A method is described which makes surface details visible and simultaneously reveals magnetic phenomena of such compacts. The size of the compacted particles mostly requires the resolving power of an electron microscope. Special replicating techniques combine shadow‐graphing and colloid application even on rough and porous surfaces.Examples are presented of such surface studies on barium ferrite and iron‐cobalt particle magnets. Surfaces of barium ferrite magnets with crystal sizes of 0.2–1 &mgr; show indications of magnetic domains smaller than observed before, sometimes with less than 0.1 &mgr;‐width. On compacts of small elongated iron‐cobalt particles, structures have become visible which indicate particle groupings varying with the methods of compacting. Colloid outlines surface pole arrangements in correlation to particle groups.

ISSN:0021-8979    

DOI:10.1063/1.1728715

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Exchange anisotropy describes a magnetic interaction across the interface between two magnetic materials. A shifted hysteresis loop, sin&thgr; torque curve, and rotational hysteresis in magnetic fields greater than 2K / Msmay result from this interaction if one of the materials is antiferromagnetic. This interaction has been found to exist between ferro‐antiferromagnetic materials, ferri‐antiferromagnetic materials, and ferri‐ferromagnetic materials. The work of various people is discussed in terms of the expected behavior in these exchange coupled systems. Some interesting results of the exchange interaction are reviewed. These include improved properties of fine particle magnets, a memory effect in a mixed ferrimagnetic spinel, a rotatable anisotropy in thin films, and an explanation for the reverse magnetization of a deposit in the earth's crust. The interfacial conditions necessary to obtain the interaction between the two magnetic systems are discussed, and it is shown that they are met in several cases. Models are presented which yield rotational hysteresis in magnetic fields greater than 2K / Msas has been found in all of the exchange coupled systems.

ISSN:0021-8979    

DOI:10.1063/1.1728716

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Neutron diffraction measurements on the ordered fcc alloys FePd3, MnPt3, and Cr0.3Pt0.7support the existence of localized magnetic moments on the Pd and Pt atoms. The individual moments were determined by combining the difference in moment, based on reasonable assumptions about the magnetic form factors, with the total moment as determined from magnetization measurements. The results for FePd3are consistent with suggestions based on magnetization measurements; at 300°K Fe has a moment of 2.73±0.13 &mgr;Band Pd of 0.51±0.05 &mgr;B. In MnPt3at 77°K a small moment on the Pt is observed (0.17±0.04 &mgr;B), with most of the moment residing on the Mn (3.60±0.09 &mgr;B). At room temperature the magnetic ordering in Cr0.3Pt0.7is ferrimagnetic, with a Cr moment (if assumed equal on both Cr and Pt sites) of 2.56±0.10 &mgr;Band a Pt moment of 0.47±0.05 oppositely aligned.

ISSN:0021-8979    

DOI:10.1063/1.1728717

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Magnetic properties of ternary Cu(0–12.5 at. %) — Mn(55–42.5 at. %) — Al(45%) alloys have been studied in relation to the change of the tetragonal structure from the binary Mn&sngbnd;Al alloy to the ternary alloys. The substitution of Mn by Cu stabilizes the ferromagnetic tetragonal phase of the Mn&sngbnd;Al alloys. An increase of the Cu content brings about a decrease of the Curie temperature, the intrinsic coercive force, and the ratio of remanence to saturation induction. Correspondingly, the introduction of Cu into Mn&sngbnd;Al alloys first reduces the axial ratioc/aof the tetragonal phase from 1.30 in the 55 at. % Mn&sngbnd;Al alloy to 1.20 at 10 at. % Cu, and then converts the tetragonal structure into body‐centered cubic at more than 10 at. % Cu. In the latter case, the ternary alloys are ferromagnetic disregarding aging conditions.

ISSN:0021-8979    

DOI:10.1063/1.1728718

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

In order to account for the magnetic properties of alloys it becomes important to determine the individual magnetic moments of the constituent atoms. This determination can be accomplished by the combination of neutron diffraction and magnetic induction measurements. Such measurements were made on the following ferromagnetic alloys: Pd3Fe, PdFe, Pd3Co, PdCo, Ni3Co, and NiCo. The average moment values were obtained from magnetic induction measurements while the differences in the atomic moments were determined from either the ferromagnetic diffuse scattering of the disordered alloys or the superlattice reflections of the ordered alloys.

ISSN:0021-8979    

DOI:10.1063/1.1728719

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Several samples of CrSxwith 1.145<x<1.200 were carefully prepared and chemically analyzed. The expected crystal structures were verified by x‐ray analysis, which failed to indicate the presence of any impurities. Conductivity measurements indicated the existence of metallic conductivity in these samples between 77° and 300°K. Magnetic measurements supported the suggestion that the ferrimagnetism of CrSxmaterials arises from the presence of the Cr5S6phase.The magnetization curves forx=1.194 demonstrated the following salient properties of the Cr5S6phase: a ferrimagnetic Curie point of 305°K, a peak magnetization of 6.9 emu/gm in an external field of 8500 oe, a ferrimagnetic‐antiferromagnetic transition over a 5° temperature interval, a transition temperature in the neighborhood of 158°K which is subject both to thermal hysteresis and to shifting by an external magnetic field, and an antiferromagnetic susceptibility below this transition. The transition temperature, as well as the peak magnetization, decreases with increasing chromium content. A simple model is tentatively proposed, and appears to be consistent with the observed behavior.

ISSN:0021-8979    

DOI:10.1063/1.1728720

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Magnetization and electrical resistivity measurements on an iron‐rhodium alloy of approximate composition FeRh, having CsCl‐type structure, confirm recent x‐ray and neutron diffraction evidence for a first‐order antiferromagnetic‐ferromagnetic transition at about 350°K. From 350°K down to 77°K, an essentially constant, field‐independent susceptibility of about 1×10−4emu/g is observed. Above 350°K, the alloy behaves like a normal ferromagnet with a Curie point of 675°K and a saturation magnetization (extrapolated to 0°K) of 130 emu/g. This &sgr;0value is equivalent to an average atomic moment of 1.85 &mgr;B, or to 3.85 &mgr;Bper iron atom if the rhodium moment is assumed to be zero. Alternatively, if the iron moment is taken to be 2.2 &mgr;B, the rhodium moment has to be about 1.5 &mgr;B. Qualitative results are given for the effect of high magnetic fields and pressures on the first‐order transition temperature of this alloy.

ISSN:0021-8979    

DOI:10.1063/1.1728721

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

This work is an extension of previous work which showed that the addition of gold to Permalloy caused the precipitation of a gold‐rich phase and thereby permitted the control of the coercive force by heat treatment. This addition of a second phase to Permalloy greatly improved the switching speed of the alloy. Some compositions of gold Permalloy had switching speeds four times faster than that of the standard composition. In the present work, some of the details of the precipitation process were studied. It was found that for some specimens annealed at 650°C, the precipitation of the gold‐rich phase depended strongly upon cold working. When some alloys containing gold were quenched from the solution temperature and annealed at 650°C for 2 hr, there was little increase in coercivity and the electron micrographs showed practically no precipitation of particles of 1000‐A diameter or larger. However, when the material was quenched from the solution temperature, drastically cold rolled and annealed at 650°C, the coercivity was increased substantially and the electron micrographs revealed the presence of many particles having a diameter of 1000 A. Seven percent gold was also added to the Perminvar composition of 43% Ni‐34% Fe‐23% Co. The alloy behaved in a manner somewhat similar to the gold‐Permalloy compositions. Age‐hardening appeared in the vicinity of 600°C and square hysteresis loops were obtained. However, the threshold for rapid rotational flux reversal was substantially higher than the coercive force. Consequently, these alloys appeared less desirable for switching applications than Permalloy.

ISSN:0021-8979    

DOI:10.1063/1.1728722

出版商:AIP    

年代:1962

数据来源: AIP