摘要:

I discuss early work in magnetism at Bell Laboratories and then survey the ways in which the theory of magnetism has contributed important ideas and concepts to the theory of condensed matter.

ISSN:0021-8979    

DOI:10.1063/1.326937

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The twenty‐fifth anniversary of the annual Conference on Magnetism and Magnetic Materials offers an appropriate occasion for assessing the significance and physical basis of some unforeseen but important advances in basic magnetism since the middle 1950’s. Three of these advances are (a) the one‐ion (or crystalline field) mechanism of magnetocrystalline anisotropy energy which was synthesized from ingredients investigated in paramagnets and developed into a practical tool for ’’designing’’ ferrimagnets; (b) the recognition of the degeneracy of the spin wave spectrum in ferrimagnetic insulators and its contribution to the establishment of the previously mysterious origins of relaxation in ferrimagnetic resonance; and (c) the concept of magnetic symmetry and its role in ’’legitimizing’’ the previously ’’forbidden’’ (and unobserved) magnetoelectric effect, piezomagnetism, and other phenomena involving considerations of time reversal. Some of these advances occurred gradually rather than abruptly, and none of them appear to have conformed to a pre‐programmed schedule of short‐range goals, milestones, and payoffs. What these advances do have in common is that they were motivated by a need or desire for basic understanding and that they originated in the recognition of a hidden problem or even in unforeseen flashes of physical insight.

ISSN:0021-8979    

DOI:10.1063/1.326938

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The series of Conferences on Magnetism and Magnetic Materials, now 25 years old, has provided a successful meeting place for science and technology by deliberately bringing together physicists, engineers, chemists, and metallurgists interested in the subject. During this period new classes of magnetic materials, remarkable magnetic phenomena, many magnetic devices, and various new applications of magnetism have been discovered. This paper provides a sketch of some trends in magnetics research, particularly in materials and applications, along with recollections of the early conferences. An overview is presented of the continued developments in hard and soft magnetic materials and in a variety of magnetic memory/recording media. Included are some observations on a number of advances in materials and phenomena, also with applications potential, which did not reach the marketplace, and on relationships between exploration and goal‐oriented programs. During this quarter century, information storage and processing technology has been a major stimulus for much magnetics RD, but we are now seeing some shift toward considerations of energy and resources conservation in electromagnetic devices and applications.

ISSN:0021-8979    

DOI:10.1063/1.326939

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The 25 years that have elapsed since the first Magnetism Conference constitute a virtual history of magnetism in the U. S. At the first post‐war International Conference on Magnetism held in Grenoble in 1950 (four years prior to the first of the regular U. S. Conferences on Magnetism that were the joint creation of the Physics and Electrical Engineering communities) fewer than ten Americans participated. Then, as two years later at the Maryland Conference, the focus was on ferromagnetism, the magnetization curve, domain wall motions and a brief nod to the then relatively new phenomena of ferrimagnetism, nuclear magnetism and neutron diffraction. The only concession to theory at Maryland was an evening symposium on ferromagnetic exchange that will be remembered by those who attended primarily for the controversy engendered therein. The explosive growth of the field in the subsequent decade is probably as much a consequence of the technological emphasis on applications principally in computers as it is a result of the new theoretical powers made available by those same computers. In an aesthetic sense, the impact that magnetism research has had on kindred fields of science and engineering from lasers to DNA is testimony to the unboundedness of scientific inquiry whatever the initial motivation. It is probably in its impact on the rest of the world of science and engineering that magnetism research today holds its greatest potential.

ISSN:0021-8979    

DOI:10.1063/1.326940

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

After a brief review of recent low field experiments and the time evolution of magnetisation, new data on CuMn alloys is presented. The new data shows quite clearly that the spin glass temperatureTfdoes not mark the abrupt onset of irreversible behavior. Rather there is clear evidence for remanent magnetisation aboveTf. Comparison with earlier a.c. susceptibility data suggests that there is a continuous freezing or blocking process occurring in the range of temperature centered onTf.

ISSN:0021-8979    

DOI:10.1063/1.326941

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Low field magnetization measurements on both weakly ferromagnetic and spin‐glass like Ni‐Cu and V‐Fe alloys are presented. There is a qualitative difference between the two types of order in the temperature variation of the magnetization curves. In both cases the transition temperature Tcis signaled by a cusp in the a.c. susceptibility. For the ferromagnetic alloys, the height of the cusp is equal to the reciprocal demagnetization factor 1/N. For the spin glass alloys, the height depends on Tcitself: the smaller Tc, the lower the cusp. For both types of alloys, d.c. magnetization measurements reveal the onset of hysteresis at Tc. For the ferromagnetic alloys, the maximum slope of the hysteresis loop is equal to 1/N, independent of temperature for T?Tc. For the spin glass alloys, the maximum slope of the loop does not reach the 1/N limit at Tc, but continues to increase below Tc, almost reaching the 1/N limit at a lower temperature.

ISSN:0021-8979    

DOI:10.1063/1.326942

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

We present a series of measurements of the magnetic hysteresis properties of alloys of CuMn in the concentration range 0.06&percent; at. to 4.7&percent; at. when these are cooled well into the spin glass state i.e. T/Tg<0.3. Previous studies (1) made at higher concentrations have revealed the occurrence of a complete reversal of magnetisation in a narrow range of coercive fields and a shifted hysteresis loop. Quite unexpectedly we find that going to lower Mn concentration increases the cooperative nature of the magnetisation reversal to the point where the hysteresis becomes square and that complete, spontaneous (less than 0.1 second time constant) magnetisation jumps are observed in low magnetic field (typically‐150 gauss) together with a shifted square center. We report on the effect on the hysteresis of a number of variables: the magnetisation, the concentration, the temperature, the metallurgy, the time and the application of a static transverse magnetic field. These observations lead to the existence of a well defined anisotropy field in the spin glass state.

ISSN:0021-8979    

DOI:10.1063/1.326943

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Zero field Cu NMR of the first and fourth nearest neighbours of Mn in spin glass CuMn (concentrations from 0.4% to 4.7%) have been detected for T<Tg/5. The enhancement factor &eegr; of the rf field and NMR signal intensity, associated with the rotation of domain magnetization, has been found roughly proportional to the remanent magnetization &sgr; and is negligible when the sample has been zero field cooled. These results contradict the independent domain model for which &eegr; should be independent of &sgr;. The data reveal the existence of a well defined anisotropy field HAinduced (as &sgr;) by the cooling field Hc, the anisotropy energy being roughly a constant. Transverse susceptibility and NMR intensity analyses confirm that most Mn spins are involved in the collective response to a transverse field and that HAis defined on a macroscopic scale. Direct observation by NMR of the shifted hysteresis loops and comparison with magnetization data imply that HAincludes both an uniaxial Hax) and an unidirectional (Hd) anisotropy field, and that the magnetization reversals, when detected, occur for fields ‖Ho+Hd‖<Hax. The simultaneous occurrence of collective and irreversible phenomena in spin glasses is discussed.

ISSN:0021-8979    

DOI:10.1063/1.326944

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Magnetic susceptibility, Mo¨ssbauer, and neutron scattering studies of the same alloy sample all indicate that amorphous YFe2(a‐YFe2) exhibits spin‐glass behavior with a truly thermodynamic spin‐glass transition at TS.G.=58K. These studies show that spin‐glass behavior can persist to quite high magnetic concentrations in an amorphous alloy system which is dominated by competing ferromagnetic and antiferromagnetic exchange interactions. At a lower temperature of T?20K, the susceptibility versus T curve shows a break and the coercive field is anomalous. We interpret this phenomenon near T=20K as arising from magnetic ’’blocking’’ of individual spins or regions of correlated spins which persist below TS.G.. This produces a cluster‐freezing‐type ’’pseudo‐transition’’ which is merely a manifestation of the slowing down of spin fluctuations to a time interval comparable with the experimental susceptibility measurement time. The temperature dependence of the spin‐glass order parameter is obtained from the susceptibility results and is in close agreement with data generated by Monte‐Carlo simulations.

ISSN:0021-8979    

DOI:10.1063/1.326926

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Ground state correlations in the Ising model with random ± J bonds are investigated on a square lattice, as a function of the concentration of negative bonds. By comparing all the ground‐states of (18×18) samples, we study the size distribution of packets of solitary spins. Large packets, containing more than half the spins, are observed in most samples, even at maximum bond disorder. This finding implies a remarkable rigidity of the random frustrated system at zero temperature.

ISSN:0021-8979    

DOI:10.1063/1.326927

出版商:AIP    

年代:1979

数据来源: AIP