摘要:

Experimental properties of Kondo‐type systems in the very low temperature regime are examined using as examples both old and new data on the specific heat, susceptibility, and resistivity, primarily of copper‐based third‐row transition series alloys. Information about theTequal 0°K limit of the entropy and susceptibility of the Cu(Fe) system has been obtained, as well as data showing a specific‐heat peak at 0.45°K in a Cu(Cr) alloy of 32 ppm concentration. The low field susceptibility of Cu(Cr) has been measured down to 15 mK. An interesting systematic variation in the zero temperature limit of the resistivity is pointed out.

ISSN:0021-8979    

DOI:10.1063/1.1657529

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The rare‐earth orthoferrites are a family of canted antiferromagnets which show an unusual variety of magnetic properties. This article begins with a brief review of the ``allowed'' spin configurations compatible with the crystallographic symmetry of the orthoferrite structure and a summary of the experimentally observed spin configurations, spin reorientation temperatures, compensation temperatures, etc. We then review recent research on these materials, grouping most of the recent work into four major categories; studies of the spin reorientation transition, studies of the rare‐earth spin ordering, spectroscopic studies directed at determining magnetic interaction parameters, and studies of magnetic domains and domain walls. The spin reorientation process has been established to occur over a finite temperature range as a fourth‐order anisotropy, generally small, comes to dominate the orientational behavior in the temperature region where the usually domainant second‐order anisotropy passes through zero and changes sign. The spin reorientation has been monitored in a number of RE orthoferrites by several techniques; magnetic torque, microwave absorption, neutron diffraction, and optical spectroscopy. A few of the rare‐earth orthoferrites show RE‐RE interaction strong enough to cause an ordering of the rare‐earth ions at temperatures on the order of 2°–6°K; this ordering process has been documented by all the above techniques plus Mo¨ssbauer‐effect and magnetic‐susceptibility measurements. Spectroscopic measurements, showing the increase in RE ground‐state exchange splittings for the configurations stable at the lower temperatures, give a qualitative understanding of the mechanisms causing the spin reorientation and spin ordering processes, and promise detailed quantiative understanding of the unique magnetic behaviors seen in the orthoferrites. The low moment and high anisotropy of the orthoferrites make possible the fabrication of thin plates magnetized normal to the plate. Such plates are semitransparent, and the Faraday rotation through them can be used to study domain structures or can be used as a readout mechanism for memory and logic devices utilizing single small stable domains as the active element. Recent studies of such domains and of domain wall energy and mobility in the orthoferrites are accordingly reviewed. Finally, several investigations are summarized which utilize the orthoferrites as a vehicle for examining general properties of magnetic systems, such as critical‐point phenomena.

ISSN:0021-8979    

DOI:10.1063/1.1657530

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

In both rare‐earth and transition‐metal salts there may be appreciable interactions of high degree in the orbital or total spin angular‐momentum operators acting between the ions. These interactions may arise either from the exchange interaction between the electrons or from the electrostatic coupling between the electric multipole moments of the two atoms. The latter in turn may arise either directly through the multipole coulombic field or indirectly through the coupling of the ions to the lattice. In transition‐metal compounds the orbital moment is usually quenched, so that for interactions involving only the ground multiplet, the anisotropy introduced by the orbital effects is quite small. However, in rare‐earth insulators the rare‐earth magnetic moment has appreciable orbital content, so that the high‐rank interactions are of considerable importance. In this paper we give a brief discussion of the theory of these interactions, and we then review recent experimental work on rare‐earth compounds in which these effects have been shown to be important. Particular emphasis is given to: (a) experiments on YIG which indicate the presence of appreciable high‐degree exchange; (b) experiments on the EPR of pairs of coupled Ce3+and Nd3+ions in LaCl3and LaBr3; in these experiments it was possible both to measure the electric quadrupole‐quadrupole interaction and to isolate contributions to the interaction tensors from high‐degree exchange of the formJ+5(1)J−5(2) andJxl(1)J±5(2); (c) a series of experiments and calculations on UO2which indicate the presence of appreciable electric multipole coupling via the lattice.

ISSN:0021-8979    

DOI:10.1063/1.1657531

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

RbFeF3is a transparent ferromagnet (below 87°K) with a large Faraday rotation which permits the direct observation of magnetic domain structures in bulk crystals. An etch‐pitting solution has been developed which reveals the dislocation configuration existing in the crystals, and it has been demonstrated that RbFeF3exhibits sufficient plasticity so that arrays of dislocations can be put into crystals. The role of dislocation subboundaries and slip bands as nucleation sites for reversed magnetic domains will be demonstrated. The effectiveness of subboundaries and slip bands as pinning sites for domain walls when subjected to varying dc and ac magnetic fields are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1657532

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Measurements of the temperature dependence of domain‐wall mobility in single‐crystal plates of Y, Eu, Ho, Er, Tm, and Lu orthoferrites are given in the range from 77°–350°K. Room‐temperature values of mobility for all of these orthoferrites fall in the range from 100–1000 cm‐sec−1Oe−1. However, substantial differences in the temperature dependence of the mobility are noted among them. In EuFeO3, the mobility increases from 300 cm‐sec−1Oe−1at 350°K to almost 3000 cm‐sec−1Oe−1at 78°K while in HoFeO3it falls from 230 cm‐sec−1Oe−1at 350°K to 17 cm‐sec−1Oe−1at 100°K. Wall mobility is measured using direct stroboscopic observation of a single domain wall whose equilibrium position is modulated by a sinusoidal drive field. Both magnitude and phase of the wall displacement can be measured as a function of frequency. The domain wall is observed by means of the Faraday effect and strobing is achieved with a He&sngbnd;Ne laser beam gated on for a small fraction of the drive cycle by an electro‐optic light modulator. The wall mobility is determined from the frequency response of the wall motion which, for these orthoferrites, exhibits a simple relaxation arising from viscous damping.

ISSN:0021-8979    

DOI:10.1063/1.1657533

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Four primary stripe patterns are observed in Permalloy single‐crystal platelets with {100} surfaces in different regions of thickness and composition. All four structures involve magnetic moment rotations between easy ⟨111⟩ axes parallel to {110} planes. The first to appear with increasing thickness (0.3<D<1.0 &mgr;m) has ⟨100⟩ stripes and ⟨100⟩ net magnetization. The second and third coexist from 0.8–1.4 &mgr;m, have fine ⟨110⟩ and broad ⟨100⟩ stripes, respectively, and have different magnitudes of net magnetization. Both involve 71° rotations parallel to vertical {110} planes. The fourth (D>0.9 &mgr;m) achieves appreciable internal flux closure by forming a helical structure through rotations toward alternate ⟨111⟩ directions in passing from top to bottom surfaces at any stripe, and through having the planar surface component in alternate stripes perpendicular to the stripes. Models for 180° and 90° walls readily follow from the above scheme in conjunction with applied‐field observations.

ISSN:0021-8979    

DOI:10.1063/1.1657534

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Circular domains can be formed in GdIG by applying a train of short, fast‐risetime pulses of magnetic field. The density of these domains depends on the temperature at which they are formed. After formation, the domain size can be varied without affecting the density by changing the temperature or applying a dc bias field.

ISSN:0021-8979    

DOI:10.1063/1.1657535

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The damping of mechanical vibrations associated with the stress induced movement of magnetic domain boundaries was measured in pure nickel, pure iron, and in an iron‐3.7% silicon alloy over the temperature range of 78°–300°K. The damping was characteristically amplitude dependent with maximum values at specific values of the shear strain. The damping for nickel, as expressed by the fractional energy loss per cycle, had maxima that decreased from 0.58 at 78°K to 0.36 at 300°K. The damping maxima for iron decreased only slightly from 0.20 at 78°K to 0.16 at 300°K. However, in the case of the iron‐3.7% silicon alloy the damping maxima increased from 0.19 at 78°K to 0.28 at 300°K. The results are discussed in terms of the recently proposed mathematical model of magnetomechanical damping by Smith and Birchak. Much of the variation of the damping with temperature can be accounted for by variations of the magnetostriction with temperature.

ISSN:0021-8979    

DOI:10.1063/1.1657536

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The Kerr effect was used to observe and photograph domain structures in grain‐oriented silicon steel under the influence of longitudinal tensile stresses. In normal, unstressed oriented silicon steel the domain structure is typically a mixture of antiparallel and ``dagger'' or ``pine tree'' domains. The application of uniform tensile stress converts this mixed structure to a completely antiparallel domain structure and reduces the antiparallel domain‐wall spacing. The current work shows that the pure antiparallel structure is developed at stresses on the order of 1000 psi. This correlates well with the magnetic property changes associated with the application of uniform tensile stress. The domain‐wall spacing refinement is an inverse function of the stress level up to a stress of ≃6500 psi. Higher stresses produced no further change in the domain structure. High‐speed motion pictures (≃5000 frames/sec) revealed that the refined spacing was retained during 60 Hz excitation. This effect also would contribute to improved core losses for oriented silicon steel.

ISSN:0021-8979    

DOI:10.1063/1.1657537

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The fundamental equation of micromagnetics is analytically solved under the following assumptions: the direction of the magnetization vector depends only on one coordinate, the anisotropy energy is uniaxial and of the second degree, the magnetization lies in a plane normal to this coordinate and including the easy axis, the external magnetic field is parallel to the easy direction. One class of periodic solutions is discussed in detail, corresponding to fully magnetized domains divided from each other by means of Bloch walls; for this class every pair of values of the field and of the period selects one and only one solution. Under the consistent assumption that the period is constant during slow changes of the external field, the magnetization curves for different periods are determined. For domains much larger than one ``classical'' Bloch wall, it is possible to define a wall energy per unit area which differs from the ``classical'' formula because of a factor (1+h)½(his the ratio between the external field and the anisotropy one). The magnetization vector within the walls points alternatively in opposite directions; solutions with magnetization vector within all the walls pointing toward the same direction belong to another class and are unstable.

ISSN:0021-8979    

DOI:10.1063/1.1657538

出版商:AIP    

年代:1969

数据来源: AIP