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Polarized neutrons offer a very useful method of studying ferrimagnetic materials such as ferrites, in which the multiplicity of cations and their valence states prevent a unique determination of the spin structure when only polycrystalline material is available. This paper presents the first extensive use of polarized neutron powder‐diffraction patterns for determining the structure of a carbide of Mn and Co and of a number of mixed ferrites having the spinel structure.The ferrimagnetic carbide with composition (Mn,Co)4C is found to have a cubic structure of the Mn4N type, with one of the Mn atoms at the cube corner and the face centers being occupied by one Mn and two Co atoms in a statistical distribution. The two Mn atoms are antiferromagnetically aligned with respect to each other, the corner atom having a moment of 4.0 μB while the face center moment is 3.0 μB. The two Co atoms have a moment of 0.8 μB each and are aligned parallel to the corner Mn moment along the cube edges. The material has a Neel...

Several authors predict minima in the Peierls energy barrier to the motion of a dislocation when the barrier is considered as a function of the elastic or force‐law parameters of a crystal. The present paper reports a numerical calculation of the Peierls barrier in the Frenkel—Kontorova dislocation model modified to include the second harmonic of the force substrate. For the elastic constants of polycrystalline copper, the model predicts Peierls‐barrier minima, at stacking‐fault energies of 660, 530, 360, 210, 120, 62, 32,…erg/cm2, which range approximately from 10−5 to 10−2 times the intervening barrier maxima of about 2.5×10−9 erg/cm. This prediction is more accessible to experimental test since the stacking‐fault energy of copper can be modified by alloying with silicon. The Peierls barrier can be determined according to the Seeger interpretation of the Bordoni experiment.

Spin‐wave theory of a Heisenberg antiferromagnet predicts a zero‐point spin deviation η from the assumed ground state, namely, the Neel state. Measurements of η using NMR in a number of compounds give values substantially smaller than that given by spin‐wave theory.The object of this paper is to present the theory necessary to allow the measurement of η by magnetic scattering of thermal neutrons from an antiferromagnet containing a weak concentration c of nonmagnetic impurities. The diffuse elastic cross section for such a system is dσ/dΩ=(γe2/2mc2)2(1−z2)Nc(1−c)F2(κ) 〈Sz〉2 | Q(κ−τ) | 2, where 〈Sz〉 is the thermal average of the unperturbed host spins, Q(κ) the Fourier transform of the deviations in the z components of spin from 〈Sz〉 due to an impurity and all other symbols have their usual meaning. It is evident that η may be measured provided Q(κ) is known. We have calculated Q(0) for a body centered antiferromagnet of arbitrary spin in the linear spin‐wave approximation. This has been achieved using th...

This paper presents spectroscopic evidence for the existence of angled spin configurations in ytterbium iron garnet (YbIG) near its compensation temperature (∼7.5°K). We measured the rare earth‐iron exchange split single‐ion transitions at 23.2 and 26.5 cm−1 in the far infrared as a function of magnetic field applied along the [111] easy magnetization axis. We find that these transitions do not decrease linearly with increasing magnetic field at 8.5°K as predicted from the g factors characteristic of splittings at higher temperatures.1,2 In fact from 30–50 kOe, the spectrum consists of 4 lines 21.1, 23.3, 25.5, and 27.4 cm−1 whose average energy is not appreciably different from that of the unsplit spectrum. All our observations are consistant with the prediction of a temperature range below 20°K where the sublattice magnetizations are not collinear, but where angles are developed between the sublattice magnetizations and the applied field.3 That the average splitting is not appreciably field dependent su...

The purpose of this paper is twofold: (1) to present data on the magnetoelastic constants of rare‐earth iron garnets, a subject which for various reasons has been little studied thus far, and (2) to describe a new method for obtaining these constants which is applicable to many ferromagnetic insulators and does not require the use of strain gauges. The new method was developed from a study of the coupling introduced between long wavelength acoustic modes and long wavelength spin‐wave modes by ions in which the orbital moment is not quenched, such as rare‐earth ions, Co2+, and Fe2+. The effect of the coupling caused by such ions is observed as a magnetic field dependence of the resonant frequency of an acoustic resonator made from the given material, even above dc saturation. The acoustic Q above dc saturation is also field‐dependent. A theory has been obtained which explains the observed field dependences of both v(H) and Q(H) as well as other important features of the magnetoacoustic interaction. It will...

Superconducting magnets have established themselves as useful tools in solid‐state physics, magneto‐optical experiments, NMR, MHD, plasma, and other areas of physics. In high‐energy physics only the bubble‐chamber physicist has shown ample interest in using superconducting magnets. Reasons why there is a reluctance against large superconducting magnets in combination with high‐energy physics experiments and accelerators are discussed. However, in various areas of high‐energy physics superconducting magnets may be utilized, such as accelerator, beam‐transport, and experimental magnets. This paper summarizes physical properties of superconducting systems for experimental and beam transport magnets in quantitative form. Charging time, field uniformity, resolution, acceptance, solid angle, improvement in optical measurements accuracy, and first‐ and second‐order optics for superconducting magnets, with and without ferromagnetic return paths, will be compared to room and cryogenic magnets. Summary of experienc...

Magnetic resonance has been observed in the Ising system, DyPO4, utilizing a unique far‐infrared laser‐magnetic resonance spectrometer. Spectra were obtained at 220.23 μ (45.407 cm−1) and 171.67 μ (58.25 cm−1) at 4.2°K in applied fields (H∥C) sufficient to saturate the spin system. The radiation source was a pulsed‐discharge gas laser (using H2O or D2O). The spectrometer employed a 90‐kOe superconducting solenoid and a dual‐beam detection system which electronically integrates the pulsed signals and records directly the percentage transmission. The resonance yields three super‐imposed hyperfine patterns, due to even isotopes (I = 0); Dy(161) and Dy(163) both (I=52). All six lines of Dy(163) were resolved yielding a spin Hamiltonian constant (A/g∥) = 37.8×10−4 cm−1, while for Dy(161) only four components could be seen, giving (A/g∥) = 27.5×10−4 cm−1. The observed linewidth for all lines was 25 Oe. The slope of the splitting between the two I = 0 resonances observed at 53 340 Oe and 67 590 Oe yields g∥ = 19.3. The data indicates there is little departure from a linear Zeeman splitting at these high fields, and by extrapolation the antiferromagnetic zero field splitting is obtained. The zero field splitting agrees with the earlier spectroscopic results [J. C. Wright and H. W. Moos, Phys. Lett. 29 A, (1969).] only if an effective field of 1.3 kOe is added to the total dipolar field in the saturated state, indicating additional long‐range forces. The full version of this paper is being prepared for publication.

The local‐moment‐conduction‐electron exchange coupling J(k′k), important for all problems involving so‐called s‐d(s‐f) exchange, has been assumed always to be either a constant or at best, a function only of Q(= |k′−k|). In this paper we extend our earlier work assuming J(Q) behavior1 and examine, in some detail, the actual k′, k dependence of the exchange; these results are then applied to the spin densities obtained within lowest order Ruderman‐Kittel‐Kasuya‐Yosida (RKKY) theory. The J (k′, k) were obtained using realistic local moment wave functions (Hartree‐Fock 3d and 4f orbitals for Fe and Gd) and conduction electrons. Results, for Fe and Gd local moments, are reported and spin densities are compared for a range of assumed Fermi momentum (kF) values (0.3

Quasistatic magnetic switching in Ni‐Fe single‐crystal films for different orientations of the reversal fields were studied both experimentally by Lorentz microscopy and theoretically. It was found that the switching processes can be explained quantitatively by a theory combining the Stoner‐Wohlfarth rotational switching curve and the theoretical wall motion switching curve derived in this paper from a simple model. Eight typical switching processes are illustrated by Lorentz micrographs with explanation for two types of films; one type has a higher ratio of coercive force to biaxial anisotropy field than the other. Both wall motion switching and rotational switching were observed. In some cases, the complete switch takes place in two separate consecutive stages, each stage corresponding to a different easy axis direction 90° apart, each stage being by a different switching mechanism. Cross‐tie walls were observed in single‐crystal films in the proper thickness range. Although the films studied had nearly...

It has been suggested 1–3 that monoclinic Cu (HCOO)2·4H2O may closely approximate a two‐dimensional antiferromagnetic. Magnetic susceptibility and magnetization measurements have indicated that antiferromagnetic ordering occurs at 17°K and have also shown evidence for weak ferromagnetism.2,3 An additional broad peak in the susceptibility has been observed near 65°K.3,4 Recent ESR measurements5 have shown a unique linear temperature dependence of the paramagnetic linewidth. In order to obtain more information on the magnetic parameters characterizing this crystal, we have made AFMR measurements between 1.4° and 15°K. Resonance transitions have been observed at 9 GHz, from 32 to 36 GHz, from 50 to 80 GHz, and from 107 to 124 GHz with magnetic fields up to 26 kOe. The minimum resonance field for the low‐frequency mode in the ac plane is observed at a position (called the z axis) 3° from the c axis. The AFMR modes (frequency vs magnetic field) for the field along the z and b axis suggest that z and b are respectively the easy and intermediate axes for the spins. However, the following differences from the behavior of a normal antiferromagnet are observed: (1) Along the z direction, the high and lowfield branches intersect at a nonzero frequency. (2) Along the b axis a discontinuity is observed at 5.3 kOe at which a weak signal is observed at various frequencies between 9 and 72 GHz; (3) Above 5.3 kOe, an additional resonance is observed in the bc′ plane. We have attempted to fit the data with a two‐sublattice model with z as the easy axis and weak moment along the b axis. The model contains isotropic exchange, symmetric and antisymmetric exchange, and g‐tensor anisotropy. Good agreement between the calculation and the data is obtained for the low‐frequency mode along the z and b axes above 5.3 kOe. From this fit and the static measurements2,3 consistent values of the exchange parameters are obtained. However, below 5.3 kOe the calculated resonance frequencies are considerably lower than the experimental ones. Furthermore, this two‐sublattice model is inadequate to explain the discontinuity along the b axis and the additional resonance in the bc′ plane. A detailed paper will be published elsewhere.

The present paper analyzes a rotational model of flux reversal in ferromagnetic materials. A brief review of the experimentally established details of the flux reversal process in square loop ferrites is given. The discrepancies between the experimental results and the predictions of the domain wall motion theory are discussed. The switching coefficient, Sw, is examined in detail.The rotation model is based on the solution of the modified Landau and Lifshitz equation, but does not assume that the magnetization rotates uniformly throughout the sample. The model predicts the shape of the output voltage pulse, the relationship between the flux reversal time and the applied field, and the minimum switching coefficient that can be obtained for a given ferrite. The switching coefficients determined experimentally for ferrites, permalloy tape, and thin evaporated metal films (with zero transverse field) are within a factor of three of the minimum value of Sw predicted by this model.

The crystal structures of α‐Mn2O3 and (Mn1−xFex)2O3, x=0.017 and 0.63, have been refined from single crystal x‐ray data. Interatomic distances and angles in (cubic) (Mn0.983Fe0.017)2O3 are equal to the analogous average distances and angles in α‐Mn2O3. There is significantly less distortion of the oxygen polyhedra about 24(d) sites in (Mn0.37Fe0.63)2O3 than in (Mn0.983Fe0.017)2O3. Evidence based on these results and on magnetic studies of systems (Mn1−xMx)2O3, M≡Fe3+, Sc3+, Ga3+, Cr3+ supports the suggestion that the antiferromagnetic ordering temperatures depend on the allowed distortion of the Mn3+ ion surroundings. The paper entitled ``Structures of α‐Mn2O3, (Mn0.983Fe0.017)2O3 and (Mn0.37Fe0.63)2O3 and Relation to Magnetic Ordering'' from which this has been taken will be published in Acta Crystallographica (probably in the first half of 1971). Two other especially pertinent references are: R. W. Grant, S. Geller, J. A. Cape, and G. P. Espinosa, Phys. Rev. 175, 686 (1968); S. Geller and G. P. Espinosa...