C. M. Williams

Giant magnetostriction materials

Abstract One of the significant technical developments in magnetism of the early 1970s was the discovery of a new class of rare earth intermetallic compounds, the RFe 2 Laves phases, which were found to exhibit room temperature magnetostrictive strains approaching 2 × 10 −3 , an order of magnitude larger than any previously known. Since that time both the fundamental and technical properties of these materials have been of intense interest, and they remain the subject of active research even today. The large strains available are useful in such applications as production of high amplitude, low frequency sound waves in water, certain types of strain gages, vibration compensation and compensation for temperature induced strains in large laser mirrors. Because the performance of these materials depends critically on such fundamental properties as the magnetic anisotropy, magnetization and grain orientation of the material, there has been a very strong interplay between fundamental studies and applications. In this article we briefly review the fundamental magnetic and magnetostrictive properties of the RFe 2 Laves phases, focusing especially on the complex behavior of the anisotropy and the success of crystal field theory in explaining it. We also present neutron measurements of magnetic excitation spectra and explain how they provide an understanding of the remarkable success of mean field theory for these systems.

The magnetic and structural properties of pulsed laser deposited epitaxial MnZn–ferrite films

The magnetic and structural properties of pulsed laser deposited MnZn–ferrite films have been examined. The results show that the uniaxial anisotropy, ferromagnetic resonance linewidth and coercive force are strongly influenced by the microstructure of the films, and the saturation magnetization and first‐order magnetocrystalline anisotropy constant depend on intrinsic properties such as composition and cation site occupation. A comparison of bulk and film magnetic properties shows that the magnetic properties of the films are comparable to the bulk, which makes pulsed laser deposition ferrite films a prime candidate for thin film high‐frequency microwave device applications.

Magnetostrictive properties of Hox Tb1 −x Fe2 intermetallic compounds

The magnetostriction of several polycrystalline compounds in the Hox Tb1−x Fe2 series have been measured from 77–300°K using magnetic fields up to 17.5 kOe. The extrapolated saturation magnetostriction at room temperature varies linearly with x between 80 × 10−6 for HoFe2 and 1720 × 10−6 for TbFe2, in good agreement with the single‐ion model. At the composition x =0.85, where the lowest‐order anisotropy constant K1 is very small, the saturation magnetostriction is 325 × 10−6, suggesting that this material might be useful in magnetostrictive device applications.

Investigations of the Effects of Neutron and He3 Irradiation on the Magnetic Properties of Permalloy Thin Films

Measurements have been made of the changes in the magnetic anisotropy energy and hysteresis loop parameters of evaporated Permalloy thin films resulting from 2.2‐MeV He3 irradiation and from reactor neutron irradiation. Only minor changes were observed following an irradiation of 1.6×1014 He3 particles/cm2. However, a total irradiation of either approximately 1017 neutrons/cm2, or 8 to 9×1016 He3 particles/cm2, yielded the following general results: Regardless of the magnetic state of the sample, the uniaxial anisotropy energy Ku increased (up to 200%) when irradiations took place in zero applied field. Similar increases were noted as a result of either neutron irradiation with a saturating magnetic field along the easy axis or He3 irradiation with a saturating magnetic field either parallel or perpendicular to the easy axis. A decrease of ∼50% in Ku resulted from neutron irradiation when the saturating magnetic field was applied perpendicular to the easy axis. Rotation of the easy axis, up to 78°, occurred for either neutron or He3 irradiation with a saturating magnetic field applied perpendicular to the easy axis. The more significant hysteresis loop modifications observed were the development of constricted loops (measured in the hard direction) for most of the samples irradiated in zero applied field, and decreases in the coercive force up to 28% (measured in the easy direction) of most of the irradiated samples.Measurements have been made of the changes in the magnetic anisotropy energy and hysteresis loop parameters of evaporated Permalloy thin films resulting from 2.2‐MeV He3 irradiation and from reactor neutron irradiation. Only minor changes were observed following an irradiation of 1.6×1014 He3 particles/cm2. However, a total irradiation of either approximately 1017 neutrons/cm2, or 8 to 9×1016 He3 particles/cm2, yielded the following general results: Regardless of the magnetic state of the sample, the uniaxial anisotropy energy Ku increased (up to 200%) when irradiations took place in zero applied field. Similar increases were noted as a result of either neutron irradiation with a saturating magnetic field along the easy axis or He3 irradiation with a saturating magnetic field either parallel or perpendicular to the easy axis. A decrease of ∼50% in Ku resulted from neutron irradiation when the saturating magnetic field was applied perpendicular to the easy axis. Rotation of the easy axis, up to 78°, occurr...

Magnetization of (Y0.9R0.1)2Fe14B single crystals

Abstract Magnetization and torque magnetometry measurements have been made on single crystals of the form (Y0.9R0.1)2Fe14B, where R = Er and Tb. Pure Y2Fe14B favors the [001] at all temperatures, reflecting the anisotropy of the Fe sublattice. The addition of Er decreases the anisotropy at room temperature slightly, but at low temperatures it induces a spin reorientation in the (100) plane toward the [010], stopping at an angle of 78° from the [001]. The addition of Tb increases the anisotropy favoring the [001] at all temperatures.

Origins of magnetic anisotropy in cubic RFe2 Laves phase compounds

Detailed studies of magnetic anisotropy and spin orientations in ternary and quaternary high magnetostriction compounds of the form HoxTbyDy1−x−yFe2 have clearly demonstrated the existence of both continuous and discontinuous rotation of the easy axis between principal crystallographic directions as a function of composition and temperature. While the usual single ion crystal field model successfully explains the qualitative features of the data, there still remain significant differences between the actual and predicted compositions at which the rotations occur. We show that these differences are largely due to changes in the magnetoelastic energy with spin orientation.

Spin reorientations in single crystal HoAl2

Torque measurements have been performed on single crystal HoAl2 compound in order to examine the origin of the 20 K specific heat anomaly that occurs in polycrystalline HoAl2. The experimental results indicate the anomaly is due to a spin reorientation between the [110] and [100] in the (100) plane as the temperature is increased. Crystal field calculations suggest the transition should be of the first order; however, torque measurements show that the transition takes place continuously over a finite temperature range.

Torque measurements of the magnetic anisotropy energy of antiferromagnetic-coupled Fe/Cr/Fe layers

Various methods have been used to determine the exchange coupling, magnetocrystalline anisotropy energy, and complex switching behavior for epitaxially grown, antiferromagnetically coupled thin films of iron with interleaved chromium. In this paper, we determined the exchange coupling and magnetocrystalline anisotropy energy directly from the angle dependence of the magnetic free energy using torque magnetometry. The results are in very good agreement with ferromagnetic resonance, magnetization, and magnetoresistance measurements reported earlier on the same sample.

Chemically homogeneous fine‐grained Mn‐Zn ferrites by spray drying

Chemically and structurally uniform Mn‐Zn ferrites have been produced using powders synthesized by the spray decomposition of mixed, aqueous Mn, Fe, and Zn nitrate solutions. The particle size of the as‐formed powder depended strongly on the metal ion concentration in solution; higher concentrations produced larger sizes. The as‐formed particles are spherical, internally hollow, and consisted mostly of Fe2O3. A 900 °C, 6‐h argon treatment removed the internal void space and converted the particles mostly to the ferrite spinel phase. Sintering of compacts from heat‐treated powders produced microstructures superior to those from as‐formed powders. Uniform, fine‐grained materials, with a densification level comparable to that of commercial sintered ferrite, have been produced at the very low sintering temperature of 1100 °C. Preliminary work indicates that a higher initial permeability is obtained when a higher sintering temperature was used and the level of its disaccommodation depended on the oxygen partia...

Torque measurements on single crystal DyxTb1−xFe2 compounds

Abstract Torque measurements on single crystal Dy x Tb 1− x Fe 2 compounds reveal continuous spin reorientations along nonprincipal crystallographic directions in the (110) plane. A comparison of the experimental results with single ion model calculations suggest that the crystal field parameters for this system may be about 30% lower than those measured for ErFe 2 .