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The temperature dependence of the magnetoelastic properties of a series of germanium/cobalt co-substituted cobalt ferrite samples has been measured. Magnetostriction loops of the compositions [CO.sub.1+x][Ge.sub.x][Fe.sub.2-2x][O.sub.4] (x = 0.1, 0.3, and 0.6) were measured over a temperature range of 250-400 K. Germanium/cobalt co-substitution was found to change the Curie temperature, anisotropy, and the magnetostriction coefficients in the crystal directions of [100] and [111]. Both magnetostriction and strain sensitivity were seen to decrease in magnitude with increasing temperature. [Co.sub.1.1][Ge.sub.0.1][Fe.sub.1.8][O.sub.4] was found to have a maximum sensitivity [(d[lambda]/dH).sub.[sigma]] of 2.6 x [10.sup.-9] [A.sup.-1] x m, which is twice the value obtained for pure cobalt ferrite, without any decrease in maximum magnetostriction in the linear region. Index Terms--Cobalt ferrite, germanium substitution, magnetostriction, stress sensor.

The temperature dependence of the magnetic anisotropy of magnetoelastic chromium-substituted cobalt ferrites (Co[Cr.sub.x][Fe.sub.2-x][O.sub.4] with 0 [less than or equal to] x [less than or equal to] 0.8) was investigated over the temperature range 10-400 K. The first-order cubic anisotropy coefficient [K.sub.1] was calculated by fitting the high-field regimes of the major hysteresis loops to the law of approach to saturation, which is based on the assumption that at sufficiently high field only rotational processes remain. It was found that anisotropy increases substantially with decreasing temperature from 400 to 150 K. Below 150 K, it appears that even under a maximum applied field of 5 T, the anisotropy of Co[Fe.sub.2][O.sub.04] and Co[Cr.0.2] [Fe.sub.1.8][O.sub.4] is so high that it prevents complete approach to saturation. In general, at fixed temperature the magnetic anisotropy decreases with increasing Cr content, which could be explained based on the one-ion model and the results of Mossbauer studies: According to the one-ion anisotropy model, [Co.sup.2+] ions in octahedral sites of the spinel structure are responsible for the high anisotropy of cobalt ferrite, and substitution of [Cr.sup.3+] for [Fe.sup.3+] appears to displace [Co.sup.2+] ions from octahedral sites to tetrahedral sites. Index Terms--Cobalt ferrite, ferrite, law of approach (LoA), magnetic anisotropy.

We report dramatic improvements in both magnetostriction level and strain derivative of polycrystalline cobalt ferrite as a result of magnetic annealing. Magnetostrictive cobalt ferrite composites have potential for use in advanced magnetomechanical stress and torque sensors due to their high sensitivity of magnetization to applied stresses and high levels of magnetostriction. Results show that annealing cobalt ferrite at 300 [degrees]C in air for 36 h under a dc field of 318 kA/m (4 kOe) induced a uniaxial anisotropy with the easy axis being along the annealing field direction. Under hard axis applied fields, the maximum magnetostriction measured along the hard axis at room temperature increased in magnitude from -200 x [10.sup.-6] to -252 x [10.sup.-6] after annealing. The maximum strain derivative [(d[lambda]/dH).sub.max], which is related to stress sensitivity, increased from 1.5 x [10.sup.-9] [A.sup.-1]m to 3.9 x [10.sup.-9] [A.sup.-1]m. The results can be interpreted in terms of the effects of induced uniaxial anisotropy on the domain structure and magnetization processes. Index Terms--Induced anisotropy, magnetic annealing, magnetostrictive cobalt ferrite, magnetoelastic stress sensor.

The thermal expansion and magnetostriction of polycrystalline and single-crystal [Pr.sub.5] [Ni.sub.2] [Si.sub.3] were investigated over the temperature range 5-300 K. The results show two magnetic phase transitions, one corresponding to the Curie temperature and the other at a lower temperature exhibiting characteristics of a spin reorientation transition. In the temperature range below the Curie temperature, the thermal expansion in single-crystal samples exhibited a temperature dependent anisotropy. Specifically negative and positive magnetic contributions to the thermal expansion were observed along the directions parallel and perpendicular to the c axis respectively. Index Terms--Magnetostriction, rare earth, spin reorientation, single crystal, thermal expansion.

For the first time, both the magnetoelectronic properties and the magnetic domain structure of a magnetic tunnel junction (MTJ) have been characterized simultaneously. In situ tunneling magnetoresistanee measurements for a spin-dependent tunnel junction were directly compared to magnetic domain behavior imaged using magnetic force microscopy (MFM) under the same applied fields, allowing for detailed investigation of electronic behavior relative to magnetic behavior. From the results, it was found that the resistance in the hysteretic field range (less than the coercivity of Hc = 800 A/m or 10 Oe) corresponded with the complex magnetic domain behavior observed in the MFM images and qualitatively matched micromagnetically modeled junctions. Index Terms--Domain structures, magnetic force microscopy (MFM), magnetoresistance, spin-dependent transport.

The effects of pure shear stress on magnetization in nickel have been studied with a view to developing a theoretical understanding of the Matteucci effect, which is of great technological interest in the search for viable magnetic torque sensors. The effects of pure shear stress on a planar magnetic material and those due to torque applied to a rod are analogous in the sense that they can be described in terms of a pair of tensile and compressive stresses acting at 45[degrees] to the neutral axis of the plate or the rod axis. In this work the responses of nickel to shear stresses were studied under various dc applied fields. A model description was developed as a generalization of the theory of magnetomechanical effect to include shear stresses. This theory can be used as an aid to interpretation of the experimental results. Index Terms--Magnetomechanical effect, Matteucci effect, shear stress.

Cobalt ferrite has been shown to be an excellent candidate material for high sensitivity magnetic stress sensors due to its large magnetomechanical effect and high sensitivity to strain. However, near room temperature, the material exhibits some magnetomechanical hysteresis, which becomes negligible for temperatures of 60[degrees]C and above. Measurements indicate that doping the cobalt ferrite with silicon lowers the Curie temperature of the material. It was also found that the Curie temperature of the material depends on the fabrication and processing procedure. These results offer the possibility of decreasing the room temperature magnetomechanical hysteresis through control of the Curie temperature, thereby altering the temperature dependence of magnetic and magnetomechanical properties. Index Terms--Cobalt ferrite, Curie temperature, magnetoelastic materials, stress sensors.

Single crystal [Gd.sub.5][(Si.sub.~0.5][Ge.sub.~0.5]).sub.4] was studied using thermal expansion measurements in the vicinity of the magnetic-structural phase transition. A strain gauge was attached on the a-b plane oriented to measure the strain along the b axis and a magnetic field of H = 0.8 x [10.sup.6] A/m (B = 1 Tesla) was applied along this same b axis. The sample was then rotated through 45[degrees], 90[degrees], 120[degrees], 165[degrees], and 180[degrees] from the direction of the applied magnetic field and the thermal expansion measurements were repeated. It was found that there was an unexpected variation in the temperature at which the sudden discontinuity in strain occurred. The results seem to suggest that the transition temperature changes with direction of applied field relative to the crystal lattice. This surprising result is attributed to the combined effects of the first order phase transformation and the presence of an anisotropy 'field,' which differs with angle. This combination of factors, therefore, results in a different strength of effective field being experienced by the magnetic moments on the Gd atoms depending on their orientation. Comparing this with the previously observed field dependence of the Curie temperature in this material provides a possible explanation for this highly unusual observation. Index Terms--Anisotropy field, first-order transformation, magnetocaloric effect, thermal expansion.

Measurements of thermal expansion of single-crystal [Gd.sub.5]([Si.sub.l.95][Ge.sub.2.05]) during cooling and heating were conducted for the first time. A very steep change in strain with temperature was observed when the material underwent a phase transformation. This was an unusual simultaneous magnetic and structural phase transformation from a ferromagnet with an orthorhombic crystal structure below the transition temperature [T.sub.c] to a paramagnet with a monoclinic crystal structure above [T.sub.c]. This transition temperature [T.sub.c] was found to depend on the magnetic field, and to exhibit hysteresis depending on whether the material was being cooled or heated. In the absence of a magnetic field, [T.sub.c] was 267 K on cooling and 269 K on heating. However, when the material was subjected to a magnetic induction B in the range 0-2.5 tesla (T), the transition temperatures, on both cooling and heating, were found to increase linearly with temperature by about 4.8 K/T. This rate of change of transition temperature with magnetic field was in good agreement with calculations based on the assumption that the additional energy due to the magnetic field can suppress the thermal vibration of Gd atoms and that the additional thermal energy per Gd atom needed to cause the phase transition to occur is equal to the additional magnetic energy of each Gd atom caused by the magnetic field. Index Terms--First-order transformation, hysteresis, phase transitions, single crystal, thermal expansion.

[Gd.sub.5] ([Si.sub.2][Ge.sub.2]) and related compounds with similar (nearly equal Si-to-Ge ratio) composition exhibit large magnetoresponsive properties including a giant magnetocaloric effect, colossal magnetostriction, and giant magnetoresistance near a structural-magnetic phase transition that occurs close to ambient temperature [1]. Magnetic force microscopy (MFM) and vibrating sample magnetometry (VSM) measurements on single-crystal samples of these materials indicate that the easy magnetization axis is the b-axis of the orthorhombic magnetic phase--perpendicular to the slabs. In fact, the MFM image of a surface perpendicular to the b-axis is quite similar to domain patterns perpendicular to the easy axis of Co and other highly anisotropic magnetic materials. Therefore, it appears that [Gd.sub.5] [([Si.sub.x][Ge.sub.1-x]).sub.4] may require modeling similar to other multilayers and superlattices of rare-earth metals with one or more nonmagnetic constituents that exhibit long-range magnetic order across nonmagnetic layers. Many of the important phenomena of these Gd compounds could be explained by the interaction of localized Gd magnetic moments across the covalent bonding between atomic slabs, adapting models already suggested for other similar materials [2]. Index Terms--Magnetic force microscopy (MFM), magnetic materials, magnetic measurements, rare earths.