P. I. Williams

Temperature dependence of magnetic anisotropy of Ga-substituted cobalt ferrite

The temperature dependence of magnetization, magnetic anisotropy, and coercive field of gallium-substituted cobalt ferrite was investigated for a series of compositions of CoGaxFe2−xO4 (0 ⩽ x ⩽ 0.8). Hysteresis loops were measured for each sample over the range of −5 T ⩽ μ0H ⩽ 5 T for selected temperatures between 10 and 400 K. The magnetization at 5 T and low temperatures was found to increase for the lower Ga contents (x = 0.2 and 0.4) compared to pure CoFe2O4, indicating that at least initially, Ga3+ substitutes predominantly into the tetrahedral sites of the spinel structure. The high field regions of these loops were modeled using the law of approach to saturation, which represents the rotational process, together with an additional linear forced magnetization term. The first order cubic magnetocrystalline anisotropy coefficient K1 was calculated from curve fitting to these data. It was found that K1 decreased with increasing Ga content at all temperatures. Both anisotropy and coercivity increased substantially as temperature decreased. Below 150 K, for certain compositions (x = 0, 0.2, 0.4), the maximum applied field of μ0H = 5 T was less than the anisotropy field and, therefore, insufficient to saturate the magnetization. In these cases, the use of the law of approach method can lead to calculated values of K1 which are lower than the correct value.

Magnetic and Magnetomechanical Properties of CoAl

The potential of CoAlxFe2-xO4 (x = 0.1 to 0.9) for magnetomechanical stress sensor applications has been studied. It was found that the plot of magnetostriction against applied magnetic field changed shape as Al content increased. Furthermore, substituting with Al3+ resulted in the increase of the sensitivity of strain to applied magnetic field (dlambda/dH)max. The maximum magnetostriction for the Al-doped (x = 0.1) was 34% lower than un-doped Co-ferrite but the (dlambda/dH)max was 112% higher. The magnitude of magnetostriction obtained for Al-doped samples is still sufficient for stress sensor and actuator applications. Results were compared with those obtained for Ge-, Ga-, Cr- and Mn-doped Co-ferrites. It was found that in terms of improving strain response to applied field whilst maintaining significant amplitude of magnetostriction CoAlxFe2-xO4 has promising potential for magnetomechanical stress sensor applications.

_{\rm x}

The temperature dependence of magnetic properties of a series of aluminium-substituted cobalt ferrite, with the general composition of CoAlxFe2-xO4 (x = 0, 0.1, 0.2, 0.5), has been studied. It was observed that the magnetization, at an applied field of mu0 H = 5 T and a temperature of 10 K, does not change significantly for small amounts of Al, then reduces sharply for larger amounts of Al. Magnetic hysteresis loops were measured over a field range of -5 T les mu0H les 5 T at temperatures between 10 and 400 K. The high field regions of these loops were modelled using the Law of Approach to saturation, which can be used to calculate the magnetocrystalline anisotropy using its description of the processes of rotation of domain magnetizations against anisotropy and forced magnetization. It was found that the first-order cubic anisotropy of these materials decreased with increasing temperature. It was also observed that the anisotropy of these materials decreased with increasing Al-content at a given temperature.

Fe

New applications for transcranial magnetic stimulation are developing rapidly for both diagnostic and therapeutic purposes. Therefore, so is the demand for improved performance, particularly in terms of the ability to stimulate deeper regions of the brain and to do so selectively. The coil designs that are used presently are limited in their ability to stimulate the brain at depth and with high spatial focality. Consequently, any improvement in coil performance would have a significant impact in extending the usefulness of TMS in both clinical applications and academic research studies. New and improved coil designs have been developed, modeled, and tested as a result of this work. A large magnetizing coil, 300 mm in diameter and compatible with a commercial TMS system, has been constructed to determine its feasibility for use as a deep brain stimulator. This coil, used in a Helmholtz configuration, can produce 105 V/m at the surface of the head and 93 V/m at a depth of 15.2 mm compared to a single turn 60 mm coil which produces 82.6 V/m at the surface and only 15 V/m at 15.2 mm. The results of this work have suggested directions that could be pursued in order to further improve the coil designs.

_{2 - {\rm x}}

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_1+xGe_xFe_2-2xO₄ (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_1.1Ge_0.1Fe_1.8O₄ was found to have a maximum sensitivity (dlambda/dH)_sigma of 2.6 times 10^-9 A^-1.m, which is twice the value obtained for pure cobalt ferrite, without any decrease in maximum magnetostriction in the linear region.

O

The variations in magnetization and magnetic anisotropy of Ge4+/Co2+ cosubstituted cobalt ferrite with temperature were investigated for a series of compositions Co1+xGexFe2−2xO4 (0≤x≤0.4). The magnetization at 5 T and low temperature were observed to increase for all Ge/Co cosubstituted samples compared to pure CoFe2O4. Hysteresis loops were measured for each sample over the magnetic field range of −5 T to +5 T for temperatures in the range of 10–400 K. The high field regions of these loops were modeled using Law of Approach to saturation, which represents the rotational and forced magnetization processes. The first order cubic magnetocrystalline anisotropy coefficient K1 was calculated from these fits. K1 decreased with increasing Ge content at all temperatures. Anisotropy increased substantially as temperature decreased. Below 150 K, for certain compositions (x=0, 0.1, 0.2, and 0.3), the maximum applied field of μ0H=5 T was less than the anisotropy field and therefore insufficient to saturate the magne...

_{4}

The effect of temperature variation on the magnetoelastic properties of the CoAlxFe2−xO4 system (for x=0.0, 0.2, 05, and 0.7) has been studied. Substitution of Al3+ for Fe3+ was found to lower the Curie temperature of the samples and to change the lattice parameter. The magnitude of peak to peak magnetostriction, strain sensitivity, magnetomechanical hysteresis, and relative contributions of λ111 and λ100 to the resultant magnetostriction were found to vary with temperature. The results show the possibility of tailoring the magnetoelastic properties of highly magnetostrictive cobalt ferrite at temperatures of interest by cation substitution.

for Stress Sensor and Actuator Applications

Dynamic domain observations have been made using a high-magnification Kerr microscope in combination with a high-speed light-intensified CMOS camera. Results have demonstrated nonrepeatable domain wall motion in electrical steel and amorphous ribbon at 1- and 50-Hz magnetizing frequencies.

Temperature Dependence of Magnetic Properties of CoAl

Abstract Argon arc melting has been used to produce partially amorphous alloys based on the Fe 72 Ga 2 Al 5 P 11 B 4 C 6 composition. Samples were produced in various forms including cylindrical shapes. They have been found to exhibit semi-hard magnetic properties. These properties are discussed in terms of the observed microstructures and the fabrication parameters.

_{\rm x}

A rotating cryostat vacuum system has been used to deposit Si3Fe97 magnetic films. Observations indicate that inplane magnetic anisotropyand coercivityis dependent on the ty pe of substrate and the deposition method used. The findings are discussed with concern to scaling up the technique for the production of strip shaped components. r 2002 Elsevier Science B.V. All rights reserved.