The temperature variation of magnetic anisotropy and coercive field of magnetoelastic manganese-substituted cobaltferrites (CoMnxFe2−xO4 with 0⩽x⩽0.6) was investigated. Major magnetic hysteresis loops were measured for each sample at temperatures over the range 10-400 K, using a superconducting quantum interference device magnetometer. The high-field regimes of the hysteresis loops were modeled using the law of approach to saturation equation, based on the assumption that at sufficiently high field only rotational processes remain, with an additional forced magnetization term that was linear with applied field. The cubic anisotropy constant K1 was calculated from the fitting of the data to the theoretical equation. It was found that anisotropy increases substantially with decreasing temperature from 400 to 150 K, and decreases with increasing Mn content. Below 150 K, it appears that even under a maximum applied field of 5 T, the anisotropy of CoFe2O4 and CoMn0.2Fe1.8O4 is so high as to prevent complete approach to saturation, thereby making the use of the law of approach questionable in these cases. The temperature variation of magnetic anisotropy and coercive field of magnetoelastic manganese-substituted cobalt ferrites ͑CoMn x Fe 2−x O 4 with 0 ഛ x ഛ 0.6͒ was investigated. Major magnetic hysteresis loops were measured for each sample at temperatures over the range 10-400 K, using a superconducting quantum interference device magnetometer. The high-field regimes of the hysteresis loops were modeled using the law of approach to saturation equation, based on the assumption that at sufficiently high field only rotational processes remain, with an additional forced magnetization term that was linear with applied field. The cubic anisotropy constant K 1 was calculated from the fitting of the data to the theoretical equation. It was found that anisotropy increases substantially with decreasing temperature from 400 to 150 K, and decreases with increasing Mn content. Below 150 K, it appears that even under a maximum applied field of 5 T, the anisotropy of CoFe 2 O 4 and CoMn 0.2 Fe 1.8 O 4 is so high as to prevent complete approach to saturation, thereby making the use of the law of approach questionable in these cases.

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This paper reports on a design for a coil for transcranial magnetic stimulation. The design shows potential for improving the penetration depth of the magnetic field, allowing stimulation of subcortical structures within the brain. The magnetic and induced electric fields in the human head have been calculated with finite element electromagnetic modeling software and compared with empirical measurements. Results show that the coil design used gives improved penetration depth, but also indicates the likelihood of stimulation of additional tissue resulting from the spatial distribution of the magnetic field. This paper reports on a design for a coil for transcranial magnetic stimulation. The design shows potential for improving the penetration depth of the magnetic field, allowing stimulation of subcortical structures within the brain. The magnetic and induced electric fields in the human head have been calculated with finite element electromagnetic modeling software and compared with empirical measurements. Results show that the coil design used gives improved penetration depth, but also indicates the likelihood of stimulation of additional tissue resulting from the spatial distribution of the magnetic field.

We have investigated the magnetic and magnetoelasticproperties of a series of Cr-substituted cobaltferriteCoCrxFe2−xO4 (x=0.0-0.79) samples. Substitution of Cr for some of the Fe in cobaltferrite reduced the Curie temperature, and the effect is more pronounced than that observed in Mn-substituted cobaltferrite samples. Cr substitution also caused the maximum magnetostriction to decrease at a greater rate than substitution of the same amount of Mn. The maximum of the strain derivative, dλ/dH, was reached for x=0.38. The behavior of the Curie temperature of Cr-substituted and Mn-substituted cobaltferrites has been analyzed using the Néel molecular field model and compared with recent Mössbauer spectroscopy results. We have investigated the magnetic and magnetoelastic properties of a series of Cr-substituted cobalt ferrite CoCr x Fe 2−x O 4 ͑x = 0.0-0.79͒ samples. Substitution of Cr for some of the Fe in cobalt ferrite reduced the Curie temperature, and the effect is more pronounced than that observed in Mn-substituted cobalt ferrite samples. Cr substitution also caused the maximum magnetostriction to decrease at a greater rate than substitution of the same amount of Mn. The maximum of the strain derivative, d / dH, was reached for x = 0.38. The behavior of the Curie temperature of Cr-substituted and Mn-substituted cobalt ferrites has been analyzed using the Néel molecular field model and compared with recent Mössbauer spectroscopy results.

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Substitution of other metals for Fe in cobalt ferrite has been proposed as a method to tailor the magnetic and magnetoelastic properties for sensor and actuator applications ͓H. Zheng et al., Science 303, 661 ͑2004͔͒. However, to understand the effect of Cr substitution, one needs atomic-level information on the local environments and interactions of the transition-metal ions. In this study, Mossbauer spectroscopy was used to investigate the local environments of the Fe atoms in these materials. A series of five powder samples with compositions CoCr x Fe 2−x O 4 ͑x = 0.0 to 0.8͒ was investigated using transmission geometry. Results show two distinct six-line hyperfine patterns, indicating Fe in A and B spinel sites. Increasing Cr concentration is seen to decrease the hyperfine field strength for both A and B sites, as well as increasing the width of those distributions. Results for Cr substitution show generally similar behavior to a prior study using Mn; however, Cr substitution has more pronounced effects: the hyperfine fields decrease and distribution widths increase at greater rates for Cr substitution, and the differences between A and B site behavior are more pronounced. Results are consistent with a model in which Cr has an even stronger B-site preference than Mn, and displaces more of the Co from the B to the A sites.