This paper reports on the behavior of single crystal Tb5Si2.2Ge1.8 in the vicinity of its order-disorder and order-order phase transition from a higher temperature paramagnetic∕monoclinic state to a lower temperature ferromagnetic∕orthorhombic state. Measurements have been made of thermal and field induced changes in strain along the crystallographic a axis. The material exhibits large strains of up to 1500ppm when a magnetic field is applied to it in its paramagnetic state but much smaller strains when a field is applied to it in its ferromagnetic state. These field induced strains are different from conventional magnetostriction because they result mostly from the change in crystal structure. As a result of this the field induced strain changes that accompany the phase transitions of this material are not fully reversible. The shape and slope of the strain versus magnetic field curves were distinctly different depending on whether the material started from above the Curie temperature (where the application of a magnetic field of sufficient strength induced a structural phase transformation) or started from below the Curie temperature (where the application of a field merely stabilized the existing magnetic order).

The Tb5(SixGe4−x) alloy system is similar to the better known Gd5(SixGe4−x), except it has a more complex magnetic and structural phase diagram. Gd5(SixGe1−x)4 has received much attention recently due to its giant magnetocaloric effect, colossal magnetostriction and giant magnetoresistance in the vicinity of a first order combined magnetic-structural phase transition. The magnetostriction changes that accompany the phase transitions of single crystal Tb5(Si2.2Ge1.8) have been investigated at temperatures between 20 and 150K by measurements of magnetostriction along the a axis. Over this temperature range the shape and slope of the magnetostriction curves change, indicative of changes in the magnetic state, crystal structure, and magnetic anisotropy. The results appear to indicate a phase transition that occurs near 106K (onset-completion range of 116–100K). The steepness of the strain transition, its unusual hysteresis, and its temperature dependence appear to indicate a first order phase transition which is activated by applied magnetic field in addition to temperature (see Fig. 1). Magnetostriction measurements at temperature below the transition region appear to indicate a magnetostriction of small overall magnitude (about 30×10−6) but high anisotropy, with anistropy showing considerable temperature dependence.

The temperature variation of magnetic anisotropy and coercive field of magnetoelastic manganese-substituted cobalt ferrites (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.

Metal bonded cobalt ferrite composites have been shown to be promising candidate materials for use in magnetoelastic stress sensors, due to their large magnetostriction and high sensitivity of magnetization to stress. However previous results have shown that below 60°C the cobalt ferrite material exhibits substantial magnetomechanical hysteresis. In the current study, measurements indicate that substituting Mn for some of the Fe in the cobalt ferrite can lower the Curie temperature of the material while maintaining a suitable magnetostriction for stress sensing applications. These results demonstrate the possibility of optimizing the magnetomechanical hysteresis of cobalt ferrite-based composites for stress sensor applications, through control of the Curie temperature.

The magnetic anisotropy and magnetic phase transition of a single crystal Tb/sub 5/(Si/sub 2.2/Ge/sub 1.8/) is investigated by of M-H and M-T measurements along the a, b and c axes. The transition from paramagnetic to ferromagnetic is observed at T = 110 K . Below this transition temperature, M-H curves show very strong anisotropy which is believed to be due to complex spin configuration. M-H measurements at T = 110 K show that the a axis is the easy axis, and that the saturation magnetization is 200 emu/g. The b axis is the hard axis, which needs an external magnetic field much higher than 2 T to saturate the magnetization in that direction, indicating a high magnetocrystalline anisotropy. The c axis is of intermediate hardness.

The magnetic anisotropy and magnetic phase transition of a single crystal Tb5(Si2.2Ge1.8) is investigated by of M-H and M-T measurements along the a, b and c axes. The transition from paramagnetic to ferromagnetic is observed at T = 110 K . Below this transition temperature, M-H curves show very strong anisotropy which is believed to be due to complex spin configuration. M-H measurements at T = 110 K show that the a axis is the easy axis, and that the saturation magnetization is 200 emu/g. The b axis is the hard axis, which needs an external magnetic field much higher than 2 T to saturate the magnetization in that direction, indicating a high magnetocrystalline anisotropy. The c axis is of intermediate hardness.

Understanding the effect of Mn substitution for Fe in Co ferrite presents a challenge because there are three different transition-metal ions distributed among two distinct crystallographic and magnetic sublattices with complicated superexchange and anisotropic interactions. In this study, a series of six powder samples with compositions Co1.0MnxFe2−xO4 were investigated using transmission Mössbauer spectroscopy. Mössbauer spectroscopy provides an excellent tool for probing the local environment of the Fe atoms present in such materials. Results show two sets of six-line hyperfine patterns for all samples, indicating the presence of Fe in both A and B sites. Identification of sites is accomplished by evidence from hyperfine distribution width, integrated intensity, and isomer-shift data. Increasing Mn concentration was found to decrease the hyperfine field strength at both sites, but at unequal rates, and to increase the distribution width. This effect is due to the relative strengths of Fe–O–X superexchange (X=Fe, Co, or Mn) and the different numbers of the next-nearest neighbors of A and B sites. Results are consistent with a model of Mn substituting into B sites and displacing Co ions onto A sites.

Improvements of both magnetostriction level and strain derivative of pure cobalt ferrite by magnetic annealing are reported. CoFe/sub 2/O/sub 4/ samples are annealed at 300/spl deg/C in air for 36 hours in a vibrating sample magnetometer with a furnace. VSM measurements indicate a uniaxial anisotropy induced by magnetic anneal, with the easy axis being along the magnetic annealing field direction. Coercivities are measured to be 2.6 kA/m and 6.9 kA/m along the easy and hard axes, respectively. After magnetic annealing, the magnitude of the maximum magnetostriction is increased. This is attributed to the induced uniaxial anisotropy, by which domain magnetization tends to align along the easy axis in the absence of any applied field. When a magnetic field is applied along the hard axis, re-orientation of domain magnetization from the induced easy directions towards the field direction involves more rotation and non-180/spl deg/ domain wall processes.