Temperature Dependence of Magnetic Properties of CoAl$_{\rm x}$Fe$_{2-{\rm x}}$O$_{4}$

The temperature dependence of magnetic properties of a series of aluminium-substituted cobalt ferrite, with the general composition of CoAl_xFe_2-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.

I. Introduction

Highly magnetostrictive cobalt ferrite materials have generated recent interest as candidate materials for torque/stress sensor and actuator applications [1]–[4]. The materials have attractive properties of high magnetostriction , low hysteresis, high chemical stability, mechanical strength and above all high magnetostrictive strain sensitivity . The high magnetostrictive strain sensitivity implies that these materials can produce high magnetostriction at low fields, which points towards a lower magnetizing current requirement in actuator applications [5]. The magnetoelastic properties of these materials are related to their magnetic properties. It has been found that a lower anisotropy leads to higher magnetostrictive stain sensitivity. In the past, several substitutions of Mn³⁺ [3], Cr³⁺ [4], Ga³⁺ [6] and Ge⁴⁺/Co²⁺ [7] for some of the Fe³⁺ have been found to improve their magnetic and consequently magnetoelastic properties.