Magnetic and magnetoelastic properties of Ge/Co co-substituted cobalt ferrite*

The highly magnetostrictive material cobalt ferrite and its derivatives based on substitution of cations have been shown to have extreme sensitivity of their magnetization to stress. In order to control their properties (e.g. magnetostriction, magnetic anisotropy, strain derivative, and hysteresis) substitution of specific cations are needed. We have shown Ge$^{+4}$/Co$^{+2}$ co-substituted cobalt ferrites (Co$_{1+x}$Ge$_{x}$Fe$_{2-2x}$O$_{4})_{ }$to have very interesting combinations of magneto-elastic properties. In the present study the variation of magnetic anisotropy with composition and temperature was investigated, for the new germanium/cobalt co-substituted cobalt ferrite Co$_{1+x}$Ge$_{x}$Fe$_{2-2x}$O$_{4}$.. Hysteresis loops were measured in the range T = 10 - 400 K, with \textit{$\mu $}$_{0}H_{max}$~=~0 - 5~T. The high field regions of these loops were then fitted to the Law of Approach to Saturation (LA) for cubic materials as given by $M=M_{s}$[1-(8/105)($K_{1}$/\textit{$\mu $}$_{0}M_{s}H)^{2}$], plus a linear forced magnetisation term. Values for first order cubic anisotropy constant $K_{1}$ were calculated and it was found that anisotropy increased as temperature decreases for all compositions. At most temperatures anisotropy decreased with increase in $x$.