Coercivity of anisotropic $\gamma$-Fe$_2$O$_3$ particles at high temperatures

The coercivity of five different samples of anisotropic $\gamma$-Fe$_2$O$_3$ particles is studied in the temperature range 0$^\circ$–600$^\circ$C. It is found that their relative coercive force $h_c=H_c(T)/H_c(0)$ is almost a linear function of the relative magnetization of the particles $m_s=M_s(T)/M_s(0)$, where $H_c (0)$ and $M_s (0)$ are the values of $H_c$ and $M_s$ of the particles at 0$^\circ$C. It is experimentally found that $h_c=\beta m_s+\alpha$, where $\beta$ = 1.103 $\pm$ 0.015 and $\alpha$ = -0.114 $\pm$ 0.009. This character of the dependence of $h_c$ on $m_s$ suggests that, at high temperatures, $H_c$ of anisotropic $\gamma$-Fe$_2$O$_3$ particles can depend on both their shape anisotropy and other factors. It is assumed that, as the temperature increases, anisotropic $\gamma$-Fe$_2$O$_3$ particles in a zero magnetic field are divided into small structurally and magnetically unstable nanoclusters with magnetization spontaneously changing its direction. As a result, $H_c$ disappears near the Curie temperature, although the saturation magnetization of the particles in a field of 1 T is still retained at this temperature.