Magnetic order-disorder transition in highly dilute ferrite-spinels

The character of the magnetic transition in three samples of dilute ferrites with the spinel structure is studied: Li$_{0.2}$Zn$_{0.6}$Fe$_{2.2}$O$_4$(Li-Zn), Mg$_{1.5}$FeTi$_{0.5}$O$_4$(Mg-Ti), and Ni$_{0.2}$Zn$_{0.8}$Fe$_2$O$_4$(Ni-Zn). According to Mossbauer investigations, not only ordered but also disordered spins are present in the first two samples (mictomagnetism), while all the 3d cations are magnetically active in the Ni-Zn sample. Values of the Curie temperature and spontaneous magnetization are determined from the dependence of $\frac{H}{\sigma}(\sigma ^2)$. It is established that in samples including magnetically inactive ions either the relations $(\sigma_s / \sigma_{s_0}^2 = \xi (1-T/\Theta)$ is not satisfied (the Mg-Ti sample has a linear dependence of $\sigma _s/\sigma$ on $T/\Theta$) or the coefficient in the Li-Zn sample is considerably smaller than in the Ni-Zn sample. It is found that it is only in dilute Mg-Ti ferrite that the displacement of the maximum on the curve of ${\left | \chi _{dif} \right |}_{H=const}(T)$ with increase in field $H$ occurs in the direction of lower temperatures, i.e., as in material with ordering of "spin-glass" type. It is concluded that transition from a paramagnetic to a noncollinear ferromagnetic state may occur in dilute ferrites.