Theory of Magnetization Temperature Dependence in Ferrimagnetics

Recent advancements in spintronics and fundamental physical research have brought increased attention to the rare-earth-based magnetically ordered materials. One of the important properties of these materials is the temperature dependence of the spontaneous magnetization $M(T)$. Recently, a successful framework was proposed for the theoretical description of M(T) across the entire temperature range from zero to the Curie temperature in simple cubic ferromagnetics, EuO and EuS. We extend this approach to compute and analyze $M(T)$ for multi-sublattice collinear ferrimagnetics such as Yttrium Iron Garnet Y₃Fe₅ O₁₂. We analyzed and generalized for multi-sublattice collinear ferrimagnetics two well-known approximations describing $M(T)$. The first approach is the Bloch-3/2 law, which describes the suppression of $M(T)$ due to spin-wave excitation, and is valid in the low-temperature limit T << Tc. The second one is Weiss's mean-field approximation, which provides a reasonable description of $M(T)$ near Tc. Using a single tuning parameter, we combine these two approaches to describe $M(T)$ for any 0<T<Tc. The theoretical result for $M(T)$ aligns well with our measurements and the previously available experimental data across the entire temperature range. We also demonstrate that experimental and theoretical dependences $M(T)$ follow the mean-field prediction √Tc - T for almost all temperatures.