Towards the theory of ferrimagnetism II

The present paper is a sequel to the paper by Karchev (2008 J.Phys.:Condens.Matter {\bf 20} 325219). A two-sublattice ferrimagnet, with spin-$s_1$ operators $\bf{S_{1i}}$ at the sublattice $A$ site and spin-$s_2$ operators $\bf{S_{2i}}$ at the sublattice $B$ site, is considered. Renormalized spin-wave theory, which accounts for the magnon-magnon interaction, and its extension are developed to describe the two ferrimagnetic phases $(0,T^*)$ and $(T^*,T_N)$ in the system, and to calculate the magnetization as a function of temperature. The influence of the parameters in the theory on the characteristic temperatures $T_N$ and $T^*$ is studied. It is shown that, increasing the inter-sublattice exchange interaction, the ratio $T_N/T^*>1$ decreases approaching one, and above some critical value of the exchange constant there is only one phase $T_N = T^*$, and the magnetization-temperature curve has the typical Curie-Weiss profile. When the intra-exchange constant of sublattice with stronger intra-exchange interaction increases the $Ne\grave{e}l$ temperature increases while $T^*$ remains unchanged. Finally, when the magnetic order of the sublattice with smaller magnetic order decreases, $T^*$ decreases. The theoretical predictions are utilize to interpret the experimentally measured magnetization-temperature curves.