| Two-sublattice ferrimagnet, with spin-s1 operators S1i at the sublattice A site and spin-s2 operators S2i at the sublattice B site, is considered. The magnon of the system, the transversal fluctuation of the total magnetization, is a complicate mixture of the transversal fluctuations of the sublattice A and B spins. As a result, the magnons' fluctuations suppress in a different way the magnetic orders of the A and B sublattices and one obtains two phases. At low temperature (0,T*) the magnetic orders of the A and B spins contribute to the magnetization of the system, while at the high temperature (T*,TN), the magnetic order of the spins with a weaker intra-sublattice exchange is suppressed by magnon fluctuations, and only the spins with stronger intra-sublattice exchange has non-zero spontaneous magnetization. The T* transition is a transition between two spin-ordered phases in contrast to the transition from spin-ordered state to disordered state (TN-transition). There is no additional symmetry breaking, and the Goldstone boson has a ferromagnetic dispersion in both phases. A modified spin-wave theory is developed to describe the two phases. All known Neel's anomalous M(T) curves are reproduced, in particular that with "compensation point". The theoretical curves are compared with experimental ones for sulpho-spinel MnCr2SSe and rare earth iron garnets. |
|