Towards the theory of ferrimagnetism

Abstract

A two-sublattice ferrimagnet, withspin-s1 operatorsS1i at the sublatticeA site and spin-s2 operators S2i at the sublattice B site, is considered. The magnon of the system, the transversal fluctuation ofthe total magnetization, is a complicate mixture of the transversal fluctuations of thesublattice A and B spins. As a result, the magnons’ fluctuations suppress in a different way themagnetic 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 hightemperature (T*,TN), the magnetic order of the spins with a weaker intra-sublattice exchangeis suppressed by magnon fluctuations, and only the spins with strongerintra-sublattice exchange have non-zero spontaneous magnetization. TheT* transition is a transition between two spin-ordered phases in contrastto the transition from the spin-ordered state to the disordered state(TN-transition). There is no additional symmetry breaking, and the Goldstoneboson has a ferromagnetic dispersion in both phases. A modified spin-wavetheory is developed to describe the two phases. All known Neel’s anomalousM(T) curves are reproduced, in particular that with ‘compensation point’. Thetheoretical curves are compared with experimental ones for sulfo-spinelMnCr2S4−xSex and rare earth iron garnets.