SPIN-WAVES AT LOW-TEMPERATURES IN 2-SUBLATTICE HEISENBERG FERROMAGNETS AND FERRIMAGNETS WITH DIFFERENT SUBLATTICE ANISOTROPIES

The Hamiltonian for two-sublattice Heisenberg ferromagnets and ferrima gnets with different sublattice anisotropies, which is applicable for rare-earth-transition-metal (R-T) intermetallics, is established. In o rder to study spin-waves in easy plane or easy cone configuration, a t ransformation of spin-vector coordinates is performed by rotating the quantization axis frame by Eulerian angles and accordingly the Hamilto nian. Spin-wave spectra at low temperatures of the present system are determined by performing the standard Holstein-Primakoff transformatio n and a four-step diagonalizing procedure consisting of two coupled Cu llen transformations, an extended Bogoliubov transformation, two indep endent Bogoliubov transformations and two independent Holstein-Primako ff transformations. The results for the ground states of the easy axis , the easy plane and the easy cone configurations are compared with th ose obtained by the mean-field theory. The border lines between the di fferent spin structures are derived in either the purr classical limit or the large-exchange limit. Continuous transitions, accompanied with the continuous change of the angle between the averaged sublattice ma gnetizations, are found in both cases. It is found that splittings of the spin-wave spectra of the two-sublattice Heisenberg ferromagnets or ferrimagnets exist. A gap can appear in the spin-wave spectra, depend ing on the competition among the exchange and the anisotropies. Other physical properties, such as sublattice magnetization and specific hea t, are discussed also.