Phase diagram of a strongly anisotropic biaxial ferromagnet and the spectra of coupled magnetoelastic waves

Abstract

The spectral and thermodynamic properties of a biaxial ferromagnetic crystal are investigated at different temperatures. It is shown that, depending on the direction of the external magnetic field, in the case of a large single-ion anisotropy (much greater than the exchange interaction) there can be both a reorientation phase transition and a transition involving a decrease in the modulus of the magnetization vector. In the first case the elastic and magnetic subsystems actively interact, which leads to the softening of a transverse polarized phonon mode at the point of the orientational phase transition; in the second case the magnon and phonon branches do not interact, and the phase transition takes place through a magnon mode. The phase diagram of a highly anisotropic biaxial ferromagnet is constructed for an arbitrary orientation of the external magnetic field and for arbitrary temperatures.