Magnetic-Field-Induced Phase Transitions in Uniaxial Ferromagnets

Abstract

Phase transitions in uniaxial ferromagnets with both single-ion anisotropy and anisotropic exchange interactions in the presence of an external magnetic field along the axis are discussed using the molecular-field approximation. Landau's theory of phase transition is employed to obtain the phase boundaries and to discuss the nature of the phase transitions. Phase diagrams have been obtained for ferromagnets with a hard axis both at $T=0\mathrm{}$ and at finite temperatures for all cases as the single-ion anisotropy varies relative to the exchange interactions. It is shown that for systems in an external magnetic field with large single-ion anisotropy a phase with spins canted away from the axis prevails between two critical values which are temperature dependent. At $T=0\mathrm{}$, the phase transition from the quenched-moment state to the canted state is second order; the phase transition from the canted state to the fully aligned state can be either first order or second order depending upon the relative magnitudes of the anisotropic exchange interactions. For systems with smaller values of single-ion anisotropy so that there is always an ordered phase at low temperatures, an external magnetic field can induce the transition from the canted phase to the paramagnetic phase. The phase transition again can be either first order or second order.