The field-driven reorientation transition of an anisotropic ferromagnetic monolayer is studied within the context of a finite-temperature Green’s-function theory. The equilibrium state and the field dependence of the magnon energy gap $E_0$ are calculated for static magnetic field $H$ applied in plane along an easy or hard axis. In the latter case, the in-plane reorientation of the magnetization is shown to be continuous at $T=0$, in agreement with free-spin-wave theory, and discontinuous at finite temperature $T>0$, in contrast with the prediction of mean-field theory. The discontinuity in the orientation angle creates a jump in the magnon energy gap, and it is the reason why, for $T>0$, the energy does not go to zero at the reorientation field. approach. Above the Curie temperature $T_C$, the magnon energy gap $E_0\left(H\right)$ vanishes for $H=0$ in both the easy and hard cases. As $H$ is increased, the gap is found to increase almost linearly with $H$, but with different slopes depending on the field orientation. In particular, the slope is smaller when $H$ is along the hard axis. Such a magnetic anisotropy of the spin-wave energies is shown to persist well above $T_C(T\approx 1.2T_C)$.

• Received 21 March 2005

DOI:https://doi.org/10.1103/PhysRevB.72.014454