Anisotropy effects on the magnetic excitations of a ferromagnetic monolayer below and above the Curie temperature

M. G. Pini; P. Politi; R. L. Stamps

doi:10.1103/PhysRevB.72.014454

paper

Anisotropy effects on the magnetic excitations of a ferromagnetic monolayer below and above the Curie temperature

arXiv:cond-mat/0503538 · doi:10.1103/PhysRevB.72.014454

Abstract

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 a 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. Above the Curie temperature $T_{C}$ , the magnon energy gap $E_{0} (H)$ vanishes for H=0 both in the easy and in the hard case. 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 $T_{C}$ ( $T \approx 1.2 T_{C}$ ).

Final version accepted for publication in Physical Review B (with three figures)

Anisotropy effects on the magnetic excitations of a ferromagnetic monolayer below and above the Curie temperature

Abstract

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