Spin-wave resonance in semi-infinite anisotropic ferromagnets

Abstract

We apply the Green's function formalism in the Random Phase Approximation to calculate the power absorbed by a semi-infinite Heisenberg ferromagnet, upon the excitation of surface spin waves by an external radiation field (spin wave resonance) at finite temperatures. The system consists of magnetic layers in the (1 1 1) direction of an f.c.c. lattice, with nearest neighbour isotropic exchange interactions in the bulk and between the surface and the bulk. We suppose that in the surface plane there is a uniaxial anisotropy in the exchange interaction. A uniform magnetic field is applied in the easy magnetic axis, which is parallel to the surface plane. The magnetic component of the radiation field is polarized perpendicularly to the static field and to the magnetization. We determined the self-consistent magnetization profile and the renormalized magnon energy spectrum at finite T. We assume that at the third plane the magnetization attains its bulk value. We show that for reasonable values of the exchange anisotropy in the surface, absorption obtained at elevated temperatures by excitation of “optical” spin-wave modes, with energies (for k ‖ = 0 ) above the bulk continuum spectrum. From the temperature dependence of the power absorption we can obtain estimates of the exchange anisotropy at the surface.