Strong influence of magnetic surface anisotropies on the Damon-Eshbach mode frequency of ultrathin Fe(110) layers (abstract)

Abstract

Epitaxially grown Fe(110) layers on W(110) with thicknesses between 8 and 150 Å have been investigated in situ by Brillouin scattering. A strong increase of the frequency of the Damon-Eshbach (surface spin wave) mode by a factor of up to 3 has been observed with decreasing layer thickness below 60 Å. The frequency increase is accompanied by a strong line width broadening. In good agreement with earlier observations,1–3 the change over of the easy axis of the saturation magnetization from [001] to [1-10] could directly be observed at a critical thickness dc=95±5 Å with an applied residual magnetic field of 30 G. The anomalous behavior of the Damon-Eshbach mode is quantitatively described by a model fit, which takes explicitly into account magnetic surface anisotropies. The magnetic in-plane surface anisotropy constant Ks,p=2.4×10−2 erg cm2 has been calculated from the literature value of the bulk anisotropy constant K1, the residual applied magnetic field and the critical thickness dc, using the homogeneous magnetization approximation.2 The fitted value of the magnetic out-of-plane anisotropy constant Ks=2.8±0.7 erg/cm2 is smaller than the value obtained for the case of Fe(110) on GaAs(110).1 While for Fe thicknesses larger than 30 Å the film quality is not very crucial for the spin-wave spectra, 10-Å-thick films show a strong increase of the Damon-Eshbach mode frequency with increasing density of steps on the surface. Since these steps are aligned parallel to the [001] direction as observed by LEED, we attribute the frequency increase to additional uniaxial surface anisotropies induced by the steps. On the other hand, the formation of islands leads to a frequency decrease due to reduced anisotropies.