Exchange-dominated surface spin waves in thin yttrium-iron-garent films

The study of exchange-dominated nonpropagating surface spin-wave modes observed in thin single-crystal yttrium-iron-garnet films is reported. The angle and temperature dependences of these modes and creation of the surface pinning condition necessary for their existence by the controlled annealing of the films are discussed. The data are consistent with a modified Puszkarski surface-inhomogeneity model in which the surface-anisotropy field has a tensorial form. At the substrate surface the in-plane component of this field, $K_{\parallel }^s$, is large and negative and the perpendicular component, $K_{\perp }^s$, is large and positive. The degree of localization varies with the annealing history of the sample, and etching experiments confirm a decay length as small as 500 Å when the magnetization lies in the plane of the film. As the magnetization is rotated out of the film plane, a "critical angle" at which the uniform mode is excited is observed, and at the perpendicular orientation the surface mode has been transformed to the first body mode of the film. With decreasing temperature, $K_{\parallel }^s$ is observed to decrease in magnitude and pass through zero between 100 and 200 K. At 4 K, $K_{\parallel }^s$ is large and positive. These data suggest that a psuedodipolar exchange interaction may be occurring between the Fe and Gd atoms at the substrate interface. By annealing at 1200 °C, a similar surface effect is produced at the free surface of the film having $K_{\parallel }^f<0\mathrm{}$ and $K_{\perp }^f>0\mathrm{}$. However, the temperature dependence of the free-surface anisotropy field indicates that the origin of the interaction has a very different microscopic basis.