Magnetostatic mode excitations in EuS

Abstract

The first observation of many well-resolved magnetostatic (MS) modes in a Heisenberg ferromagnet is reported. The experiments were carried out on a thin, nearly square, platelet of EuS (${T}_{c}=16.7$ K) at $X$-band frequencies and at 4.2 K. The experimental values of the resonance fields ${H}_{r}$ of the various MS modes (for static field $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}$ applied in the plane of the disk) are used to test the Damon-Eshbach theory modified to include the demagnetizing fields in a finite disk. Except for the (1,1) and (1,3) modes which appear as surface MS modes, all the other modes ($1, {n}_{z}$) with ${n}_{z}=5, 7, 9, \dots{}, 41$ appear as volume MS modes. The experimental values of ${H}_{r}(1, {n}_{z})$ agree (within \ifmmode\pm\else\textpm\fi{}3%) with the values predicted by the modified Damon-Eshbach theory. However, for ${n}_{z}>31$, there is increasing deviation between theory and experiment leading to an absorption edge whose field position is about 8% lower than the predicted value. This is not fully understood. The linewidth of the modes varies approximately as ($\frac{1}{{n}_{z}}$). For $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}$, at an angle $\ensuremath{\theta}$ out of the plane of the disk, the observed resonance fields of the first several modes varies as ${H}_{r}(\ensuremath{\theta})=\frac{{H}_{r}(0)}{cos\ensuremath{\theta}}$ for $\ensuremath{\theta}<75\ifmmode^\circ\else\textdegree\fi{}$. An explanation for this result is given.