Spin Waves in GaMnAs/GaAs Superlattices

We report on the spin-wave behavior of GaMnAs/GaAs superlattices as a function of the thickness of the nonmagnetic layer. Spin-wave frequencies were determined from time-domain magnetic Kerr measurements using a standard pump-probe setup. We studied two superlattices grown by molecular beam epitaxy on GaAs (001) substrates and, for comparison purposes, a single 120-nm-thick Ga$_{0.955}$Mn$_{0.045}$As layer. The superlattices consist of 6 repetitions of 20-nm-thick Ga$_{0.955}$Mn$_{0.045}$As layers separated by either $d~$=~3 or 6~nm of GaAs. For magnetic fields $H \quad <$~0.22 T, applied along the magnetic easy axis [100], the thin film and the superlattices exhibit a nearly $d$-independent mode of frequency \textit{$\nu $ }$\cong $ 5 + 33\textit{${\rm H}$} (GHz), whereas the sample with $d~$=~6~nm shows a second oscillation at \textit{$\nu $ }$\cong $ 7 + 33\textit{${\rm H}$} (GHz), which we assign to a higher-order confined spin-wave. The existence of this mode is tentatively ascribed to a reduction in the coupling between neighboring magnetic layers.