Enhancement of In-Plane Magnetic Anisotropy Through Compensation in Ga$_{1-x}$Mn$_{x}$P:S

Ga$_{1-x}$Mn$_{x}$P is a ferromagnetic semiconductor (FS) in which exchange is mediated by localized holes [Scarpulla \textit{et al.}, Phys. Rev. Lett. \textbf{95,} 207204 (2005)]. As is the case for the prototypical FS Ga$_{1-x}$Mn$_{x}$As, there exists a uniaxial magnetic anisotropy between in-plane $<$110$>$-type directions with the magnetic easy axis lying near the in-plane [1-10] direction [Bihler \textit{et al.}, Phys. Rev. B \textbf{75,} 214419 (2007)]. Here we report the effect of compensation of Mn acceptors by sulfur donors on the in-plane uniaxial magnetic anisotropy in Ga$_{1-x}$Mn$_{x}$P as measured by both ferromagnetic resonance (FMR) and SQUID magnetometry. Raising the S concentration increases the magnitude of the uniaxial magnetic anisotropy between in-plane $<$110$>$-type directions. While the [1-10] direction remains the easy axis in the plane of the film, ``wasp-waisted'' hysteresis loops develop in the [110] direction as the S concentration increases. The wasp-waisted loops are modeled whereby magnetization reversal occurs by a combination of coherent spin rotation and noncoherent spin switching. Finally, by comparing FMR and SQUID data we extract domain wall formation energies as a function of compensation.