All optical FMR of ferromagnetic (Ga,Mn)As with various Mn contents

We have investigated all-optical FMR of non-thermal origin [1] on (Ga,Mn)As with various Mn contents $x$ with time-resolve MO signals obtained by a single-wavelength pump and probe (P{\&}P) technique, and found clear dependence of $x$ on the effective magnetic field on ferromagnetically coupled Mn spins. (Ga,Mn)As samples $x$ = 0.02-0.11 were grown by molecular beam epitaxy. Measurements were performed for both as-grown and annealed samples. In-plane magnetization easy axis was [1-10] for the $x$ = 0.11 sample, whereas it was $<$100$>$ for the rest. Samples were magnetized prior to P{\&}P measurements. Polarization of probe pulses was carefully controlled with respect to the direction of $M$. Pump power was varied between 3.4 - 34 $\mu $J/cm$^{2}$. Precession of magnetization was clearly observed at 10-40 K in most of samples. The period of the oscillation becomes longer with increasing $x$ value, indicating a decrease in an effective magnetic field $H_{eff}$ = $\hbar \omega $ / \textit{g$\mu $}$_{B}$ on Mn spins from 0.2 to 0.1 Tesla with increasing $x$. Assuming $H_{eff}=J_{pd}$ with constant $J_{pd}$, the observed trend suggests the reduction in the mean spin density of holes $$ with increasing $x$. [1] Y. Hashimoto and H. Munekata, arXiv: 0707.4055v2; H. Takechi \textit{et al.}, phys. stat. sol. (c) \textbf{3}, 4267 (2006).