A grate field-dependent change of magnetic damping in Fe/(Ga,Mn)As

Reported here is the field dependence of the magnetic damping in the photo-induced precession of magnetization in three different samples, a simple (Ga,Mn)As, two hybrids Pt/(Ga,Mn)As and Fe/(Ga,Mn)As. The Mn content $x $is $x$ = 0.045 for all cases. In (Ga,Mn)As, the precession frequency \textit{$\omega $} increases and the precession lifetime\textit{ $\tau $} decreases with increasing a lateral, external field applied along the easy axis, whereas the \textit{$\omega \tau $} product is hardly changed. Similar trend is observed in Pt/(Ga,Mn)As, except that the \textit{$\omega \tau $} product is smaller than that of (Ga,Mn)As. An inverse value of the \textit{$\omega \tau $} product, so called the Gilbert damping constant $\alpha $, is $\alpha $ = 0.1 and 0.15, respectively, for (Ga,Mn)As and Pt/(Ga,Mn)As. The enhanced magnetic damping in Pt/(Ga,Mn)As can be understood qualitatively in terms of the spin pumping. In Fe/(Ga,Mn)As, the \textit{$\omega \tau $} product around the zero field is even smaller than that of Pt/(Ga,Mn)As, being indicative of a larger damping ($\alpha $ $\sim $ 0.26), whereas the \textit{$\omega \tau $} product increases steeply with an external field. At around 400 Oe and higher, the \textit{$\omega \tau $} product saturates at the value comparable to that of a simple (Ga,Mn)As. Taking magnetization data into account, a great field-dependent change in damping could be attributed to the spin-wave excitation at the Fe/(Ga,Mn)As interface caused by non-parallel magnetization configuration between Fe and (Ga,Mn)As in microscopic scale.