Direct observation of the magnetic ordering process in the ferromagnetic semiconductor Ga_1−xMn_xAs via soft x-ray magnetic circular dichroism

In order to understand the mechanism of the ferromagnetism in the ferromagnetic semiconductor Ga$1−x$Mn$x$As [(Ga,Mn)As], we have investigated the magnetic behavior on a microscopic level through systematic temperature (⁠$T$⁠)- and magnetic field (⁠$H$⁠)-dependent soft x-ray magnetic circular dichroism (XMCD) experiments at the Mn $L2,3$ absorption edges. The $T$ and $H$ dependences of XMCD intensities have been analyzed using a model consisting of the ferromagnetic (FM), paramagnetic, and superparamagnetic (SPM) components. Intriguingly, we have found a common behavior for the ferromagnetic ordering process in (Ga,Mn)As samples with different Mn concentrations (4% and 10.8%) and different Curie temperature (⁠$TC$⁠) values (65, 120, and 164 K). In particular, the SPM component develops well above $TC$⁠, indicating that local FM regions are formed well above $TC$⁠. The present findings indicate that the onset of ferromagnetic ordering is triggered by local electronic states around the substitutional Mn ions rather than uniform electronic states considered by mean-field theories. Insight into the most representative ferromagnetic semiconductor, (Ga,Mn)As, provided by the present study will be an important step in understanding the mechanism of ferromagnetic ordering in various ferromagnetic semiconductor families.