Mn 3d electronic configurations in Ga xMnx As ferromagnetic semiconductors and their influence on magnetic ordering

We applied x ray absorption spectroscopy and x ray magnetic circular dichroism XMCD at the Mn 2p 3d resonances to study the Mn 3d electronic configuration and the coupling of Mn 3d magnetic moments in various Ga1 amp; 8722;xMnxAs films. The homogeneity of the Mn depth profile throughout the Ga1 amp; 8722;xMnxAs film was tested by additional structure sensitive x ray resonant reflectivity measurements. In all investigated Ga1 amp; 8722;xMnxAs films the electronic and magnetic configuration of the Mn impurities varies throughout the Mn doped layer. This inhomogeneity is caused by the surface segregation of nonferromagnetic Mn in a d5 configuration. X ray resonant reflectivity data show that the accumulation of nonferromagnetic Mn near the surface is strongly enhanced by low temperature annealing. By XMCD we identified the Mn species responsible for the long range ferromagnetic coupling. It is characterized by an Mn 3d5 3d6 mixed valence acceptor state that is unchanged at all investigated Mn concentrations, ranging from 1 to 6 . Additional nonferromagnetic Mn occurs in the bulk of high concentration samples. We discuss a model in which the latter is due to antiferromagnetic Mn Mn nearest neighbor pair