Magnetic materials based on Mn-doping of GaAs are favorable systems for exploring spin phenomena in the III-V family of semiconductors. The Mn, incorporated on the Ga sublattice, introduces both unpaired spins, and charge carriers (holes) which mediate a ferromagnetic interaction. The ferromagnetic Curie temperature TC increases with the substitutional Mn concentration and is above 180K for carefully prepared samples. The concentration of holes also plays a key role in determining the magnetic properties, leading to novel control of ferromagnetism using light or electric fields. However, the character of the states near the Fermi level, in particular whether the Mn-induced impurity states are detached or merged with the valence band, is much debated.
X-ray magnetic circular dichroism (XMCD) measurements have proved particularly useful for understanding the detailed magnetic order in Mn-doped GaAs, as they allow quantitative and separate determination of the properties of unfilled states with Mn 3d, As 4p and Ga 4p character. As K edge XMCD measurements reveal a significant orbital polarization of the As 4p valence states, of opposite sign to the net magnetization, which increases linearly with TC. The orbital moment of the Ga 4p states is at least an order of magnitude smaller. In Mn L3 edge XMCD, a pre-edge peak is identified as a signature of a partial Mn 3d character of the Fermi level states, from its unique symmetry and doping dependence.
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