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Peering Deep into Spintronic Materials:  Dilute Magnetic Semiconductors

Schematic of the HARPES technique, in which bulk valence electronic states are probed with high-energy x-rays of several keV to generate angle-resolved photoemission spectra (lower right).
A team of researchers headed by Chuck Fadley has investigated the bulk electronic structure of the prototypical dilute magnetic semiconductor gallium manganese arsenide using a new technique called HARPES, for Hard x-ray Angle-Resolved PhotoEmission Spectroscopy. Their findings help resolve a long-standing question about the material’s ferromagnetism, which they find arises from both of the two different mechanisms that have been proposed to explain it.

When the common semiconductor gallium arsenide is doped with manganese, the result is a ferromagnetic material known as a dilute magnetic semiconductor. This class of materials would be well-suited for further development into spintronic devices if the mechanisms behind their ferromagnetism were better understood.

  The two prevailing theories behind the origin of ferromagnetism in GaMnAs and other dilute magnetic semiconductors claim that ferromagnetism is mediated either by electrons in the GaAs valence band (p-d exchange) or by electrons in a separate manganese impurity band (double exchange). Fadley and coworkers used HARPES to peer deep into the material, beyond the surface layers that most other techniques, e.g. conventional low-energy ARPES, are limited to. There the team found evidence of both mechanisms, painting a new picture of ferromagetism in this class of materials. The team’s results also suggest that the HARPES technique should be broadly applicable to many new classes of materials in the future.  

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A. X. Gray, J. Minár, S. Ueda, P. R. Stone, Y. Yamashita, J. Fujii, J. Braun, L. Plucinski, C. M. Schneider, G. Panaccione, H. Ebert, O. D. Dubon, K. Kobayashi & C. S. Fadley, “Bulk electronic structure of the dilute magnetic semiconductor Ga1−xMnxAs through hard X-ray angle-resolved photoemission,” Nature Materials 11, 957-962 (2012). doi:10.1038/nmat3450