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Nearly non-magnetic valence band of the ferromagnetic semiconductor GaMnAs

Abstract

The origin of ferromagnetism in the prototype ferromagnetic semiconductor GaMnAs is controversial because of an insufficient understanding of its band structure and Fermi level position. This is a major issue for further development of this material for future semiconductor spintronics. Here, using a unique method combining a precise etching technique and resonant tunnelling spectroscopy, applied to a variety of surface GaMnAs layers, we can elucidate the universal valence-band (VB) picture of GaMnAs. We find that the VB structure of GaAs is almost perfectly maintained and does not merge with the impurity band for any of the GaMnAs samples, with Mn concentrations ranging from 6 to 15%, that we examined. Furthermore, the exchange splitting of the VB is found to be very small (only several millielectronvolts), even in GaMnAs with a high Curie temperature (154 K). Our findings shed light on the precise mechanism behind ferromagnetism in GaMnAs; a subject that has been debated for more than a decade.

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Figure 1: VB conduction model of GaMnAs versus VB picture of GaMnAs obtained in our study.
Figure 2: General device structure of the GaMnAs heterostructure and the principle of our method.
Figure 3: d2I/dV2V curves of the fabricated GaMnAs-based devices for various GaMnAs thicknesses d.
Figure 4: VB structures assumed in our calculation.
Figure 5: Calculated resonant peaks obtained by the transfer matrix method with the 6×6 k·p theory for the fabricated devices and the colour-coded d2I/dV2 intensities obtained in the experiments as functions of −V and d.
Figure 6: d2I/dV2V curves of the fabricated GaMnAs-based devices with the various GaMnAs thicknesses d in the small bias (−V <0.1 V) region.

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Acknowledgements

This work was partly supported by Grants-in-Aid for Scientific Research, the Special Coordination Program for Promoting Science and Technology, FIRST Program of the JSPS, PRESTO of the JST and Asahi Glass Foundation.

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Contributions

Device fabrication: S.O., K.T.; experiments; K.T., S.O.; data analysis and theory: S.O., K.T.; writing and project planning: S.O. and M.T.

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Correspondence to Shinobu Ohya or Masaaki Tanaka.

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The authors declare no competing financial interests.

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Ohya, S., Takata, K. & Tanaka, M. Nearly non-magnetic valence band of the ferromagnetic semiconductor GaMnAs. Nature Phys 7, 342–347 (2011). https://doi.org/10.1038/nphys1905

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