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Ferromagnetic semiconductors have been a very active research field in the last couple of years. They promise to combine the versatility of magnetic effects with the ability to electrically tune the properties of semiconductors. The ferromagnetic semiconductor best studied to date is GaMnAs, typically containing a few percent of Manganese. The Manganese atoms act as acceptors, and the holes generated mediate the ferromagnetic coupling between the half-filled 3d shells of the Mn.

GaMnAs has the highest Curie temperature of any ferromagnetic semiconductor reported so far. This temperature, below which a material is ferromagnetic, is 173 K, corresponding to –100°C. Therefore, one of the major research areas is the search for ferromagnetic semiconductors with a higher Curie temperature. We have studied two of the main candidates in more detail: In GaMnN, we pointed out that the Mn acceptor level becomes deep prohibiting effective doping and carrier-mediated ferromagnetism. In GeMn, we observed the formation of a spin-glass phase. While indicative of magnetic ordering, the slow relaxation typical of glasses does not encourage the use of this material for applications. In collaboration with colleagues in Berkeley, we therefore currently concentrate on the effect of alloying P or Sb to GaMnAs.

The second major research area is the tuning of the magnetic properties of this class of materials. We have pioneered the use of hydrogen to tune the magnetic properties of ferromagnetic semiconductors. Hydrogen is known to passivate acceptors, so that the introduction of hydrogen removes the holes required for the carrier-mediated ferromagnetism. We have demonstrated the effect of hydrogen on GaMnAs and GaMnP so far, where incorporation of hydrogen leads to a complete suppression of ferromagnetism, and currently focus on the understanding of the Mn-H complexes formed using techniques such as electron spin resonance, particle-induced X-ray emission, X-ray standing waves and X-ray absorption fine structure. In addition, we study in detail the possibility to manipulate the magnetic properties of ferromagnetic semiconductors via mechanical stress, and have demonstrated both the reversible continuous reorientation as well as the nonvolatile switching of the magnetic easy axis of GaMnAs thin films.

• The Mn3+/2+ acceptor level in group III nitrides
  Applied Physics Letters 81 5159 (2002)
  T. Graf, M. Gjukic, M. S. Brandt, M. Stutzman, O. Ambacher
  
  
• Spin wave resonance in Ga1-xMnxAs
  Applied Physics Letters 82 730 (2003)
  S.T.B. Goennenwein, T. Graf, T. Wassner, M. S. Brandt, M. Stutzmann, J.B. Philipp, R. Gross, M. Krieger, K. Zurn, P. Ziemann, A. Koeder, S. Frank, W. Schoch, A. Waag
  
  
• Prospects for carrier-mediated ferromagnetism in GaN
  Feature article, physica status solidi B 239 277 (2003)
  T. Graf, S.T.B. Goennenwein, M. S. Brandt
  
  
• Hydrogen control of ferromagnetism in a dilute magnetic semiconductor
  Physical Review Letters 92 227202 (2004)
  S.T.B. Goennenwein, T. A. Wassner, H. Huebl, M. S. Brandt, J.B. Philipp, M. Opel, R. Gross, A. Koeder, W. Schoch, A. Waag
  
  
• Angle-dependent magnetotransport in cubic and tetragonal ferromagnets: Application to (001)- and (113)A-oriented (Ga,Mn)As
  Physical Review B 74 205205 (2006)
  W. Limmer, M. Glunk, J. Daeubler, T. Hummel, W. Schoch, R. Sauer, C. Bihler, H. Huebl, M. S. Brandt, S.T.B. Goennenwein
  
  
• Spin-glass-like behavior of Ge: Mn
  Physical Review B 74 045330 (2006)
  C. Jaeger, C. Bihler, T. Vallaitis, S.T.B. Goennenwein, M. Opel, R. Gross, M. S. Brandt
  
  
• Ga1−xMnxAs/piezoelectric actuator hybrids: A model system for magnetoelastic magnetization manipulation
  Physical Review B 78 045203 (2008)
  C. Bihler, M. Althammer, A. Brandlmaier, S. Geprägs, M. Weiler, M. Opel, W. Schoch, W. Limmer, R. Gross, M. S. Brandt, and S. T. B. Goennenwein
  
  

• Walther-Meissner-Institut
• Universität Ulm
• Universität Regensburg
• Lawrence Berkeley National Laboratory
• European Synchrotron Radiation Facility