My research interests concern the physics of ferromagnetic semiconductors (FMS), which unite the long-range magnetic ordering characteristic of ferromagnets with the versatile properties of conventional semiconductors. This class of multifunctional materials therefore is very attractive from a fundamental physics point of view. Moreover, novel spinelectronic devices can be realized using FMS, in which the electronic functionality is directly linked to magnetic properties such as the magnetization orientation.
The ferromagnetic ordering in Mn-doped semiconductors is generally mediated by holes. In the prototypical III-V FMS Ga1-xMnxAs, the transition metal Mn incorporates substitutionally on the Ga site in the oxidation state +3 and acts as a relatively shallow acceptor, leading to the formation of Mn2+(3d5) with a spin S=5/2 and a hole h+ in the valence band (cf. figure). Therefore, ferromagnetic Ga1-xMnxAs usually exhibits a metallic conductivity.
My work especially addresses the characterization and tuning of the magnetic properties of FMS. I have used ferromagnetic resonance (FMR) spectroscopy to study the magnetic properties of GaMnAs thin films on nonstandard substrates such as GaAs(110) and (311), GaAs:Mn thin films with ferromagnetic MnAs clusters, as well as GaMnP thin films. Measurements of the anisotropy of the FMR together with simulations of the free energy surface yield the magnetic anisotropy constants of the thin films. A particular advantage of FMR for the investigation of magnetic anisotropy is the short measurement time required, in particular compared to superconducting quantum interference device (SQUID) magnetometry.
As illustrated in the figure, the magnetic properties can be tuned via the incorporation of hydrogen. Hydrogen passivates the Mn acceptors and thus induces a strong decrease of hole concentration. Since the holes mediate the exchange interaction between the Mn localized magnetic moments hydrogenation leads to the suppression of both conductivity and ferromagnetism, which has been observed in GaMnAs and GaMnP. To investigate the structure of the Mn-H complexes we have used a variety of techniques comprising electron spin resonance (ESR), high-resolution x-ray diffraction (HRXRD), x-ray absorption fine structure analysis (XAFS), particle induces x-ray emission (PIXE), and x-ray standing waves (XSW).
Furthermore, we have shown that the magnetic anisotropy of GaMnAs can be controlled via the application of external strain by a piezoelectric actuator. This piezo-voltage control of magnetization orientation is directly transferable to other ferromagnetic/piezoelectric hybrid structures, paving the way to innovative multifunctional device concepts.
This work has been performed in collaboration with the Walther-Meissner-Institut, Universität Ulm, Universität Regensburg, Universität Gießen, Lawrence Berkeley National Laboratory, and the European Synchrotron Radiation Facility.
Compensation-dependent in-plane magnetization reversal processes in GaMnPS
Phys. Rev. B 78, 214421 (2008)
P. Stone | C. Bihler | M. Kraus | M. A. Scarpulla | J. W. Beeman | K. M. Yu | M. S. Brandt | O. D. Dubon
GaMnAs/piezoelectric actuator hybrids: A model system for magnetoelastic magnetization manipulation
Phys. Rev. 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 | S. Goennenwein
In situ manipulation of magnetic anisotropy in magnetite thin films
PHYSICAL REVIEW B 77 10 104445 (2008)
A. Brandlmaier | S. Geprags | M. Weiler | A. Boger | M. Opel | H. Huebl | C. Bihler | M. S. Brandt | B. Botters | D. Grundler | R. Gross | S. T. B. Goennenwein
Local structure of Mn in hydrogenated GaMnAs
Phys. Rev. B 78, 235208 (2008)
C. Bihler | G. Ciatto | H. Huebl | G. Martinez-Criado | P. J. Klar | K. Volz | W. Stolz | W. Schoch | W. Limmer | F. Filippone | A. Amore Bonapasta | M. S. Brandt
Piezo-voltage control of magnetization orientation in a ferromagnetic semiconductor
phys. stat. sol. (RRL) 2, 96 (2008)
S. Goennenwein | M. Althammer | C. Bihler | A. Brandlmaier | S. Geprägs | M. Opel | W. Schoch | W. Limmer | R. Gross | M. S. Brandt
Electrically detected ferromagnetic resonance
APPLIED PHYSICS LETTERS 90 16 162507 (2007)
S. T. B. Goennenwein | S. W. Schink | A. Brandlmaier | A. Boger | M. Opel | R. Gross | R. S. Keizer | T. M. Klapwijk | A. Gupta | H. Huebl | C. Bihler | M. S. Brandt
Hydrogen passivation of nitrogen in GaNAs and GaNP alloys: How many H atoms are required for each N atom?
APPLIED PHYSICS LETTERS 90 021920 (2007)
I. A. Buyanova | W. M. Chen | M. Izadifard | S. J. Pearton | C. Bihler | M. S. Brandt | Y. G. Hong | C. W. Tu
Magnetocrystalline anisotropy and magnetization reversal in Ga1-xMnxP synthesized by ion implantation and pulsed-laser melting
Phys. Rev. B 75, 214419 (2007)
C. Bihler | M. Kraus | H. Huebl | M. S. Brandt | S. T. B. Goennenwein | M. Opel | M. A. Scarpulla | P. R. Stone | R. Farshchi | O. D. Dubon
Angle-dependent magnetotransport in cubic and tetragonal ferromagnets: Application to (001)- and (113)A-oriented (Ga,Mn)As
PHYSICAL REVIEW B 74 20 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
Magnetic anisotropy in (Ga,Mn)As on GaAS(113)As studied by magnetotransport and ferromagnetic resonance
MICROELECTRONICS JOURNAL 37 12 1490-1492 (2006)
W. Limmer | M. Glunk | J. Daeubler | T. Hummel | W. Schoch | C. Bihler | H. Huebl | M. S. Brandt | S. T. B. Goennenwein | R. Sauer
Unusual effects of hydrogen on electronic and lattice properties of GaNP alloys
PHYSICA B-CONDENSED MATTER 376 568-570 (2006)
I. A. Buyanova | M. Izadifard | T. Seppanen | J. Birch | W. M. Chen | S. J. Pearton | A. Polimeni | M. Capizzi | M. S. Brandt | C. Bihler | Y. G. Hong | C. W. Tu