College Park, Maryland      June 6 - 10 , 2004

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MP43: Annealing-Dependent Phenomena in Ga_1-xMn_xAs

B. J. Kirby (Department of Physics & Astronomy, University of Missouri), J. A. Borchers (NIST Center for Neutron Research), J. J. Rhyne (Los Alamos Neutron Science Center, Los Alamos National Laboratory; Department of Physics & Astronomy, University of Missouri), S. G. E. te Velthuis, A. Hoffmann (Materials Science Division, Argonne National Laboratory), K. V. O'Donovan (NIST Center for Neutron Research; Department of Materials Science and Engineering, University of Maryland), T. Wojowicz (Department of Physics, University of Notre Dame; Institute of Physics of the Polish Academy of Sciences), X. Liu, W. L. Lim, J. K. Furdyna (Department of Physics, University of Notre Dame)

We have used polarized neutron reflectometry (PNR) to study the dilute ferromagnetic semiconductor Ga_1-xMn_xAs. There is great interest in the development of high Curie temperature (T_C) ferromagnetic semiconductors for use in spintronics applications. Ga_1-xMn_xAs is a possible candidate for such applications, with T_C exceeding 150 K in some cases. The ferromagnetic behavior in this material originates from coupling between spin-5/2 Mn²⁺ ions substituting for Ga. These substitutional Mn ions (MnGa) are acceptors, generating holes that mediate the ferromagnetic exchange. However, Mn_Ga are known to be partially compensated by Mn atoms that exist at interstitial sites in the lattice (MnI). MnI align antiferromagnetically with Mn_Ga, and are double donors, fighting the ferromagnetic ordering. A very interesting property of this material is that post-growth annealing greatly increases the magnetization and T_C in Ga_1-xMn_xAs thin films, as was first shown through conventional magnetometry.

PNR is a powerful experimental tool that can be used to establish magnetic and chemical depth profiles for thin films. We have used PNR to further examine the mechanisms through which annealing enhances the ferromagnetic ordering in Ga_1-xMn_xAs. This work has shown that annealing leads not only to an increased total magnetization, but also to a much more homogeneous depth distribution of magnetization. Additionally, these measurements have revealed that annealed films posses a non-magnetic surface layer with a composition different from the rest of the film. This result strongly corroborates the idea that annealing enhances the ferromagnetism by redistributing MnI to the surface of the film. Additional aspects of this work include studies of changes in the Ga_1-xMn_xAs magnetic and chemical depth profiles induced by varying Mn concentration, varying film thickness, and the addition of surface capping layers.

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