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March 21, 2007

Magnetic and Chemical Non-Uniformity in Ga_1-xMn_xAs as Probed with Neutron and X-Ray Reflectivity

B.J. Kirby^1,2,3, J.A. Borchers³, J.J. Rhyne², K.V. O’Donovan^3,4, S.G.E. te Velthuis⁵, S. Roy⁶, C. Sanchez-Hanke⁷, T. Wojtowicz^8,9, X. Liu⁹, W.L. Lim⁹, M. Dobrowolska⁹, and J.K. Furdyna^9
1Department of Physics and Astronomy, University of Missouri-Columbia, Columbia, MO; ²Manuel Lujan Jr. Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM; ³Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD; ⁴Department of Physiology and Biophysics, University of California-Irvine, Irvine, CA; ⁵Materials Science Division, Argonne National Laboratory, Argonne, IL; ⁶Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, CA; ⁷National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY; ⁸Institute of Physics of the Polish Academy of Sciences, Warsaw, Poland; ⁹Department of Physics, University of Notre Dame, Notre Dame, IN

Artificial magnetic semiconductor materials could play a key role in future spin-electronics, or “spintronic” devices. We have used x-ray and polarized neutron reflectometry to study the mechanisms through which post-growth annealing increases the ferromagnetic transition temperature (TC ) of manganese-doped gallium arsenide. Our combined studies suggest that annealing liberates Mn from interstitial sites throughout the Ga_1-xMn_xAs film, and allows them to migrate to the film surface and oxidize – a process that drastically increases TC, and alters the distribution of the magnetic moment.

Brian Kirby

Magnetic semiconductors are of great importance to the development of spin-electronics (spintronics) technology, as they can be used to exert magnetic control of electrical current in devices. While suitable ferromagnetic semiconductors cannot be found in nature, non-magnetic semiconductor materials can be made ferromagnetic by replacing a fraction of the atoms in the crystal lattice with magnetic atoms. For example, low-temperature molecular beam epitaxy is used to produce manganese-doped gallium arsenide (Ga_1-xMn_xAs, x>10%), in which long-range order among Mn at Ga sites give rise to ferromagnetism. This material is attractive because GaAs is commonly found in modern electronics. However, room temperature ferromagnetism is necessary for practical device applications, but the ferro transition temperature (T_c) of Ga_1-xMn_xAs is only around 70 K. The unstable growth conditions that allow for Mn to enter Ga sites also lead to small concentrations of Mn at interstitial sites in the lattice. These unwanted interstitial Mn impurities are known to oppose ferromagnetic ordering, and are partially responsible for the low T_c of as-grown Ga_1-xMn_xAs. However, T_c can be more than doubled by a careful post-growth annealing.

Fully understanding how this annealing process works could lead to room temperature ferromagnetism in Ga_1-xMn_xAs. To that aim, we used x-ray and polarized neutron reflectometry (PNR) to study how annealing altered a series of Ga_1-xMn_xAs thin films. Following growth, pieces were cleaved off and annealed – resulting in sets of as-grown/annealed pairs. Figure 1 shows magnetic and chemical depth profiles for one such set, deduced from PNR measurements taken on the NG-1 Polarized Beam Reflectometer at the NIST Center for Neutron Research. For this set, annealing was found to increase T_c from 60-120 K. The as-grown models are in blue, the annealed are in red. The chemical profiles are shown above the break in the vertical axis, and the magnetic profiles are shown below. Annealing is observed to increase the net magnetization, change the depth distribution of magnetic moment, and add a layer of foreign material to the film surface. Since the chemical profile is flat, the as-grown magnetization gradient cannot be explained by changes in the concentration of Mn at Ga sites, but can be explained by small changes in the depth-dependent concentration of interstitial Mn impurities.

Figure 1. Scattering length density models of the as-grown (blue) and annealed (red) films, as deduced from polarized neutron reflectometry. The chemical depth profiles are shown above the break in the vertical axis, the magnetic depth profiles are shown below.

Figure 2. X-ray reflectivity for the as-grown (blue) and annealed (red) films taken at NSLS beamline X13A. The annealed film features pronounced oxygen and manganese peaks, while the as-grown film does not.

Energy-dependent x-ray reflectometry measurements of these samples were taken on beamline X13A of the National Synchrotron Light Source at Brookhaven National Laboratory, and are shown in Figure 2. For the annealed piece, there are pronounced peaks at the oxygen and manganese edges that are absent for the as-grown piece. Since this technique is most sensitive to material near the film surface, these results indicate that the layer of foreign material added by annealing is rich in Mn and O. Non-scattering work by other researchers has shown evidence that the positive effects of annealing are due to removal of interstitial Mn. Combined, our neutron and x-ray results corroborate that idea, suggesting that annealing liberates Mn from interstitial sites throughout the Ga_1-xMn_xAs film, and allows them to migrate to the film surface and oxidize – a process that drastically increases T_c, and alters the distribution of magnetic moment.

BEAMLINE
X13A

FUNDING
National Science Foundation
Missouri University Research Reactor Graduate Fellowship
U.S. Department of Energy

PUBLICATION
B.J. Kirby, J.A. Borchers, J.J. Rhyne, K.V. O’Donovan, S.G.E. te Velthuis, S. Roy, C. Sanchez-Hanke, T. Wojtowicz, X. Liu, W.L. Lim, M. Dobrowolska, J.K. Furdyna, “Magnetic and Chemical Nonuniformity in Ga_1-xMn_xAs Films as Probed by Polarized Neutron and X-Ray Reflectometry,” Physical Review B 74, 245304 (2006).

FOR MORE INFORMATION
Brian Kirby
Center for Neutron Research
National Institute of Standards and Technology
Email: bkirby@nist.gov