Magnetic and chemical nonuniformity in Ga[1-x]Mn[x]As films as probed by   polarized neutron and x-ray reflectometry
  We have used complementary neutron and x-ray reflectivity techniques to examine the depth profiles of a series of as-grown and annealed Ga[1-x]Mn[x]As thin films. A magnetization gradient is observed for two as-grown films and originates from a nonuniformity of Mn at interstitial sites, and not from local variations in Mn at Ga sites. Furthermore, we see that the depth-dependent magnetization can vary drastically among as-grown Ga[1-x]Mn[x]As films despite being deposited under seemingly similar conditions. These results imply that the depth profile of interstitial Mn is dependent not only on annealing, but is also extremely sensitive to initial growth conditions. We observe that annealing improves the magnetization by producing a surface layer that is rich in Mn and O, indicating that the interstitial Mn migrates to the surface. Finally, we expand upon our previous neutron reflectivity study of Ga[1-x]Mn[x]As, by showing how the depth profile of the chemical composition at the surface and through the film thickness is directly responsible for the complex magnetization profiles observed in both as-grown and annealed films. 
B.J. Kirby, J.A. Borchers, J.J. Rhyne, K.V. O'Donovan, S.G.E. te   Velthuis, S. Roy, Cecilia Sanchez-Hanke, T. Wojtowicz, X....