Controlling the Curie temperature in the ferromagnetic semiconductor ( Ga,Mn )As through location of Fermi level in the

1. Controlling the Curie temperature in the ferromagnetic semiconductor (Ga,Mn)As through location of Fermi level in the impurity band Margaret Dobrowolska, University of Notre Dame,DMR 1005851 Description of achievement: Understanding of the origin of ferromagnetism in (Ga,Mn)As – one of the most promising materials for spintronic technology – is of obvious importance, as it can serve as a guide for strategies to boost its ferromagnetism (i.e., its Curie temperature TC) to a technologically relevant range. In our work we have resolved the long-standing controversy of how the ferromagnetic Mn-Mn interaction is mediated in (Ga,Mn)As, by showing that (contrary to the widely-held view of many scientists) the holes which control the Curie temperature in this material are located in an impurity band rather than in the valence band. Our work also showed that it is the location of the Fermi level and the degree of localization of the holes within this impurity band that drives the value of the Curie temperature. Methodology: Systematic study of the relationship between TC in (Ga,Mn)As and two key parameters: the hole density (p), and the concentration of magnetically-active Mn (xeff), determined by channeling Rutherford backscattering (RBS) and channeling particle-induced X-ray emission (PIXE). Illustration of results in figure on the right: (a) Schematic of RBS and PIXE, which exploit the passage of high-energy alpha particles to measure the concentration of substitutional (xsub) and interstitial (xi) Mn impurities. (b) Data showing that TC does not follow the prediction of the widely-held view in which ferromagnetism is mediated by itinerant holes within the valence band. (c) Schematic diagram of the impurity band created by manganese doping of (Ga,Mn)As, showing location of the Fermi level which controlsTC in this material. PIXE a) RBS [110] [111] Energy Conduction band EF Valence band c) b)

2. DMR 1005851: Electron spin Effects in Semiconductor Nanostructures Margaret Dobrowolska, University of Notre Dame,DMR 1005851 Education: Our program typically involves three graduate students and two undergraduate students each year. These students tare exposed to a wide range of materials fabrication and characterization techniques, thus preparing them for the US manpower needs in the area of semiconductor science and technology. We also conduct a broad program of international scientific exchanges. Currently we are hosting Prof. Sanghoon Lee from Korea University on his sabbatical. Moreover, Dr. K. Dziatkowski of Warsaw University and Prof. T. Wojtowicz of the Polish Academy of Sciences will be our guests on extended visits to conduct research in our group. Societal Impact: Our group has continually acted as a resource of materials for other groups. We are currently interacting with at least 15 other institutions around the world by providing them with magnetic semiconductor specimens for their research; and at least ten graduate students in institutions other than Notre Dame are currently doing their doctoral research on materials which we have provided. The understanding of these materials obtained in our laboratory is thus automatically of benefit to our collaborators, whose research depends not only on the specimens that we provide, but on the intellectual input from our group in the form of characterization and general understanding of the properties of these materials.