Selective Doping of Antiferromagnetic Semiconductors

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Technical: This project aims for synthesis of a room temperature magnetic semiconductor, exploring a new strategy by starting with an antiferromagnetic semiconductor, and preferentially substituting nonmagnetic donors into one sublattice. Current approaches to synthesize these materials focus on doping semiconductors with a transition metal (TM) such as Mn. The magnetic ion is responsible both for providing a localized magnetic moment and a hole (e.g. in GaMnAs). The exchange interaction amongst the localized d electrons of Mn, mediated by the holes, leads to ferromagnetic ordering. In this approach only one type of doping can be obtained, e.g., p-type for Mn doped III-V's, and the concentration required to achieve Curie temperatures above 300K is higher than the solubility of Mn. In this project the approach is to emulate ferromagnetic semiconductors by selective doping of an antiferromagnetic (AFM) semiconductor with nonmagnetic dopants--to preferentially incorporate a nonmagnetic dopant, i.e., Cu, into one of the magnetic sublattices of the AFM semiconductor, resulting in local reduction of the magnetic moment, and in an overall net ferrimagnetic moment. The total magnetization of such doped AFM material is controlled by the number of dopants, and the choice of dopants, so both spin-polarized electrons and holes can be introduced. This approach may evade the problem of solubility limit and permit the independent control of different carrier types in magnetic semiconductors. The scope of earlier research is expanded in this project to investigate the properties of a new class of material: TMGeV2, by varying the group V element (As, P, and Sb) and TM (Cr, Fe, Co, and Ni), through DFT calculations, initially. Guided by the theoretical predictions, experimental efforts will focus on compounds that, when doped, yield ferrimagnetic semiconductors with transition temperature in excess of 300 K and have independently controllable spin-polarized electrons and holes. Epitaxial thin alloy films will be grown by MBE. Their structures will be characterized using high resolution transmission electron microscopy and related diffraction techniques. Magnetic properties will be investigated by temperature and field dependent magnetization measurements, Hall effect and magnetoresistance. Electronic and magnetic properties will be determined by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Advanced Photon Source, Argonne National Laboratory (ANL). Magnetic domain structures and their evolution will be obtained by scanning electron microscopy with polarization analysis (SEMPA) at the Center for Nanoscale Materials Research, Oak Ridge National Laboratory (ORNL). Non-technical: The project addresses basic research issues in a topical area of electronic/photonic materials science with high technological relevance. It is considered a high risk/high potential pay-off project. If successful, magnetic semiconductors with transition temperature in excess of 300 K and independently controllable carriers will be realized. The project involves training of graduate and undergraduate students in the synthesis and characterization of magnetic semiconductors using facilities at the PI's labs, as well as at ANL and ORNL. The interdisciplinary nature of the research and the combined theoretical/experimental approach provide additional opportunities for graduate and undergraduate students to broaden their educational experience. An RET program will be continued with local high schools to expose high school students to spintronics, and to inspire their interests in science in general.