Technical: The development of ferromagnetic, group III-V semiconductors has provided the potential for a class of multifunctional materials that exhibit magnetic, magneto-optical, and semiconducting properties. These materials offer the potential for manipulating both spin and charge. Their utilization, however, has been impeded due to their low transition (Curie) temperatures. Recently there have been reports of ferromagnetic semiconductors with Curie temperatures in excess of 300 K. There is growing evidence that disorder plays an essential role in stabilizing ferromagnetism in certain ferromagnetic semiconductors. The question arises as to the detailed mechanism for stabilizing the high temperature ferromagnetic phase and the nature of the magnetic species. In this project, narrow bandgap indium based III-V ferromagnetic semiconductor thin films will be developed, including InMnAs and InMnSb and their solid solutions. Of specific interest is what role transition metal atomic clusters play in stabilizing high temperature ferromagnetism in these semiconductors and the nature of the short range order. Furthermore the role of free carrier concentration on the ferromagnetic phase stability will be examined. During the project, epitaxial thin alloy films will be synthesized by metalorganic vapor phase epitaxy (MOVPE). Semiconductor alloys with different manganese concentrations will be prepared to determine the role disorder plays in stabilizing ferromagnetism. Recent models based on spinodal decomposition forming ferromagnetic clusters will be tested. Experimental characterization techniques to be used include temperature and field dependent magnetization measurements, Hall effect, and magnetoresistance. The magneto-optical Kerr effect (MOKE) and its spectral dependence over the mid infrared to visible region will be used to determine the nature and magnitude of the exchange interaction in the alloys. X-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) at the Advanced Photon Source will be used to determine the magnetic properties of the elements comprising the alloys. Extended x-ray fine structure analysis (EXAFS), analytical electron microscopy, and an electrode atom probe with atomic scale resolution will be used to determine cluster size and distribution. Comparisons will be made between cluster sizes determined by structural and magnetic measurements.
The project addresses basic research issues in a topical area of materials science with high technological relevance, and is expected to provide scientific understanding of ferromagnetic semiconductor materials with potential applications for spin based devices such as spin valves, magnetic random access memories, and quantum computation. The project provides training of graduate and undergraduate students in an interdisciplinary topic, consisting of semiconductor physics and magnetism. The project also includes educational outreach activities to local schools and collaboration with national laboratories, which provide enhanced learning opportunities to students.
The utilization of electron spin for information processing has been the subject of widespread research for several decades.Much of the research has focused on the development of materials that are both semiconducting and magnetic. While this type of material rarely exists in nature, new compounds that exhibit both these properties have been synthesized in the laboratory using advanced thin film deposition techniques. These deposition techniques enable the fabrication of spintronic (spinelectronic) devices. Much of the research has involved the alloy gallium manganese arsenide (GaMnAs). This alloy, however, is ferromagnetic at cryogenic temperatures limiting its applicability. We have been studying other semiconductor alloysthat have higher magnetic transition temperatures. Based upon our theoretical predictions, we studied the narrow band gap semiconductors indium manganese arsenide (InMnAs)and indium manganese antimonide (InMnSb).Both of these alloys showed ferromagnetism above room temperature. The Curie transition temperatures are 57°C and 287°C for InMnAs and InMnSb, respectively. The high transition temperature makes them potentially useful for practical spintronic devices. The question arises as to the factors that stabilize the ferromagnetism at high temperatures. The prevailing theory isthat free carriers in the magnetic semiconductors stabilize the ferromagnetic phase through interactions with the magnetic ions. The distribution of magnetic ions in the semiconductor is of paramount importance. Possibilities include: randomly distributed, disordered, or as nanoprecipitates. Our finding indicates that the magnetic ions are disordered at concentrations of several atomic percent. For higher concentration, the magnetic ions form compounds that are present in the semiconductor as nanoprecipitates. These nanoprecipitates can be ferromagnetic. As to why our alloys are ferromagnetic, results from differences in synthetic techniques. We use metalorganic vapor phase epitaxy (MOVPE). Since we form the alloy thin films at higher temperatures, the films have a higher degree of structural perfection. This leads to materials with improved electrical and magnetic properties. Since the existence of high temperature ferromagnetism results from the presence of free carriers, highly conductive semiconductors are required. Our electrical measurements indicate that this is the case. Thin film samples of our magnetic semiconductor have been provided to a number of external collaborators world-wide for advanced microstructural and magnetic property analyses. Collaborations with faculty in the United States: Virginia Polytech and MIT, Europe: Oxford University, York University and Eindhoven University of Technology and Japan: University of Tokyo have been established and numerous publications have resulted. With the availability of the InMnAs semiconductor thin films, the applicability of these materials for spintronic devices was explored. Magnetic p-n junction diodes and magnetic bipolar transistors were demonstrated that exhibit unique properties. New types of spin logic circuits were proposed and a collaboration to develop these circuits has been undertaken.
