Disorder Effects on Magnetism in Dilute Magnetic Semiconductors

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Technical Description: Group III-V compound dilute magnetic semiconductors (DMS) offer the potential for manipulating both spin and charge and a class of materials that exhibit magnetic, magneto-optical and semiconducting properties. Their utilization, however, has been impeded due to their low magnetic transition (Curie) temperatures. It was demonstrated that InMnSb alloys prepared by metalorganic vapor phase epitaxy exhibit a Curie temperature in excess of 400 K. Mn dopant atoms were imaged using cross-sectional scanning tunneling microscopy (STM). No clustering of Mn was noted but positional disorder was observed. The inhomogeneous distribution of magnetic ions on the atomic scale is proposed to play a dominant role in the magnetic behavior.The objective of this research project centers on determining the magnetic phase diagram of DMS in the dilute concentration limit. The alloy system of interest is the narrow-gap DMS InMnSb. The applicability of a theoretical model for describing the magnetic properties of disordered DMS is investigated. The major questions to be answered are (a) why these alloys have high Curie temperatures and (b) what the role of disorder of Mn dopants is on the transition temperature. In the project, epitaxial thin alloy films are deposited by metalorganic vapor phase epitaxy. Alloys with Mn concentrations of less than 1% and carrier concentrations of lower than 10^19 cm-3 are synthesized. The role of disorder in stabilizing the ferromagnetic phase is investigated. It is proposed that the ferromagnetic phase is stabilized at carrier concentrations 2 x 10^17 cm-3, which is above the metal-insulator transition carrier concentration. Experimental characterization techniques used include: transmission electron microscopy, temperature dependent magnetization measurements, Hall effect, resistivity and magnetoresistance measurements. Low-temperature cross-sectional STM and spin-tunneling STM at the Technical University at Eindhoven are used to determine the nature of Mn substitution in InSb and its magnetic properties at the atomic level.Non-technical Description: As to broader impact, alloy films prepared by metalorganic vapor phase epitaxy are provided to colleagues at universities in the US and abroad and Argonne National Laboratories for characterizing their structural, magneto-optical and magnetic properties. An extended collaboration is established between Northwestern University and Technical University of Eindhoven on the imaging of single dopant atom in dilute magnetic semiconductors. The project involves the training of post-doctoral, graduate and undergraduate students in the synthesis and property measurements of ferromagnetic semiconductors. Graduate students are involved in STEM outreach through the HANDS program at the Haven School in Evanston, Illinois. The magnetic semiconductor materials have potential technological importance for spintronic devices such as spin valves, bipolar magnetic junction transistors, spin diode logic circuits and quantum computation devices.