Transition Metal Doped Sb 2 Te 3 and Bi 2 Te 3 Diluted Magnetic Semiconductors Ctirad Uher DMR By low temperature MBE technique, we were able to extend the limit of solubility of Cr in Sb 2 Te 3 and fabricated thin films that are ferromagnetic to temperatures of at least 190 K. The ferromagnetic state was confirmed by anomalous Hall effect measurements and by robust hysteresis loops. We fabricated a simple trilayer structure consisting of two outside layers of Sb 2-x Cr x Te 3 with different coercive fields (different Cr content x) and a spacer layer of pure, diamagnetic Sb 2 Te 3 and explored Interlayer Exchange Coupling (IEC) by carefully studying the shape of the hysteresis loops. While thick spacer layers and high temperatures lead to two-step transitions, hysteresis loops of structures with thin spacer layer measured at low temperatures reveal a one step transition, demonstrating the existence of IEC. Having previously observed that both vanadium and chromium can, on their own, stimulate ferromagnetism in the Sb 2 Te 3 matrix, we attempted to double dope Sb 2 Te 3 single crystals with V and Cr inquiring whether such conditions could lead to a cooperatively enhanced Curie temperature. Although we did not observe a dramatic enhancement in the Curie temperature, the double-doped Sb 2 Te 3 crystals showed higher remanent magnetization. University of Michigan Reference: (1)Z. Zhou, Y.-J. Chien, and C. Uher, Thin Film Dilute Ferromagnetic Semiconductors Sb 2-x Cr x Te 3 with a Curie Temperature up to 190K, Phys. Rev. B 74, (2006). (2)Z. Zhou, Y.-J. Chien, and C. Uher, Ferromagnetic Interlayer Exchange Coupling in Semiconductor SbCrTe/Sb 2 Te 3 /SbCrTe Trilayer Structures, Appl. Phys. Letters 89, (2006). (3)C. Drasar, J. Kasparova, P. Lostak, X. Shi, and C. Uher, Transport and Magnetic Properties of Diluted Magnetic Semiconductors Sb 1.98-x V 0.02 Cr x Te 3, Phys. Stat. Solidi (b) 244, 2202 (2007). (4)J. Horak, P. Lostak, C. Drasar, J. Navratil, and C. Uher, Defect Structure of Fe-doped Sb 2 Te 3 Single Crystals, J. Solid State Chemistry 189, 909 (2007). Trilayer structure for studying IEC. The fields H c1 and H c2 are the respective coercive fields. Sb 2-x Cr x Te 3 50 nm Sb 2 Te nm Sb 2-y Cr y Te 3 50 nm Sapphire substrate ferromagnetic, H C2 nonmagnetic ferromagnetic, H C1 Easy axis

Applied field H Without IECWith IEC Stage 1 H c1 <H c2 <H Applied field H Stage 2 H c1 < |-H| <H c2 Applied field H Stage 3 H c1 <H c2 < |-H| Applied field H Stage 4 H c1 < H <H c2 Mechanism of IEC The change in Hall resistivity hysteresis loops from two step to one step transitions points toward IEC taking place in samples with a smaller spacer thickness (especially at lower temperatures). The coupling between magnetic ions separated by a spacer layer is likely indirect and free carrier mediated. To further confirm this interesting property, new F/N/F trilayer samples consisting of spacers of different types (insulator, etc.) and with different carrier concentrations are being prepared. A better understanding of the mechanism of ferromagnetism in tetradymite-type semiconductors is expected to come out of this investigation. [In the four hysteresis plots above, Cr concentration in the top layer is x=.34 and in the bottom layer y=.19.] Transition Metal Doped Sb 2 Te 3 and Bi 2 Te 3 Diluted Magnetic Semiconductors Ctirad Uher DMR University of Michigan Sb 1.66 Cr 0.34 Te 3 /Sb 2 Te 3 /Sb 1.81 Cr 0.19 Te 3

Education: This research has been a thesis project of Y.-J. Chien. Just last week, Yi-Jiunn passed his oral defense with distinction and will stay in my lab for a couple of months to assist with training of my new student, Lynn Endicott, a second year student in our Physics Graduate Program. Although not funded by this grant, my postdoc, Dr. Xun Shi, is much interested in thin film growth of magnetic structures and is actively participating in the project. During the past academic year and this summer, I supported and much enjoyed the presence in my laboratory of two talented undergraduate students, Chris Lawrence (academic 2006/07) and Steven Moses (summer of 2007). Chris is now a graduate student in the Physics Department at Michigan State University, and Steve is a physics major in our department and continues to work in my laboratory about 10 hours a week. Outreach: As chair of the Department during , I organized and supported a series of public lectures called Saturday Morning Physics. These lectures are given by our postdocs (Fall term) and faculty (Winter term) and attract a wide and diverse audience (attendance in excess of 250 each Saturday morning, football season or not, spanning from middle school kids to senior citizens). These lectures highlight research work conducted in our department, and my group is an active participant. The past three years, I taught Physics 160 Honors, an intro physics course for the most talented students our university attracts. As an integral part of the course, I introduced group projects on modern physics topics, among them spintronics. Students in groups of 4-5 students have 3 months to prepare a proper scientific report and then present their finding to the class. The topic of spintronics is one of the most successful presentations partly because the students have a chance to visit my laboratory and observe various stages of research on our thin film diluted magnetic semiconductors. I also give an annual mini-colloquium for our first and second year graduates and the topic of spintronics attracted Lynn Endicott to join my research group this summer. I collaborate with chemists from the University of Pardubice in the Czech Republic (Prof. P. Lostak’s group)—the collaboration that has led to the discovery of ferromagnetism in bulk crystals of transition metal-doped tetradymite-type semiconductors (e.g., V-doped Sb 2 Te 3 ) and the work upon which this research project was originally based—and this interaction continues to contribute to the training of materials scientists in the Czech Republic. University of Michigan Transition Metal Doped Sb 2 Te 3 and Bi 2 Te 3 Diluted Magnetic Semiconductors Ctirad Uher DMR