时间：2017年7月12日上午10点
地点：唐仲英大楼A213
Direct evidence of ferromagnetism in a quantum anomalous Hall system
Weida Wu
Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA, Piscataway, NJ, United States.
The quantum anomalous Hall (QAH) systems are of great fundamental interest and of potential application because of dissipationless conduction without external magnetic field[1-3]. The QAH effect has been realized in magnetically doped topological insulator thin films[4-6]. However, full quantization requires extremely low temperature (<100 mK) in the initial works, and was significantly improved in heterostructures with modulation doping or co-doping of magnetic elements[7]. Improved ferromagnetism was suggested in these thin films, yet a direct evidence of long-range ferromagnetism is lacking. In this talk, I will present results of direct visualization of long-range ferromagnetic order in thin films of Cr and V co-doped (Bi,Sb)2Te3 using low-temperature magnetic force microscopy with in-situ transport. The magnetization reversal process reveals a typical ferromagnetic domain behavior, i.e., domain nucleation and domain wall propagation, in contrast to much weaker magnetic signals observed in the end members, possibly due to superparamagnetic behavior [8]. The gating dependence of magnetic reversal process indicates multiple mechanisms of exchange interactions. The observed long-range ferromagnetic order resolves one of the major challenges in QAH systems, and paves the way to high-temperature dissipationless conduction with magnetic topological insulators.
References:
[1] F. D. M. Haldane, Phys. Rev. Lett. 61, 2015-2018 (1988). [2] M. Onoda and N. Nagaosa, Phys. Rev. Lett. 90, 206601 (2003). [3] R.Yu, et al., Science 329, 61-64 (2010). [4] C.-Z. Chang, et al., Science 340, 167-170 (2013). [5] J. G. Checkelsky, et al., Nat. Phys. 10, 731 (2014). [6] X. Kou, et al., Phys. Rev. Lett. 113, 137201 (2014). [7] M. Mogi, et al., Appl. Phys. Lett. 107, 182401 (2015). [8] E. O. Lachman, et al., Sci. Adv. 1, e1500740 (2015).
Brief bio:
Dr. Weida Wu is an associate professor at the Department of Physics and Astronomy at Rutgers University. He received his BS degree from University of Science and Technology of China in 1998, MS from Northwestern University in 1999, and Ph.D. in Physics from Princeton University in 2004. He did post-doc at University of Texas at Austin in 2004-2006. Dr. Wu joined Rutgers University first as research assistant professor in 2006-2007, the as assistant professor in 2007-2012. He was promoted to associate professor with tenure in 2013. Dr. Wu’s research covers a broad range of correlated phenomena, including multiferroics and magnetoelectrics, topological quantum materials, CMR manganites, etc. For his accomplishments, Dr. Wu received CAREER Award from US NSF in 2009 and Early Career Award from US DOE in 2011. His research program is currently funded by both NSF and DOE.
