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CAMP Seminar: Direct evidence of ferromagnetism in a quantum anomalous Hall system

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Weida Wu, Rutgers University
12 September 2017 from 3:30 PM to 4:30 PM
339 Davey Laboratory
Contact Name
Cui-Zu Chang
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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 QAHE has been realized in magnetically doped topological insulator thin films[4-6]. However, full quantization requires extremely low temperature (<50 mK) in the initial works, though it was significantly improved with modulation doping or co-doping of magnetic elements [7, 8]. Improved ferromagnetism was indicated in these thin films, yet a direct evidence of long-range ferromagnetic order is lacking. In this talk, I will present 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 observed in Cr doped TI films [9]. The gate dependence of magnetic reversal process confirms that the long range ferromagnetic order is robust against bulk carrier modulation. The observed long-range ferromagnetic order resolves one of the major challenges in QAH systems, and paves the way to high-temperature dissipationless conduction by exploring magnetic topological insulators.


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[4] C.-Z. Chang, et al., Science 340, 167-170 (2013).

[5] J. G. Checkelsky, et al., Nat. Phys. 10, 731 (2014).

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[7] M. Mogi, et al., Appl. Phys. Lett. 107, 182401 (2015).

[8] Y. Ou, et al, submitted to Adv. Mater., (2017)

[9] E. O. Lachman, et al., Sci. Adv. 1, e1500740 (2015).