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Direct evidence of ferromagnetism in a quantum anomalous Hall system

Abstract

Quantum anomalous Hall (QAH) systems are of great fundamental interest and potential application because of their dissipationless conduction without the need for an external magnetic field1,2,3,4,5,6,7,8,9. The QAH effect has been realized in magnetically doped topological insulator thin films10,11,12,13,14. However, full quantization requires extremely low temperature (T < 50 mK) in the earliest works, athough it has been significantly improved by modulation doping or co-doping of magnetic elements15,16. Improved ferromagnetism has been shown in these thin films, yet direct evidence of long-range ferromagnetic order is lacking. Herein, we 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 typical ferromagnetic domain behaviour—that is, domain nucleation and possibly domain wall propagation—in contrast to much weaker magnetic signals observed in the endmembers, possibly due to superparamagnetic behaviour17,18,19. The observed long-range ferromagnetic order resolves one of the major challenges in QAH systems, and paves the way towards high-temperature dissipationless conduction by exploring magnetic topological insulators.

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Fig. 1: Schematic of the in situ transport set-up and the Cr concentration (y) dependence of σxy and Hc/FWHMMR.
Fig. 2: The magnetization reversal process at 5 K at the neutral point \({{\bf{\it V}}}_{{\bf{g}}}^{{\bf{0}}}\).
Fig. 3: Gate dependence of ferromagnetic behaviour.
Fig. 4: Gate dependence of ρyx and Hc and schematic band structure.

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Acknowledgements

We thank C. Chang for helpful discussions and P. Sass for proofreading the manuscript. This work at Rutgers is supported by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, US Department of Energy under Award numbers DE-SC0008147 and DE-SC0018153. The work at Tsinghua University is supported by the National Natural Science Foundation of China and the Ministry of Science and Technology of China.

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W.Wu, K.H. and Y.W. conceived the project. W.Wu and W.Wa designed the MFM experiment. W.Wa performed MFM experiments with in situ transport measurements, and analysed the data. Y.O. synthesized the MBE films under the supervision of K.H. and Q.X. C.L. and Y.W. carried out transport characterization of the films. W.Wu and W.Wa wrote the manuscript with inputs from all authors.

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Correspondence to Weida Wu.

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Wang, W., Ou, Y., Liu, C. et al. Direct evidence of ferromagnetism in a quantum anomalous Hall system. Nature Phys 14, 791–795 (2018). https://doi.org/10.1038/s41567-018-0149-1

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