Atomic-Scale Magnetism of Cr-Doped Bi2Se3 Thin Film Topological Insulators

• Published in: ACS nano 2015-10, Vol.9 (10), p.10237-10243
• Main Authors: Liu, Wenqing, West, Damien, He, Liang, Xu, Yongbing, Liu, Jun, Wang, Kejie, Wang, Yong, van der Laan, Gerrit, Zhang, Rong, Zhang, Shengbai, Wang, Kang. L
• Format: Article
• Language: English
• ISSN: 1936-0851; 1936-086X
• DOI: 10.1021/acsnano.5b03980

Magnetic doping is the most common method for breaking time-reversal-symmetry surface states of topological insulators (TIs) to realize novel physical phenomena and to create beneficial technological applications. Here we present a study of the magnetic coupling of a prototype magnetic TI, that is, Cr-doped Bi2Se3, in its ultrathin limit which is expected to give rise to quantum anomalous Hall (QAH) effect. The high quality Bi2–x Cr x Se3 epitaxial thin film was prepared using molecular beam epitaxy (MBE), characterized with scanning transimission electron microscopy (STEM), electrical magnetotransport, and X-ray magnetic circularly dichroism (XMCD) techniques, and the results were simulated using density functional theory (DFT) with spin–orbit coupling (SOC). We observed a sizable spin moment m spin = (2.05 ± 0.20) μB/Cr and a small and negative orbital moment m orb = (−0.05 ± 0.02) μB/Cr of the Bi1.94Cr0.06Se3 thin film at 2.5 K. A remarkable fraction of the (CrBi–CrI)3+ antiferromagnetic dimer in the Bi2–x Cr x Se3 for 0.02 < x < 0.40 was obtained using first-principles simulations, which was neglected in previous studies. The spontaneous coexistence of ferro- and antiferromagnetic Cr defects in Bi2–x Cr x Se3 explains our experimental observations and those based on conventional magnetometry which universally report magnetic moments significantly lower than 3 μB/Cr predicted by Hund’s rule.