文摘
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鈥?i>xCrxSe3 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鈥搊rbit coupling (SOC). We observed a sizable spin moment mspin = (2.05 卤 0.20) 渭B/Cr and a small and negative orbital moment morb = (鈭?.05 卤 0.02) 渭B/Cr of the Bi1.94Cr0.06Se3 thin film at 2.5 K. A remarkable fraction of the (CrBi鈥揅rI)3+ antiferromagnetic dimer in the Bi2鈥?i>xCrxSe3 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鈥?i>xCrxSe3 explains our experimental observations and those based on conventional magnetometry which universally report magnetic moments significantly lower than 3 渭B/Cr predicted by Hund鈥檚 rule.