KOU ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX www.acsnano.org A C XXXX American Chemical Society Interplay between Different Magnetisms in Cr-Doped Topological Insulators Xufeng Kou, † Murong Lang, † Yabin Fan, † Ying Jiang, ‡ Tianxiao Nie, † Jianmin Zhang, §,^ Wanjun Jiang, † Yong Wang, ‡ Yugui Yao, § Liang He, †, * and Kang L. Wang †, * † Department of Electrical Engineering, University of California, Los Angeles, California 90095, United States, ‡ Center for Electron Microscopy and State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China, § School of Physics, Beijing Institute of Technology, Beijing 100081, China, and ^ Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China T he strong spinÀorbit coupling and time-reversal-invariant symmetry give rise to a new class of materials known as the topological insulators (TIs). 1À7 Fea- tured by the unique massless Dirac fermions on the boundaries, TIs are anticipated for dissipationless spin-dependent transport even at relative high temperatures. As a result, huge progress in recent years has been focused on the time-reversal-symmetry (TRS) protected TI systems. 8À11 Alternatively, if ad- ditional ferromagnetic orders are introduced, the original topologically nontrivial state will be driven into the TRS-breaking realm. 12 Associated physical phenomena like quantum anomalous Hall Effect (QAHE) and exotic par- ticles (dyons, axions and majorana fermions) are anticipated. 13À22 All of such discoveries will further broaden the research and applica- tion scopes of topological insulators. Introducing magnetic impurities to the surface or bulk of topological insulators has been proven to be an effective way to open a gap of the surface states. 12,23À27 To under- stand the magnetic origin, it has been proposed that in magnetic TI systems, ferromagnetic moments can be developed through two major mechanisms: the van Vleck mechanism and the Ruderman- Kittel-Kasuya-Yosida (RKKY) coupling. 12,14,28 In the former case, due to the large spin susceptibility of the valence electrons in the band-inverted TI materials, the magnetic ions can thus be directly coupled through these local valence electrons without the assistance of the itinerant electrons. Conse- quently, this “bulk ferromagnetism” is inde- pendent of the carrier density. 14 On the other hand, neighboring magnetic ions can also be coupled through the mediation of conduction carriers and this kind of cou- pling is referred to as the carrier-mediated RKKY interaction. 29À31 Recently, these two magnetic mechanisms were independently * Address correspondence to heliang@ee.ucla.edu, wang@ee.ucla.edu. Received for review July 23, 2013 and accepted September 26, 2013. Published online 10.1021/nn4038145 ABSTRACT Breaking the time-reversal-symmetry of topological insulators through magnetic doping has led to exotic physical discov- eries. Here, we report the gate-dependent magneto-transport measure- ments on the Cr-doped (Bi x Sb 1Àx ) 2 Te 3 thin films. With effective top-gate modulations, we demonstrate the presence of both the hole-mediated RKKY coupling and carrier-independent van Vleck magnetism in the magnetic TI systems. Most importantly, by varying the Cr doping concentrations from 2% to 20%, we unveil the interplay between the two magnetic orders and establish the valid approach to either enhance or suppress each individual contribution. The electric-field-controlled ferromagnetisms identified in the Cr-doped TI materials will serve as the fundamental step to further explore the TRS-breaking TI systems, and it may also help to expand the functionality of TI-based device for spintronics applications. KEYWORDS: topological insulator . magnetic doping . van Vleck magnetism . electric-field-controlled ferromagnetism ARTICLE