December 29, 2015

Carrier-mediated ferromagnetism in a magnetic topological insulator

A group of researchers in Japan and China identified the requirements for the development of new types of extremely low power consumption electric devices by studying Cr-doped (Sb, Bi)2Te3 thin films.

At extremely low temperatures, an electric current flows around the edge of the film without energy loss, and under no external magnetic field. This attractive phenomenon is due to the material's ferromagnetic properties; however, so far, it has been unclear how the material gains this property. For the first time, researchers have revealed the mechanism by which this occurs. "Hopefully, this achievement will lead to the creation of novel materials that operate at room temperature in the future," said Akio Kimura, a professor at Hiroshima University and a member of the research group.

Nature Communications - Carrier-mediated ferromagnetism in the magnetic topological insulator Cr-doped (Sb,Bi)2Te3


Magnetically doped topological insulators, possessing an energy gap created at the Dirac point through time-reversal-symmetry breaking, are predicted to exhibit exotic phenomena including the quantized anomalous Hall effect and a dissipationless transport, which facilitate the development of low-power-consumption devices using electron spins. Although several candidates of magnetically doped topological insulators were demonstrated to show long-range magnetic order, the realization of the quantized anomalous Hall effect is so far restricted to the Cr-doped (Sb,Bi)2Te3 system at extremely low temperature; however, the microscopic origin of its ferromagnetism is poorly understood. Here we present an element-resolved study for Cr-doped (Sb,Bi)2Te3 using X-ray magnetic circular dichroism to unambiguously show that the long-range magnetic order is mediated by the p-hole carriers of the host lattice, and the interaction between the Sb(Te) p and Cr d states is crucial. Our results are important for material engineering in realizing the quantized anomalous Hall effect at higher temperatures.

The experimental observation of the quantum anomalous Hall (QAH) effect a hallmark of topologically non-trivial states in magnetic topological insulators (TIs) has stimulated unprecedented research activities in the field of materials showing topologically protected surface states. While the origins of such strongly coupled magnetism in TIs are still under debate, Checkelsky et al. reported the suppression of ferromagnetism in Mn-doped Bi2Te3−ySey by increasing carrier densities, suggesting a Dirac-fermion-mediated origin for the surface ferromagnetism in TIs. In contrast, for the long-range ferromagnetic order in a Cr-doped (BiySb1−y)2Te3 film, Chang et al. demonstrated an independence of Curie temperature (TC) with carrier density, typically ~30–35K. To settle this conflict on the role of carriers in magnetic TIs, the microscopic origin of this magnetism needs to be studied systematically for various TC, rather than simply chasing a high one. One should certainly be guided by the analogy with conventional dilute magnetic semiconductors where extrinsic magnetism, for instance, from the clustering of a magnetic dopant, also gives rise to the elevation of the TC. Indeed, the aggregation of Cr dopants in Bi2Se3, which resulted in an energy gap opening for Dirac surface states even without long-range magnetic order, has been reported very recently. By spin-polarized scanning tunnelling microscopy, Yang et al. revealed in Cr0.05Sb1.95Te3 that the spin polarization of the surface states lies in the surface plane, and deviates from the bulk states being oriented along the out-of-plane easy axis. These results suggest that the surface magnetism in the Cr-doped TIs might not simply follow the bulk one. Since the observation of the QAH effect with Cr-doped (Sb,Bi)2Te3 system is restricted less than 100 mK the development of ferromagnetic TIs with much higher TC are strongly desired. Apart from the complex surface magnetism, to raise the bulk TC would lead to a better stabilization of surface ferromagnetism. Therefore, the first important step in realizing the QAH at higher temperature would be to investigate the driving mechanism of ferromagnetism.

In this work, we identify the element-resolved magnetism in the Cr-doped magnetic TI (Sb,Bi)2Te3 using X-ray magnetic circular dichroism (XMCD) combined with photoemission spectroscopy and a first-principles calculation. We find that, with increasing Cr concentration, the bulk ferromagnetism is more stabilized in the Cr-doped (Sb,Bi)2Te3 system. More importantly, we have detected the magnetic moments not only for the Cr dopant d states but also for the Sb and Te p states in the host lattice; they are found to be absent on the Bi-site, suggesting that the formation of long-range magnetic order is mainly mediated by both Te and Sb p-hole carriers.

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