Introducing a magnetic dopant into a topological insulator can give rise to ferromagnetic ordering which can break timereversal symmetry, realizing dissipationless electronic states in the absence of an external magnetic field. Assessment and control of the magnetic state can translate into novel future applications in quantum computing. We provide a detailed study of the magnetic state in Cr doped Sb<sub>2</sub>Te<sub>3</sub> thin films using terahertz time-domain spectroscopy (THz-TDS) and electrical transport. The temperature dependent behavior of the THz conductance of Cr<sub>x</sub>Sb<sub>2-x</sub>Te<sub>3</sub> thin films with <i>x</i> = 0.15 exhibits a clear insulator-metal transition at 40 K, indicating the onset of ferromagnetic order in the Cr<sub>x</sub>Sb<sub>2-x</sub>Te<sub>3</sub> at the TC (40 K). Moreover, the magneto-transport measurements showed anomalous Hall behavior below 40 K, demonstrating the consistency between the electrical and optical measurements. The direct correlation obtained between the carrier density and ferromagnetism in the magnetically doped topological insulators films, using the THz optical technique, strongly suggests a carrier-mediated RKKY coupling scenario. Our non-contact method of using THz radiation to investigate ferromagnetism and the consistency between optical and electrical measurements pave the way to realise exotic quantum states for spintronic and low energy magneto-electronic device applications.
The surface of a topological insulator harbors exotic topological states, protected against backscattering from disorder by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomena in condensed matter physics, such as the spin Hall effect, but also lays the foundation for applications from quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly distinguish topological surface states from bulk states. Employing terahertz time-domain spectroscopy, we study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi<sub>2</sub>Se<sub>3</sub>) allowing for the deconvolution of the surface state response from the bulk. Our measurement of carrier dynamics give an optical mobility exceeding 2100 cm<sup>2</sup>/V•s at 4 K, indicative of a surface-dominated response, and a scattering lifetime of ~0.18 ps and a carrier density of 6×10<sup>12</sup> cm<sup>-2</sup> at 4 K for the Bi<sub>2</sub>Se<sub>3</sub> film. The sample was further processed into a Hall bar device using two different etching techniques, a wet chemical etching and Ar<sup>+</sup> ion milling, which resulting in a reduced Hall mobility. Even so, the magneto-conductance transport reveals weak antilocalization behavior in our Bi<sub>2</sub>Se3 sample, consistent with the presence of a single topological surface state mode.