Systematic Study of Ferromagnetism in CrxSb2−xTe3 Topological Insulator Thin Films using Electrical and Optical Techniques

Abstract

Ferromagnetic ordering in a topological insulator can break time-reversal symmetry, realizing dissipationless electronic states in the absence of a magnetic field. The control of the magnetic state is of great importance for future device applications. We provide a detailed systematic study of the magnetic state in highly doped CrxSb2−xTe3 thin films using electrical transport, magneto-optic Kerr effect measurements and terahertz time domain spectroscopy, and also report an efficient electric gating of ferromagnetic order using the electrolyte ionic liquid [DEME][TFSI]. Upon increasing the Cr concentration from x = 0.15 to 0.76, the Curie temperature (Tc) was observed to increase by ~5 times to 176 K. In addition, it was possible to modify the magnetic moment by up to 50% with a gate bias variation of just ±3 V, which corresponds to an increase in carrier density by 50%. Further analysis on a sample with x = 0.76 exhibits a clear insulator-metal transition at Tc, indicating the consistency between the electrical and optical measurements. The direct correlation obtained between the carrier density and ferromagnetism - in both electrostatic and chemical doping - using optical and electrical means strongly suggests a carrier-mediated Ruderman-Kittel-Kasuya-Yoshida (RKKY) coupling scenario. Our low-voltage means of manipulating ferromagnetism, and consistency in optical and electrical measurements provides a way to realize exotic quantum states for spintronic and low energy magneto-electronic device applications.

Temperature dependence of the ferromagnetic response in CrxSb2-xTe3 topological insulator thin films investigated using terahertz spectroscopy and magneto-transport.

“…The carrier densities (and mobilities) in the THz measurements increase (decrease) roughly linearly with the Curie temperature Tc (Tc for x = 0.41, 0.58, 0.76 is obtained from Ref. 39] yielding the carrier dependence of the magnetic ordering temperature. It is interesting to note that the values of p obtained from the THz measurements tend to be smaller than those deduced from the Hall transport measurements.…”

“…It is interesting to note that the values of p obtained from the THz measurements tend to be smaller than those deduced from the Hall transport measurements. We used the bulk hole effective mass m * = 0.78 me in calculating the values for the THz measurements 39 . Due to the anisotropic nature of the upper and lower valence bands in Sb2Te3 the effective mass varies from m * = 0.034 me to m * = 1.24 me 40,41 .…”

Two‐Dimensional Metal Telluride Atomic Crystals: Preparation, Physical Properties, and Applications

“…Barnes et al. [ 178 ] reported the Cr x Sb 2‐ x Te 3 thin films with a magnetic state by the terahertz time domain spectroscopy and magneto‐optic Kerr effect measurements. Results indicate that a high‐efficiency electric gating of the ferromagnetic order was realized by using ionic liquid electrolyte.…”

A low-temperature Kerr effect microscope for the simultaneous magneto-optic and magneto-transport study of magnetic topological insulators

“…The (Cr,V):Sb 2 Te 3 and Cr:Sb 2 Te 3 TI thin films were grown on c-plane sapphire substrates by molecular beam epitaxial (MBE) co-deposition with a base chamber pressure of 1 × 10 −10 mbar [33,36,51]. Sb and Te elements were evaporated from 6N purity sources in standard effusion cells, while Cr and V elements were from 4N purity sources in high temperature cells.…”