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.
Highlights
The term “topological insulator” (TI) was first introduced in 2007 to generalize the two-dimensional quantum spin Hall state to three dimensions[1,2]
The evolution of the anomalous Hall effect (AHE) with temperature and magnetic field can be linked to the magnetization, M, of the sample through the following relationship[14]: ρxy = RoB + RsM, where ρxy is the total Hall resistivity, B is the applied magnetic field, Ro = 1/nec (e being the electron charge and n the carrier concentration) is the ordinary Hall coefficient arising from the Lorentz force experienced by electrons/holes, and Rs is the anomalous Hall coefficient
We present a comprehensive comparison of the variation in ferromagnetism in highly doped CrxSb2−xTe3 films (x ranging from 0.15 to as high as 0.76) through gated electrical transport, magneto-optical Kerr measurements and terahertz time domain spectroscopy
Summary
Transition metal doping in TIs is predicted to break TRS which opens a surface gap at the Dirac point[5,6] This has led to the recent experimental observations of the topological magnetoelectric effect[7], induction of a magnetic monopole[8] and the quantum anomalous Hall effect (QAHE)[9]. The evolution of the AHE with temperature and magnetic field can be linked to the magnetization, M, of the sample through the following relationship[14]: ρxy = RoB + RsM, where ρxy is the total Hall resistivity, B is the applied magnetic field, Ro = 1/nec (e being the electron charge and n the carrier concentration) is the ordinary Hall coefficient arising from the Lorentz force experienced by electrons/holes, and Rs is the anomalous Hall coefficient On dividing this aceatioqcnvcumdiutaypartianσootxnoTenyme.dbnAeayttslσρeoρxrx2Ouxxymy,s=w iHneaaRsρtρlOexil2xxoBexcn,,toσhhonxTeayfdtnσu=tcxhAceyetwρeixve2nxiqiρl+atlxuybyiaρnlrxit2ydneiosig≈rnpeteehcρρcxtexx2tlyxpyifvri=geeeolsldysvR.eeaosTBrnnth+ρdhxe2txesthReeaseMmdrmdepwsiaetuigirtvlanhtitetuiσytnxriToys:eaσf=dtxtiAehyopσen=xeOHyMnσa+dxlTbeylnyσc−cuxcAyera,σrriwxeneOynfhuMt=eslrl,yettRohρrσsx2MaexxbOtyme.iaFsoen,rxvtdotihnrmσeagxcAtyttothheatdareiselodHetrihqdraeueilnlcaoctatrloirydoyninfndHr,oauaarmcnly-l magnetotransport data. We conclude that the ferromagnetic order in the films is via the carrier-mediated RKKY mechanism
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