Dirac gap opening and Dirac-fermion-mediated magnetic coupling in antiferromagnetic Gd-doped topological insulators and their manipulation by synchrotron radiation

A M Shikin,D A Estyunin,E V Shevchenko,Y Takeda,K A Kokh,O E Tereshchenko,A V Koroleva,S Kumar,E F Schwier,A Kimura,T Yoshikawa,Y Saitoh,K Shimada,Yu I Surnin

doi:10.1038/s41598-019-41137-w

Abstract

A new kind of magnetically-doped antiferromagnetic (AFM) topological insulators (TIs) with stoichiometry Bi1.09Gd0.06Sb0.85Te3 has been studied by angle-resolved photoemission spectroscopy (ARPES), superconducting magnetometry (SQUID) and X-ray magnetic circular dichroism (XMCD) with analysis of its electronic structure and surface-derived magnetic properties at different temperatures. This TI is characterized by the location of the Dirac gap at the Fermi level (EF) and a bulk AFM coupling below the Neel temperature (4–8 K). At temperatures higher than the bulk AFM/PM transition, a surface magnetic layer is proposed to develop, where the coupling between the magnetic moments located at magnetic impurities (Gd) is mediated by the Topological Surface State (TSS) via surface Dirac-fermion-mediated magnetic coupling. This hypothesis is supported by a gap opening at the Dirac point (DP) indicated by the surface-sensitive ARPES, a weak hysteresis loop measured by SQUID at temperatures between 30 and 100 K, XMCD measurements demonstrating a surface magnetic moment at 70 K and a temperature dependence of the electrical resistance exhibiting a mid-gap semiconducting behavior up to temperatures of 100–130 K, which correlates with the temperature dependence of the surface magnetization and confirms the conclusion that only TSS are located at the EF. The increase of the TSS’s spectral weight during resonant ARPES at a photon energy corresponding to the Gd 4d-4f edge support the hypothesis of a magnetic coupling between the Gd ions via the TSS and corresponding magnetic moment transfer at elevated temperatures. Finally, the observed out-of-plane and in-plane magnetization induced by synchrotron radiation (SR) due to non-equal depopulation of the TSS with opposite momentum, as seen through change in the Dirac gap value and the k∥-shift of the Dirac cone (DC) states, can be an indicator of the modification of the surface magnetic coupling mediated by the TSS.

Highlights

In recent years, magnetically-doped topological insulators (TIs) attracted great interest due to the possible realization of the Quantum Anomalous Hall effect (QAHE)[1,2,3,4,5,6,7] and topological magneto-electric effect[6,7] caused by magnetic impurities with a high out-of-plane magnetic anisotropy[8,9,10,11,12]
We have shown that the electronic structure of Gd-doped topological insulator with stoichiometry Bi1.09Gd0.06Sb0.85Te3 is characterized by a gapped Dirac cone located at the Fermi Level without additional crossing of either valence or conduction band
We further demonstrate the persistence of the Dirac Gap to temperatures as high as room temperatures, which are well above the bulk magnetic transition

Summary

Introduction

Magnetically-doped TIs attracted great interest due to the possible realization of the Quantum Anomalous Hall effect (QAHE)[1,2,3,4,5,6,7] and topological magneto-electric effect[6,7] caused by magnetic impurities with a high out-of-plane magnetic anisotropy[8,9,10,11,12]. Kou et al.[18,19] who studied the mechanism of ferromagnetic (FM) ordering and its connection with the DP position relative to the EF, showed that a shift of the Dirac gap towards EF is followed via the transition from a hole-mediated RKKY coupling to an electric-field-independent bulk van Vleck ferromagnetism. It means that classical RKKY-type FM coupling through itinerant carriers cannot be directly applied for analysis of the conditions of the QAHE realization and cannot be used for detailed analysis of the developed surface magnetism when the Dirac gap is located at the EF. It was observed that the developed magnetism can persist up to room temperature, despite the fact that the

Methods

Conclusion