Magnetic Cr doping of Bi2Se3: Evidence for divalent Cr from x-ray spectroscopy

A. I. Figueroa, G. van der Laan, L. J. Collins-McIntyre, S.-L. Zhang, A. A. Baker, S. E. Harrison, P. Schönherr, G. Cibin, and T. Hesjedal
Phys. Rev. B 90, 134402 – Published 3 October 2014

Abstract

Ferromagnetically doped topological insulators with broken time-reversal symmetry are a prerequisite for observing the quantum anomalous Hall effect. Cr-doped (Bi,Sb)2(Se,Te)3 is the most successful materials system so far, as it combines ferromagnetic ordering with acceptable levels of additional bulk doping. Here, we report a study of the local electronic structure of Cr dopants in epitaxially grown Bi2Se3 thin films. Contrary to the established view that the Cr dopant is trivalent because it substitutionally replaces Bi3+, we find instead that Cr is divalent. This is evidenced by the energy positions of the Cr K and L2,3 absorption edges relative to reference samples. The extended x-ray absorption fine structure at the K edge shows that the Cr dopants substitute on octahedral sites with the surrounding Se ions contracted by Δd=0.36 Å, in agreement with recent band structure calculations. Comparison of the Cr L2,3 x-ray magnetic circular dichroism at T=5 K with multiplet calculations gives a spin moment of 3.64 μB/Crbulk, which is close to the saturation moment for Cr2+ d4. The reduced Cr oxidation state in doped Bi2Se3 is ascribed to the formation of a covalent bond between Cr d(eg) and Se p orbitals, which is favored by the contraction of the Cr-Se distances.

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  • Received 1 August 2014
  • Revised 19 September 2014

DOI:https://doi.org/10.1103/PhysRevB.90.134402

©2014 American Physical Society

Authors & Affiliations

A. I. Figueroa1, G. van der Laan1, L. J. Collins-McIntyre2, S.-L. Zhang2, A. A. Baker1,2, S. E. Harrison2,3, P. Schönherr2, G. Cibin4, and T. Hesjedal2,4

  • 1Magnetic Spectroscopy Group, Diamond Light Source, Didcot, OX11 0DE, United Kingdom
  • 2Department of Physics, Clarendon Laboratory, University of Oxford, Oxford, OX1 3PU, United Kingdom
  • 3Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA
  • 4Diamond Light Source, Didcot, OX11 0DE, United Kingdom

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Issue

Vol. 90, Iss. 13 — 1 October 2014

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