In-plane magnetic anisotropy of Fe atoms on Bi$_2$Se$_3$(111)

J. Honolka, A. A. Khajetoorians, V. Sessi, T. O. Wehling, S. Stepanow, J. -L. Mi, B. B. Iversen, T. Schlenk, J. Wiebe, N. Brookes, A. I. Lichtenstein, Ph. Hofmann, K. Kern, R. Wiesendanger

Published 2011-12-20, updated 2011-12-22Version 2

The robustness of the gapless topological surface state hosted by a 3D topological insulator against perturbations of magnetic origin has been the focus of recent investigations. We present a comprehensive study of the magnetic properties of Fe impurities on a prototypical 3D topological insulator Bi$_2$Se$_3$ using local low temperature scanning tunneling microscopy and integral x-ray magnetic circular dichroism techniques. Single Fe adatoms on the Bi$_2$Se$_3$ surface, in the coverage range $\approx 1%$ are heavily relaxed into the surface and exhibit a magnetic easy axis within the surface-plane, contrary to what was assumed in recent investigations on the opening of a gap. Using \textit{ab initio} approaches, we demonstrate that an in-plane easy axis arises from the combination of the crystal field and dynamic hybridization effects.