In-Plane Magnetic Anisotropy of Fe Atoms onBi2Se3(111)

Abstract

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 Bi2Se3 using local low temperature scanning tunneling microscopy and integral x-ray magnetic circular dichroism techniques. Single Fe adatoms on the Bi2Se3 surface, in the coverage range ≈ 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 ab initio approaches, we demonstrate that an in-plane easy axis arises from the combination of the crystal field and dynamic hybridization effects.Topological insulators (TI) have demanded heavy interest from the scientific community as a new class of materials illuminating fascinating yet exotic physics and offering large potential for applications in the field of spintronics [1]. TI host a gapless topological surface state (TSS) which exhibits a Dirac-cone like dispersion. However, unlike in the case of graphene, the Dirac cone is located in the center of the Brillouin-zone and the spin and momentum degrees of freedom are locked. The latter so-called topological quality of the TSS is protected by time-reversal symmetry which leads to a variety of interesting effects. For example, these materials may be a forum for Majorana fermions [2], a topological magnetoelectric effect [3], and a quantized anomalous Hall effect [4].The locking of both spin and momentum degrees of freedom leads to the suppression of 180 • elastic backscattering of TSS electrons in the absence of spin-flip processes. The robustness of these "topologically protected" processes, when introducing impurities which break time reversal symmetry, is of critical importance for spinbased transport in such materials. It has been suggested that the interaction between magnetic impurities and the topological state can cause an opening of an energy gap at the Dirac point (DP), provided that the magnetic order is oriented normal to the surface plane [5][6][7]. Nevertheless, the stability of the TSS against local magnetic perturbations is currently under heavy debate [8][9][10], since little is known about the fundamental interface effects of magnetic entities with TI surfaces as well as the static magnetic properties of impurities in these materials.In this work we present a comprehensive study of the magnetic properties of iron impurities on a prototypical TI Bi 2 Se 3 using local scanning tunneling microscopy (STM) and integral x-ray magnetic circular dichroism (XMCD) techniques. We show that the multiplet structure visible in x-ray absorption spectra reflect a high-spin state of the adsorbed Fe impurities, which are confirmed to bind at hollow sites of the surface lattice, under the influence of trigonal crystal fields given in hollow site positions of the Bi 2 Se 3 surface. In contrary to rec...

Breaking time-reversal symmetry at the topological insulator surface by metal-organic coordination networks

“…This is in agreement with angle-resolved photoemission spectroscopy (ARPES) measurements of magnetically doped bismuth chalcogenide surfaces exhibiting a feature near the DP which resembles a gap [36,37]. On the other hand, magnetic measurements found no evidence of the long-range FM order in the systems of Fe and Co adatoms at the Bi 2 Te 3 [38] and Bi 2 Se 3 [39][40][41] surfaces (the coverage in the latter case was reaching 0.9 monolayer). Accordingly, several ARPES studies report the absence of the DP gap opening in such systems [40][41][42][43][44][45].…”

“…Accordingly, several ARPES studies report the absence of the DP gap opening in such systems [40][41][42][43][44][45]. The suppression of the long-range FM order may stem from the nonuniform distribution of magnetic adsorbates on the TI surface, that tend to form clusters when their coverage is around 0.1 monolayer or higher [39,40,44,46]. This situation may be partly overcome by annealing, as it triggers the diffusion of the adsorbates inside the TI bulk, where they substitute Bi atoms [44,47,48].…”

Robust Gapless Surface State and Rashba-Splitting Bands upon Surface Deposition of Magnetic Cr on Bi2Se3

“…For example, although doping Fe and Cr into the bulk Bi 2 Se 3 crystal or thin film during the growth process leads to a gap opening at the Dirac point suggesting TRS breaking, 7,15-17 the robustness of the gapless SS against Fe deposition on the surface has been debated 18,19 and surface deposition of Cr still remains to be investigated.…”

Signatures of Dirac fermion-mediated magnetic order

“…Although numerous studies aimed to shed light on the role played by magnetic perturbations, contradictory results have been obtained and a clear picture is still missing [16][17][18][19][20][21][22][23][24][25] . Results published so far seem to suggest that surface-and bulk-doped samples behave quite differently.…”