Atomic-Scale Magnetism of Cr-Doped Bi₂Se₃ Thin Film Topological Insulators

Abstract

Magnetic doping is the most common method for breaking time-reversal-symmetry surface states of topological insulators (TIs) to realize novel physical phenomena and to create beneficial technological applications. Here we present a study of the magnetic coupling of a prototype magnetic TI, that is, Cr-doped Bi2Se3, in its ultrathin limit which is expected to give rise to quantum anomalous Hall (QAH) effect. The high quality Bi2-xCrxSe3 epitaxial thin film was prepared using molecular beam epitaxy (MBE), characterized with scanning transimission electron microscopy (STEM), electrical magnetotransport, and X-ray magnetic circularly dichroism (XMCD) techniques, and the results were simulated using density functional theory (DFT) with spin-orbit coupling (SOC). We observed a sizable spin moment mspin = (2.05 ± 0.20) μB/Cr and a small and negative orbital moment morb = (-0.05 ± 0.02) μB/Cr of the Bi1.94Cr0.06Se3 thin film at 2.5 K. A remarkable fraction of the (CrBi-CrI)(3+) antiferromagnetic dimer in the Bi2-xCrxSe3 for 0.02 < x < 0.40 was obtained using first-principles simulations, which was neglected in previous studies. The spontaneous coexistence of ferro- and antiferromagnetic Cr defects in Bi2-xCrxSe3 explains our experimental observations and those based on conventional magnetometry which universally report magnetic moments significantly lower than 3 μB/Cr predicted by Hund's rule.

Electronic structure of the chiral helimagnet and3d-intercalated transition metal dichalcogenideCr1/3NbSirica1, Mo2, Bondino3 et al. 2016Phys. Rev. B

“…A similar shoulder is reported in the Fe L 3 XAS spectrum of Fe x NbS 2 , but no claim is made as to its origin [55]. Our results suggest that the low energy shoulder in the Cr XAS spectrum originates from nonperiodically ordered Cr on the surface, which may have a lower valence as compared to bulk [56]. Additionally, the absence of sharp multiplets in the XAS spectrum in Fig.…”

“…1(b)]. The kinks in the line shape on the low BE side of each peak are consistent with the presence of a surface component, which possibly indicates surface Cr to have a lower valence as compared to bulk [56], but a definite assignment remains inconclusive. We did not attempt to fit the Cr 2p spectrum due to its complicated nature.…”

Atomic-level structural and chemical analysis of Cr-doped Bi2Se3 thin films

“…3c (data treated by Principle Component Analysis (PCA)). Similar to previous reports on the same system10, the map shows a uniform Cr distribution throughout the film without any signs of Cr diffusion into the substrate, or Cr segregation at the interface or surface of the sample (Supplementary Fig. 5).…”

“…According to first-principle calculations, doping TIs with V, Cr, Mn or Fe can break TRS and open a band gap in the surface bandstructure8. Long-range ferromagnetic order has been reported in materials such as Cr- and Sm-doped Bi 2 Se 3 791011, Mn-and Fe-doped Bi 2 Se 3 1213, and Bi 2 Te 3 1415, as well as V-, Cr- and Mn-doped Sb 2 Te 3 716. The latter system has been used to demonstrate exotic quantum phenomena such as the magnetoelectric and the quantum anomalous Hall effect (QAHE)17.…”

TCNQ Physisorption on the Topological Insulator Bi2Se3

“…The effects of impurity doping on the electronic and magnetic properties of TIs have been under debate in the last years and seem to strongly depend on the presence of intrinsic defects of the pristine crystals, on the chemical nature of the magnetic impurities,, on the type of doping (surface or bulk),,, on the possible clustering of impurities,, and, in the specific case of surface doping, on the adsorption sites, and on the orientation of the magnetic easy axis with respect to the surface plane ,,…”