Abstract
We report the growth of Mn-doped Bi2Se3 thin films by molecular beam epitaxy (MBE), investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), SQUID magnetometry and x-ray magnetic circular dichroism (XMCD). Epitaxial films were deposited on c-plane sapphire substrates by co-evaporation. The films exhibit a spiral growth mechanism typical of this material class, as revealed by AFM. The XRD measurements demonstrate a good crystalline structure which is retained upon doping up to ∼7.5 atomic-% Mn, determined by Rutherford backscattering spectrometry (RBS), and show no evidence of the formation of parasitic phases. However an increasing interstitial incorporation of Mn is observed with increasing doping concentration. A magnetic moment of 5.1 μB/Mn is obtained from bulk-sensitive SQUID measurements, and a much lower moment of 1.6 μB/Mn from surface-sensitive XMCD. At ∼2.5 K, XMCD at the Mn L2,3 edge, reveals short-range magnetic order in the films and indicates ferromagnetic order below 1.5 K.
Highlights
We report the growth of Mn-doped Bi2Se3 thin films by molecular beam epitaxy (MBE), investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), superconducting quantum interference device (SQUID) magnetometry and x-ray magnetic circular dichroism (XMCD)
Bi2Se3 films grow via van der Waals epitaxy, for which high quality films can be obtained on substrates that are not lattice-matched with the film.[22]
It is found that Mn dopants may be introduced into the host Bi2Se3 without the formation of additional parasitic phases or significant degradation to the host crystal structure up to a doping concentration of ∼7.5 at% Mn
Summary
The Dirac-like band dispersion of the topological surface state (TSS) present in chalcogenide (Bi, Sb)2(Se, Te)[3] three-dimensional topological insulators (TIs) has been subject to intense study since the discovery of its existence.[1,2,3] Application of an out-of-plane magnetic field or the presence of ferromagnetic order in a TI breaks time reversal symmetry (TRS) and may introduce a bandgap at the Dirac point,[4,5,6] opening up a plethora of exotic quantum phenomena such as the topological magneto-electric effect,[7] magnetic (image) monopole,[4], and quantum anomalous Hall effect.[8]. There is little consensus in the literature regarding the magnetic order in the Mn-doped chalcogenide TIs, despite the fact that a uniform distribution of the magnetic ordering is found by polarized neutron reflectometry[20] and muon spin rotation.[14] For example, in the case of Mn-doped Bi2Se3, a nanoscale surface segregation of Mn was found, leading to surface magnetism with a reported transition temperature on the order of 100 K, while it was 5 K for the bulk of the film.[21]. The films show high structural quality as confirmed by atomic force microscopy (AFM) and x-ray diffraction (XRD) up to a doping concentration of ∼7.5 atomic percent (at%) Mn. The magnetic properties were investigated by superconducting quantum interference device (SQUID) magnetometry and x-ray magnetic circular dichroism (XMCD). The magnetic moments, as determined by the bulk-sensitive SQUID magnetometry and surface-sensitive XMCD, disagree considerably; possible origins of this observation are discussed
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