Abstract
Magnetic doping of topological insulators (TIs) is crucial for unlocking novel quantum phenomena, paving the way for spintronics applications. Recently, we have shown that doping with rare earth ions introduces large magnetic moments and allows for high doping concentrations without the loss of crystal quality, however no long range magnetic order was observed. In Dy-doped Bi2Te3 we found a band gap opening above a critical doping concentration, despite the paramagnetic bulk behavior. Here, we present a surface-sensitive x-ray magnetic circular dichroism (XMCD) study of an in situ cleaved film in the cleanest possible environment. The Dy M4,5 absorption spectra measured with circularly polarized x-rays are fitted using multiplet calculations to obtain the effective magnetic moment. Arrott–Noakes plots, measured by the Dy M5 XMCD as a function of field at low temperatures, give a negative transition temperature. The evaporation of a ferromagnetic Co thin film did not introduce ferromagnetic ordering of the Dy dopants either; instead a lowering of the transition temperature was observed, pointing towards an antiferromagnetic ordering scenario. This result shows that there is a competition between the magnetic exchange interaction and the Zeeman interaction. The latter favors the Co and Dy magnetic moments to be both aligned along the direction of the applied magnetic field, while the exchange interaction is minimized if the Dy and Co atoms are antiferromagnetically coupled, as in zero applied field.
Highlights
Magnetic doping of topological insulators (TIs) [1], resulting in the breaking of time-reversal symmetry (TRS), is crucial for unlocking novel quantum phenomena [2]
We studied the magnetic properties of the surfaces of Dy-doped Bi2Te3 in greater detail using surface-sensitive x-ray magnetic circular dichroism (XMCD)
The magnetic order on the surfaces was studied in detail using the Arrott–Noakes analysis of the XMCD hysteresis plots as a function of temperature
Summary
Magnetic doping of topological insulators (TIs) [1], resulting in the breaking of time-reversal symmetry (TRS), is crucial for unlocking novel quantum phenomena [2]. In three-dimensional TIs, the topological surface state (TSS) is protected by TRS, and magnetic doping leading to long-range ferromagnetic order with outof-plane anisotropy is one possibility to open a gap in the TSS [3]. Such a magnetically induced band gap was first experimentally confirmed in Mn-doped Bi2Se3 using angle-resolved photoemission electron spectroscopy (ARPES) [4]. Despite the paramagnetic bulk behavior of Dy-doped films, there is the possibility of surface ferromagnetic ordering via the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction between the localized Dy moments and the TSS [1,3,15,16].
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