Magnetic ordering in Cr-doped Bi ₂ Se ₃ thin films

Abstract

We report the structural and magnetic study of Cr-doped Bi2Se3 thin films using x-ray diffraction (XRD), magnetometry and polarized neutron reflectometry (PNR). Epitaxial layers were grown on c-plane sapphire by molecular beam epitaxy in a two-step process. Highresolution XRD shows the exceptionally high crystalline quality of the doped films with no parasitic phases up to a Cr concentration of 12% (in % of the Bi sites occupied by substitutional Cr). The magnetic moment, measured by SQUID magnetometry, was found to be ∼2.1 μB per Cr ion. The magnetic hysteresis curve shows an open loop with a coercive field of ∼10 mT. The ferromagnetic transition temperature was determined to be 8.5 K analyzing the magnetizationtemperature gradient. PNR shows the film to be homogeneously ferromagnetic with no enhanced magnetism near the surface or interface.

Magnetic proximity-enhanced Curie temperature of Cr-dopedBi2Se3thin films

“…Measuring the unoxidized interior of the film, we show unequivocally that Cr enters the host matrix as nominally Cr 2+ , matching very closely the calculated multiplet structure of the spectra. Using the method of Arrott plots [18], where the isothermal magnetization is measured by XMCD as a function of field, we find a T C of (7 ± 1) K, comparable to previous bulk-sensitive magnetometry measurements on the same films [15]. After in-situ evaporation of a ferromagnetic thin film of Co onto the cleaved surface at low temperatures, we find a clean interface with no evidence of alloying or any distortion of the XMCD spectra.…”

“…The films are of high crystalline quality and free of secondary phases [15] and Cr clusters [14]. The c-axis lattice constant remains virtually unchanged for the doped films as compared to Bi 2 Se 3 , supporting the finding of substitutional doping, obtained from extended x-ray absorption fine structure (EXAFS) [14].…”

“…Nevertheless, the QAHE was not observed in Mn-doped material, probably also because an increasing interstitial incorporation of Mn is observed with increasing doping concentration [12]. Cr remains the most promising dopant to achieve the QAHE [13], as it incorporates substitutionally on the Bi site [14] and leads to long-range ferromagnetic order at ∼8.5 K [15]. Recently, it has been shown that TIs proximity coupled to a ferrimagnetic insulator show an increased T C [16], suggesting the great potential of interface engineering to improve their magnetic properties.…”

“…The growth rate was ∼0.9 nm/min at a growth temperature of 300 • C. For a detailed description of the growth procedure, see Ref. [15].…”

“…The dopant concentration was calibrated using Rutherford back-scattering spectrometry (RBS) to be 4.6 at.% Cr for the investigated sample. The samples are ferromagnetic with a coercive field of 10 mT at 2 K and T C ≈ 8.5 K, as determined by the dip in the first derivative of the temperature dependent magnetization, ∂M/∂T , measured by superconducting quantum interference device (SQUID) magnetometry [15]. The electric conductivity of the film is n type with a sheet carrier density of n = −3.16 × 10 19 cm −3 at 5 K.…”