Abstract
Breaking the time reversal symmetry of a topological insulator, for example by the presence of magnetic ions, is a prerequisite for spin-based electronic applications in the future. In this regard Mn-doped Bi2Te3 is a prototypical example that merits a systematic investigation of its magnetic properties. Unfortunately, Mn doping is challenging in many host materials—resulting in structural or chemical inhomogeneities affecting the magnetic properties. Here, we present a systematic study of the structural, magnetic and magnetotransport properties of Mn-doped Bi2Te3 single crystals using complimentary experimental techniques. These materials exhibit a ferromagnetic phase that is very sensitive to the structural details, with TC varying between 9 and 13 K (bulk values) and a saturation moment that reaches 4.4(5) μB per Mn in the ordered phase. Muon spin rotation suggests that the magnetism is homogeneous throughout the sample. Furthermore, torque measurements in fields up to 33 T reveal an easy axis magnetic anisotropy perpendicular to the ab-plane. The electrical transport data show an anomaly around TC that is easily suppressed by an applied magnetic field, and also anisotropic behavior due to the spin-dependent scattering in relation to the alignment of the Mn magnetic moment. Hall measurements on different crystals established that these systems are n-doped with carrier concentrations of ∼ 0.5–3.0 × 1020 cm−3. X-ray magnetic circular dichroism (XMCD) at the Mn L2,3 edge at 1.8 K reveals a large spin magnetic moment of 4.3(3) μB/Mn, and a small orbital magnetic moment of 0.18(2) μB/Mn. The results also indicate a ground state of mixed d4–d5–d6 character of a localized electronic nature, similar to the diluted ferromagnetic semiconductor Ga1−xMnxAs. XMCD measurements in a field of 6 T give a transition point at T ≈ 16 K, which is ascribed to short range magnetic order induced by the magnetic field. In the ferromagnetic state the easy direction of magnetization is along the c-axis, in agreement with bulk magnetization measurements. This could lead to gap opening at the Dirac point, providing a means to control the surface electric transport, which is of great importance for applications.
Highlights
One of the first studies to show the interplay between ferromagnetism and the breaking of time reversal symmetry in a TI was reported by Hor et al [6] in Mn-doped Bi2Te3
superconducting quantum interference device (SQUID) magnetometry was performed as a function of temperature and magnetic field, in different orientations with respect to the ab-plane, using a 7 T quantum design magnetic properties measurement system, with results shown in figure 5
The demonstrated ferromagnetic order in our n-type Mn-doped Bi2Te3 crystals has a similar ordering temperature to the p-type crystals [6]. This is in contrast to the well-known diluted magnetic semiconductors (DMSs) Ga1−x Mnx As, in which the carrier-induced ferromagnetism is only achieved in p-type samples, and the Curie temperature has a dependence on carrier density [54, 55]
Summary
Mn-doped Bi2Te3 single crystals, typically measuring 2–3 cm in length and 1 cm in diameter, were grown using high purity (>99.99%) Bi, Te and Mn. Transmission electron microscopy (TEM) shows the presence of nm-scale Mn-rich precipitates in the xn = 0.15 crystal (see figure 3(b)), whereas the xn = 0.09 crystal (see in figure 3(a)) does not show any evidence of precipitation This suggests that Mn incorporates in solid solution in the xn = 0.09 sample but as the concentration of Mn in the melt is increased, c/a ratio. The results clearly show a quasi-periodic μm-scale variation in c/a ratio in the xn = 0.09 crystal, whereas (for the given spatial resolution) the xn = 0.15 crystal is found to be more homogeneous (see figure 4) This interesting microstructure is thought to develop as a result of the internal stresses in the material produced by the Mn substitution, and may be a result of the local compositional variations suggested by the EDX results
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