Weak localization effect in topological insulator micro flakes grown on insulating ferrimagnet BaFe₁₂O₁₉

Abstract

Many exotic physics anticipated in topological insulators require a gap to be opened for their topological surface states by breaking time reversal symmetry. The gap opening has been achieved by doping magnetic impurities, which however inevitably create extra carriers and disorder that undermine the electronic transport. In contrast, the proximity to a ferromagnetic/ferrimagnetic insulator may improve the device quality, thus promises a better way to open the gap while minimizing the side-effects. Here, we grow thin single-crystal Sb1.9Bi0.1Te3 micro flakes on insulating ferrimagnet BaFe12O19 by using the van der Waals epitaxy technique. The micro flakes show a negative magnetoresistance in weak perpendicular fields below 50 K, which can be quenched by increasing temperature. The signature implies the weak localization effect as its origin, which is absent in intrinsic topological insulators, unless a surface state gap is opened. The surface state gap is estimated to be 10 meV by using the theory of the gap-induced weak localization effect. These results indicate that the magnetic proximity effect may open the gap for the topological surface attached to BaM insulating ferrimagnet. This heterostructure may pave the way for the realization of new physical effects as well as the potential applications of spintronics devices.

Figure 1. Device characteristics.

(a) The magnetic moments of the single crystal ferromagnetic insulator BaFe₁₂O₁₉ (BaM). The out-of-plane and in-plane magnetic moments are indicated by “|| c” and “⊥ c”, respectively. The magnetic moments in the two directions do not change as the temperature increases from 2 K to 50 K. Inset: the XRD pattern of the single crystal BaM. Only (00l) peaks related to the hexagonal phase can be observed. (b) The R-T curves of the BaM substrate only and the heterostructure of topological insulator and BaM, respectively. Inset: The scanning electron microscope image of the Sb_1.9Bi_0.1Te₃-BaFe₁₂O₁₉ heterostructure, with the current (I + and I-) and voltage (V + and V-) probes. The white points are redundant tellurium particles generated during cooling. The warping edges show the large lattice mismatch between Sb_1.9Bi_0.1Te₃ and BaFe₁₂O₁₉. The scale bar is 10 μm.

Figure 2. Magnetoconductivity of heterostructures.

(a) In perpendicular magnetic fields. (b) In parallel magnetic fields. The solid curves within 1 T in (a) are the fitting curves by using Eq. (2). The data curves at different temperatures are offset for clarity.

Figure 3. Transport properties of the control sample.

(a) The comparison of the magnetoconductivity in perpendicular fields between the control sample (topological insulator on SiO₂) and the topological insulator on BaM at 2 K. The black curve is the experimental data while the red curve is the fitting. The fitting yields  = 135 nm. (b) The Hall resistance of the topological insulator grown on the SiO₂ and BaM substrates, respectively.

Figure 4. Fitting results of the magnetoconductivity in perpendicular magnetic fields.

(a) The fitted Δ/2E_F as a function of temperature, where Δ is the gap of the surface states and is the Fermi level. Inset: a schematic illustration of the band structure of the topological insulator on BaM. The bulk conduction band, bulk valence band, and surface states are indicated by BCB, BVB, and SS, respectively. (b) The fitted phase coherence length, which is suppressed with increasing temperature. The relative change of Δ/2E_F with temperature is much smaller than that of the phase coherence length. The fitting is performed by using Eq. (2).