Emerging weak localization effects on a topological insulator–insulating ferromagnet (Bi${}_2$Se${}_3$-EuS) interface

Thin films of topological insulator Bi${}_2$Se${}_3$ were deposited directly on insulating ferromagnetic EuS. Unusual negative magnetoresistance was observed near the zero field below the Curie temperature $T_C$, resembling the weak localization effect; whereas the usual positive magnetoresistance was recovered above $T_C$. Such negative magnetoresistance was only observed for Bi${}_2$Se${}_3$ layers thinner than $t\sim 4$ nm, when its top and bottom surfaces are coupled. These results provide evidence for a proximity effect between a topological insulator and an insulating ferromagnet, laying the foundation for future realization of the half-integer quantized anomalous Hall effect in three-dimensional topological insulators.

Figure 1

(a) Magnetization of an EuS thin film in perpendicular magnetic fields and (b) its dependence on temperature. The black solid curve is a fitting to the Curie-Weiss law. (c) Schematic of a bilayer device: an EuS film was deposited on an Al${}_2$O${}_3$(0001) substrate, on top of it 15 nm titanium contacts with gradual height on the edges and a Bi${}_2$Se${}_3$ layer were sequentially deposited. (d) TEM image of the cross section of a bilayer device, showing the QL of Bi${}_2$Se${}_3$ and a smooth TI-IF interface. (e) Kerr angles measured at 10 K by a scanning Sagnac interferometer across the edge of the Bi${}_2$Se${}_3$ layer, where the $x<0$ region is bare EuS and $x>0$ is Bi${}_2$Se${}_3$ on top of EuS.

Figure 2

MR and its temperature dependence of PLD-grown Bi${}_2$Se${}_3$ thin films (TI) and Bi${}_2$Se${}_3$-EuS BL devices. (a), (b) The Bi${}_2$Se${}_3$-only samples have positive MR from WAL regardless of thicknesses. (c), (d) The BL devices with TI-layer thicknesses $t\gtrsim 4\mathrm{QL}$ behave similarly to the TI-only films, whereas with TI-layer thicknesses $t\lesssim 4\mathrm{QL}$ a distinctive WL-like negative MR is observed at low fields below the Curie temperature of EuS. The thickness limit coincides with the occurrence of coupling between the top and the bottom surfaces of a TI thin film.

Figure 3

Low-field MR of a TI-IF bilayer device (BL3, $t\approx 3\mathrm{QL}$) in magnetic fields perpendicular to the film. (a) Above $T_C$, a WAL-like positive MR sharpens with decreasing temperature. (b) Just below $T_C$, MR broadens with decreasing temperature. (c) Well below $T_C$, a WL-like negative MR emerges near the zero field. The emergence of WL below $T_C$ of the EuS indicates a clear TI-IF proximity effect. (d) MR of BL3 below $T=2$ K, with solid lines as illustrative guides. Resistance at $T\le 0.8\mathrm{K}$ was measured with two-terminal configurations.

Figure 4

(a) The WL and WAL features in BL4 broaden with increasing angles between the magnetic field and the normal of the sample plane, confirming their orbital origins. The inst is a zoomed-in version of low-field features. (b) Sheet resistances of the four TI-IF bilayer samples with WL features (BL1–4) and a PLD-grown TI-only film (TI3) measured with the van der Pauw method. The nonvanishing MR near parallel fields and the high resistances of the BL samples are consistent with the observed inhomogeneity in sample thickness.