Anomalous Hall effect in a magnetically extended topological insulator heterostructure

Constructing heterostructures of a topological insulator (TI) with an undoped magnetic insulator (MI) is a clean and versatile approach to break the time-reversible symmetry in the TI surface states. Despite a lot of efforts, the strength of the interfacial magnetic proximity effect (MPE) is still too weak to achieve the quantum anomalous Hall effect and many other topological quantum phenomena. Recently, a new approach, “magnetic extension,” was proposed to achieve strong MPE [Otrokov et al., 2D Mater. 4, 025082 (2017)]. This approach is demonstrated effective by intercalation of the MI layer to the TI [Hirahara et al., Nano Lett. 17, 3493 (2017)]. Motivated by this proposal, here we study a magnetic extension system prepared by molecular beam epitaxial growth of MnSe thin films on a topological insulator ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$. Direct evidence is obtained for intercalation of the MnSe atomic layer into a few quintuple layers of ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$, forming either a double magnetic septuple layer (SL) or an isolated single SL at the interface, where one SL denotes a van der Waals building block consisting of B-A-B-Mn-B-A-B ($\mathrm{A}={\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$, $\mathrm{B}={\mathrm{Te}}_{1-y}{\mathrm{Se}}_y$). The two types of interfaces (namely, TI/mono-SL and TI/bi-SL) have different MPE, which is manifested as distinctively different transport behaviors. Specifically, the mono-SL induces a spin-flip transition with a sharp change at a small magnetic field in the anomalous Hall effect of the TI layers, while the bi-SL induces a spin-flop transition with a slow change at large field. Our work provides a useful platform to realize the full potential of the magnetic extension approach for pursuing novel topological physics and related device applications.

Figure 1

(a) Schematic sketch of a back-gated Hall bar device fabricated on the MnSe/${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$ heterostructure. (b) Temperature dependences of sheet resistance $R_{\square }$ of heterostructures with different ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$ growth thickness of 3, 5, 8, and 13 nm, respectively. (c) Cross-section scanning transmission electron microscopy (STEM) image of the nominal MnSe/${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$(5 nm/13 nm) grown on the STO substrate. The insets are the sketches of quintuple layer (QL) and septuple layer (SL, due to intercalation of Mn to QL). QL consists of Te-Bi(Sb)-Te-Bi(Sb)-Te and SL consists of Te(Se)-Bi(Sb)-Te(Se)-Mn-Te(Se)-Bi(Sb)-Te(Se) monoatomic layers. Te(Se): yellow solid circle; Bi(Sb): purple solid circle; Mn: red solid circle. Mixed interfacial layers of SL-QL-SL-SL-QL are observed along the $c$ axis. (d) Magnetoresistances of the heterostructure shown in (c) measured at different temperatures from 1.7 to 10 K and 8-nm-thick ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$ thin film at 1.7 K. Hikami-Larkin-Nagaoka (HLN) equation fitting curves are shown by the red lines. The ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$ thin film can be well fitted to the HLN equation that describes the weak-antilocalization effect (WAL) behavior at low field, while MnSe/${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$ deviates strongly from HLN fitting.

Figure 2

(a) Gate-voltage tuning of Hall resistance $R_{\mathrm{xy}}$ in the nominal MnSe/${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$(5 nm/13 nm) heterostructure at $T=1.7\mathrm{K}$. The gate voltage is varied from −210 to 210 V with the carriers changed from $p$ type (negative slope) to $n$ type (positive slope). The red and black curves represent magnetic field sweeping to the negative and positive directions, respectively. (b) Schematic band diagrams for the top and bottom surface states at varied back-gated voltages.

Figure 3

Hall resistance $R_{\mathrm{xy}}$ (a) and in-plane sheet resistance $R_{\square }$ (b) plotted as a function of magnetic field from 1.7 to 15 K in MnSe/${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2{\mathrm{Te}}_3$. With increasing temperature, the hysteresis loop becomes smaller and almost disappears above 8 K. The scale bar is 50 Ω.

Figure 4

(a) Magnetic field dependence of anomalous Hall conductance ${\sigma }_{\mathrm{AH}}$ at $V_{\mathrm{G}}=0\mathrm{V}$. The red and black curves represent magnetic field sweeping to the negative and positive directions, respectively. The four insets are the schematic drawings of the spin-switching process in the Mn-intercalated layers at the ${\mathrm{H}}_1$, ${\mathrm{H}}_2$, ${\mathrm{H}}_3$, and ${\mathrm{H}}_4$ magnetic fields. The red arrows in the insets show the spin orientation of Mn atoms. The lower two insets show the states at ${\mathrm{H}}_1$ and ${\mathrm{H}}_2$, respectively. The upper two insets show the states at ${\mathrm{H}}_3$ and ${\mathrm{H}}_4$, respectively. (b) Magnetic field dependence of Hall conductance $\mathrm{\Delta }{\sigma }_{\mathrm{xy}}^{\mathrm{up}-\mathrm{down}}$ (the difference value of sweep-up and sweep-down Hall conductance) at different gate voltages. All the Hall data are taken from the heterostructure sample at 1.7 K. The scale bar is $0.003{\mathrm{e}}^2/\mathrm{h}$.