Two-dimensional ferromagnetic extension of a topological insulator

Inducing a magnetic gap at the Dirac point of the topological surface state (TSS) in a three-dimensional (3D) topological insulator (TI) is a route to dissipationless charge and spin currents. Ideally, magnetic order is present only at the surface, as through proximity of a ferromagnetic (FM) layer. However, experimental evidence of such a proximity-induced Dirac mass gap is missing, likely due to an insufficient overlap of TSS and the FM subsystem. Here, we take a different approach, namely ferromagnetic extension (FME), using a thin film of the 3D TI ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, interfaced with a monolayer of the lattice-matched van der Waals ferromagnet ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. Robust 2D ferromagnetism with out-of-plane anisotropy and a critical temperature of $T_c\approx 15$ K is demonstrated by x-ray magnetic dichroism and electrical transport measurements. Using angle-resolved photoelectron spectroscopy, we observe the opening of a sizable magnetic gap in the 2D FM phase, while the surface remains gapless in the paramagnetic phase above $T_c$. Ferromagnetic extension paves the way to explore the interplay of strictly 2D magnetism and topological surface states, providing perspectives for realizing robust quantum anomalous Hall and chiral Majorana states.

Figure 1

(a) Scheme of a magnetically extended 3D TI in the ferro- (FME) and paramagnetic (PME) cases. Below $T_c$, ferromagnetic order in the out-of-plane direction induces a magnetic gap in the topological surface state. (b) 3D ARPES data set of the topological surface state, measured above $T_c$ at $T=20$ K using $p$-polarized light at $h\nu =12$ eV. (c) X-ray diffraction $Q_z$ scan around the (0006) Bragg peak of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. The line-shape asymmetry is attributed to the single ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ layer, as confirmed by calculations. (d) Schematic of the layer structure. The 3D TI ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ is terminated by a single layer of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ containing the magnetically active Mn atomic sheet. (e), (f) Cross-sectional STEM image revealing the sample layer stacking: ${\mathrm{BaF}}_2$ (111) substrate, 6 quintuple-layer ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ film, single septuple-layer (SL) ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, and Te-capping layer. (g) Intensity profile analysis along the trace shown in (e).

Figure 2

(a) XAS and XMCD data confirming a stable ferromagnetic state of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. (i) XAS data set at the Mn $L_{2,3}$ edge. (ii), (iii) XMCD difference spectra at $T=3.5$ K for a saturation field of 0.5 T and in remanence. (b) XMCD hysteresis at the same temperature indicating long-range ferromagnetic order. (c) Anomalous Hall effect (AHE) in a single-layer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4/{\mathrm{Bi}}_2{\mathrm{Te}}_3$ heterostructure vs magnetic field, at several temperatures. The arrows indicate the magnetic field sweep direction. Curves are vertically shifted for clarity. A clear hysteretic behavior of the AHE is observed below $18\mathrm{K}$, with coercive fields up to $90\mathrm{m}\mathrm{T}$, confirming the ferromagnetic behavior of the single ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ layer. The inset shows a schematic of the pseudo-Hall bar geometry of the transport experiments. (d) Temperature dependence of the remnant XMCD signal yielding a critical temperature of 14.9 K. (e) Temperature dependence of the AHE amplitude at zero field extracted from the data in (c).

Figure 3

(a) ARPES data acquired below (top row) and above (bottom row) the critical temperature $T_c$ using $p$-polarized light at $h\nu =12$ eV. Data sets acquired perpendicular [(i) and (iii)] and along [(ii) and (iv)] the plane of incidence are shown. (v) and (vi) display energy distribution curves (EDC) at the the $\overline{\mathrm{\Gamma }}$ point, revealing a magnetic exchange splitting at the Dirac point below $T_c$. (b) Temperature-dependent EDC at the $\overline{\mathrm{\Gamma }}$ point, showing the spectral-weight evolution across $T_c$. (c) Momentum-dependent EDC above and below $T_c$, taken from the data sets in (a). The energy scales are relative to the respective Dirac-point position, to compensate for a small temperature-dependent energy shift. The ferromagnetic order affects the spectral weight predominantly in a narrow momentum range of $\delta k=\pm 0.01{\text{Å}}^{-1}$ around the $\overline{\mathrm{\Gamma }}$ point.