A van der Waals antiferromagnetic topological insulator with weak interlayer magnetic coupling

Abstract

Magnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential technical revolutions. By realizing a bulk van der Waals material MnBi₄Te₇ with alternating septuple [MnBi₂Te₄] and quintuple [Bi₂Te₃] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane saturation field of ~0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calculations further demonstrate that MnBi₄Te₇ is a Z₂ antiferromagnetic TI with two types of surface states associated with the [MnBi₂Te₄] or [Bi₂Te₃] termination, respectively. Additionally, its superlattice nature may make various heterostructures of [MnBi₂Te₄] and [Bi₂Te₃] layers possible by exfoliation. Therefore, the low saturation field and the superlattice nature of MnBi₄Te₇ make it an ideal system to investigate rich emergent phenomena.

Fig. 1. Magnetic and transport properties of bulk AFM MnBi₄Te₇.

a The view of the crystal structure of MnBi₄Te₇ from the [110] directions. Red arrow: Mn spins in the A-type AFM state. Blue block: edge-sharing BiTe₆ octahedra; Pink block: edge-sharing MnTe₆ octahedra, which are connected to the blue block via edge-sharing. J_⊥ is the interlayer exchange coupling. b The (00l) X-ray diffraction peaks of the cleaved ab plane of MnBi₄Te₇. Inset: A piece of MnBi₄Te₇ against 1-mm scale. c The temperature dependent field-cooled susceptibility and inverse susceptibility taken at H = 0.1 T for H || ab and H || c. Average χ is calculated by ${\chi }^{\mathrm{ave}}=\left(2{\chi }^{\mathrm{ab}}+{\chi }^{\mathrm{c}}\right)∕3$. d and e: Full magnetic hysteresis loop of isothermal magnetization taken at various temperatures for: d H || c and e H || ab. f The temperature dependent ρ_xx (I || ab) and ρ_zz (I || c). g Transverse magnetoresistance with I || ab and H || c at various temperatures.

Fig. 2. Magnetotransport of bulk AFM MnBi₄Te₇.

a The field dependent magnetization M, transverse magnetoresistivity of ρ_xx, and Hall resistivity ρ_xy at 2 K with I || ab and H || c. b The longitudinal magnetoresistivity of ρ_zz, at 2 K with I || H || c. c The field dependent magnetization M with H || ab at 2 K. d The longitudinal magnetoresistivity of ρ_xx, at 2 K with I || H || ab. e The transverse magnetoresistivity of ρ_zz, at 2 K with I || c and H || ab.

Fig. 3. Topological properties of bulk AFM MnBi₄Te₇ predicted by first-principles calculations.

a Band structure with the projection of Bloch eigenstates onto Bi-p (blue) and Te-p (red) orbitals. SOC is included. b Evolution of Wannier charge centers (WCCs) for k_z = 0, indicating a nontrivial topological invariant Z₂ = 1. c Surface spectra of (010) side surface, showing a gapless Dirac cone protected by S-symmetry.

Fig. 4. Comparison between ARPES-measured and DFT-calculated surface states.

a–c The DFT-calculated k-E map along $\overline{\mathrm{K}}\leftarrow \overline{\mathrm{\Gamma }}\to \overline{\mathrm{K}}$ on the [MnBi₂Te₄] SL termination: a surface and bulk (S+B) spectrum, b bulk only, and c surface only. d, e The experimental ARPES spectrum on the [MnBi₂Te₄] SL termination obtained with 47 eV, linear horizontal light: d along Μ←Γ→Μ, e along Κ←Γ→Κ high symmetry direction. f, g The DFT-calculated k-E map along $\overline{\mathrm{K}}\leftarrow \overline{\mathrm{\Gamma }}\to \overline{\mathrm{K}}$ on the [Bi₂Te₃] QL termination: f surface and bulk (S+B) spectrum, g surface only. h, i The experimental ARPES spectrum on the [Bi₂Te₃] QL termination obtained with 47 eV, linear horizontal light: h along Μ←Γ→Μ, i along Κ←Γ→Κ high symmetry direction. j The EDC plot at the Γ point (blue-line cut in i) showing three main peaks corresponding to the bulk conduction band, surface conduction band, and mixed surface/bulk valence band. The green curve shows the fitted Voigt profile peaks which sum to the blue curve.

Fig. 5. Experimental and theoretical constant energy slices.

a, b ARPES constant energy surfaces sliced at every 50 meV. c The same contours calculated by DFT for the [Bi₂Te₃] QL termination. The six-fold symmetric snowflake-like surfaces are seen in all cases.