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

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.

percentage is used to correct the value obtained in our magnetic property measurements.
Supplementary Table 1 and Supplementary Table 2 summarize the detailed crystal structure of MnBi4Te7 obtained from the refinement of our powder X-ray diffraction pattern.  Supplementary Figure 2 shows Heat Capacity measured at zero field near the Neel

Supplementary
Temperature. Supplementary Figure 3 shows the temperature-dependent resistivity of ( ) (I // ab) and ( ) (I // c) measured at zero field from 2 to 300 K.
Supplementary Figure 4 shows the bulk Hall Resistivity ( ). The hysteresis feature shown at 2 K vanishes near 6 K but the step-feature due to the spin-flip transition persists up to TN. As temperature further increases above TN, the step feature is broadened gradually with weaker spin fluctuations until becomes completely linear above 50 K.

Supplementary Note 2: Exchange coupling of bulk MnBi4Te7
MnBi4Te7 consists of one MnBi2Te4 layer and one Bi2Te3 layer alternately stacked.
The magnetic moments in ARPES (Supplementary Figure 7). Therefore, we studied the surface state of the apparent Dirac cone on the [MnBi2Te4] SL termination carefully with numerical simulation. In our simulations, we estimate the Fermi velocity of the simulated Dirac cone from our data, and all instrument resolution functions were assumed to be Gaussian. During the simulation, we fix E since E arises from the presence of the sample's Fermi edge; we make angle , Σ and Δ as the free parameters since Σ and Δ are unknown and angle is coupled to Σ. Then we closely monitor angle so that it is no smaller than the instrument manufacturer's specification. When we simulate a spectrum, there are several features we need to keep track of to ensure the simulation resembles the measured spectra: 1) the MDC linewidths, 2) the size of the measured gap, and 3) if there is a gap, the contrast of the gapped region to the peak of the bands above and below.

Supplementary
Our simulations help reveal the complex effect of each individual term has on the measured ARPES spectra. Increasing Σ broadens the APRES spectra primarily in energy. If the system is intrinsically gapped, a large Σ can reduce the contrast of the gap, making the gap appear smaller and even eliminating it if the gap is small enough.
On the other hand, angle is an important parameter that typically exaggerate the size of gap, or manifest one even if it does not exist.
Supplementary Figure 8(a-c) show examples of simulated ARPES E-k map, to be compared with 8(d) taken on the [MnBi2Te4] SL surface. We found that if the intrinsic gap ∆ is greater than 10 meV, to produce a spectrum similar to our experimental data, angle had to be set less than 0.1°, the instrument manufacturer's specified angular resolution, which is unlikely. Furthermore, the gapped region begins to show significant contrast that we don't see in our ARPES experiment. Therefore, we conclude that the surface state on the [MnBi2Te4] SL has a gap smaller than 10 meV, if the gap exists at all.