Novel magnetic topological materials pave the way for studying the interplay between band topology and magnetism. However, an intrinsically ferromagnetic topological material with only topological bands at the charge neutrality energy has so far remained elusive. Using rational design, we synthesized MnBi8Te13, a natural heterostructure with [MnBi2Te4] and [Bi2Te3] layers. Thermodynamic, transport, and neutron diffraction measurements show that despite the adjacent [MnBi2Te4] being 44.1 Å apart, MnBi8Te13 manifests long-range ferromagnetism below 10.5 K with strong coupling between magnetism and charge carriers. First-principles calculations and angle-resolved photoemission spectroscopy measurements reveal it is an axion insulator with sizable surface hybridization gaps. Our calculations further demonstrate the hybridization gap persists in the two-dimensional limit with a nontrivial Chern number. Therefore, as an intrinsic ferromagnetic axion insulator with clean low-energy band structures, MnBi8Te13 serves as an ideal system to investigate rich emergent phenomena, including the quantized anomalous Hall effect and quantized magnetoelectric effect.

We have systematically investigated the Sb-doping effect in the newly discovered Z$_2$ AFM topological insulator MnBi$_4$Te$_7$ through the transport and thermodynamic measurements and mapped out the doping-temperature phase diagram of Mn(Bi$_{1-x}$Sb$_x$)$_4$Te$_7$ ($x\leq 0.83$). The only Mn1 sublattice in MnBi$_4$Te$_7$ undergoes a paramagnetic to antiferromagnetic phase transition at T${_N}=12.7$ K. While T$_N$ remains almost unchanged with doping, an additional antiferromagnetic to ferromagnetic transition of the Mn1 sublattice occurs at lower temperature T$_c$ which monotonically increases and takes over T$_N$ at $x=0.68$. Meanwhile, upon doping, the Mn/Sb antisite disorders give rise to and strengthen the additional Mn2 and Mn3 sublattices. Despite no clear sign of the Mn2/Mn3-sublattice ordering at low dopings, at $x>0.12$, in concert with the antiferromagnetic to ferromagnetic transition of the Mn1 sublattice, Mn2/Mn3 sublattices develop long-range order which couples antiferromagnetically to the Mn1 sublattice, leading to a ferrimagnetic ground state below T$_c$ in this series. We argue the magnetic dilution and the ordering of the Mn2/Mn3 sublattices are responsible for the ferromagnetic switching of the Mn1 sublattice.

The search for materials to support the Quantum Anomalous Hall Effect (QAHE) have recently centered on intrinsic magnetic topological insulators (MTIs) including MnBi$_2$Te$_4$ or heterostructures made up of MnBi$_2$Te$_4$ and Bi$_2$Te$_3$. While MnBi$_2$Te$_4$ is itself a MTI, most recent ARPES experiments indicate that the surface states on this material lack the mass gap that is expected from the magnetism-induced time-reversal symmetry breaking (TRSB), with the absence of this mass gap likely due to surface magnetic disorder. Here, utilizing small-spot ARPES scanned across the surfaces of MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$, we show the presence of large mass gaps (~ 100 meV scale) on both of these materials when the MnBi$_2$Te$_4$ surfaces are buried below one layer of Bi$_2$Te$_3$ that apparently protects the magnetic order, but not when the MnBi$_2$Te$_4$ surfaces are exposed at the surface or are buried below two Bi$_2$Te$_3$ layers. This makes both MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$ excellent candidates for supporting the QAHE, especially if bulk devices can be fabricated with a single continuous Bi$_2$Te$_3$ layer at the surface.

The search for materials to support the Quantum Anomalous Hall Effect (QAHE) have recently centered on intrinsic magnetic topological insulators (MTIs) including MnBi_2Te_4 or heterostructures made up of MnBi_2Te_4 and Bi_2Te_3. While MnBi_2Te_4 is itself a MTI, most recent ARPES experiments indicate that the surface states on this material lack the mass gap that is expected from the magnetism-induced time-reversal symmetry breaking (TRSB), with the absence of this mass gap likely due to surface magnetic disorder. Here, utilizing small-spot ARPES scanned across the surfaces of MnBi_4Te_7 and MnBi_6Te_10, we show the presence of large mass gaps (  100 meV scale) on both of these materials when the MnBi_2Te_4 surfaces are buried below one layer of Bi_2Te_3 that apparently protects the magnetic order, but not when the MnBi_2Te_4 surfaces are exposed at the surface or are buried below two Bi_2Te_3 layers. This makes both MnBi_4Te_7 and MnBi_6Te_10 excellent candidates for supporting the QAHE, especially if bulk devices can be fabricated with a single continuous Bi_2Te_3 layer at the surface.