  The interplay between topology and magnetism is essential for realizing novel
topological states including the axion insulator, the magnetic Weyl semimetal,
etc. An intrinsically ferromagnetic topological material with only the
topological bands at the charge neutrality energy has so far remained elusive.
By rationally designing the natural heterostructure consisting of [MnBi2Te4]
septuple layers and [Bi2Te3] quintuple layers, we report MnBi8Te13 as the first
intrinsic ferromagnetic topological material with clean low-energy band
structure. Based on the thermodynamic, transport and neutron diffraction
measurements, our data show that despite the adjacent [MnBi2Te4] being 44.1
{\AA} apart, MnBi8Te13 manifests long-range ferromagnetism below 10.5 K with
strong coupling between magnetism and charge carriers. Our first-principles
calculations and angle-resolved photoemission spectroscopy measurements further
demonstrate that MnBi8Te13 is an intrinsic ferromagnetic axion state.
Therefore, MnBi8Te13 serves as an ideal system to investigate rich emergent
phenomena, including the quantized anomalous Hall effect and quantized
topological magnetoelectric effect.
