  The intrinsic magnetic topological insulators MnBi$_2$X$_4$ (X = Se, Te) are
promising candidates in realizing various novel topological states related to
symmetry breaking by magnetic order. Although much progress had been made in
MnBi$_2$Te$_4$, the study of MnBi$_2$Se$_4$ has been lacking due to the
difficulty of material synthesis of the desired trigonal phase. Here, we report
the synthesis of multilayer trigonal MnBi$_2$Se$_4$ with alternating-layer
molecular beam epitaxy. Atomic-resolution scanning transmission electron
microscopy (STEM) and scanning tunneling microscopy (STM) identify a
well-ordered multilayer van der Waals (vdW) crystal with septuple-layer base
units in agreement with the trigonal structure. Systematic thickness-dependent
magnetometry studies illustrate the layered antiferromagnetic ordering as
predicted by theory. Angle-resolved photoemission spectroscopy (ARPES) reveals
the gapless Dirac-like surface state of MnBi$_2$Se$_4$, which demonstrates that
MnBi$_2$Se$_4$ is a topological insulator above the magnetic ordering
temperature. These systematic studies show that MnBi$_2$Se$_4$ is a promising
candidate for exploring the rich topological phases of layered
antiferromagnetic topological insulators.
