  We report direct visualization of spin-flip transition of the surface layer
in antiferromagnet MnBi4Te7, a natural superlattice of alternating MnBi2Te4 and
Bi2Te3 layers, using cryogenic magnetic force microscopy (MFM). The observation
of magnetic contrast across domain walls and step edges confirms that the
antiferromagnetic order persists to the surface layers. The magnetic field
dependence of the MFM images reveals that the surface magnetic layer undergoes
a first-order spin-flip transition at a magnetic field that is lower than the
bulk transition, in excellent agreement with a revised Mills' model. Our
analysis indicates an enhancement of the order parameter in the surface
magnetic layer, implying robust ferromagnetism in the single-layer limit. The
direct visualization of surface spin-flip transition not only opens up
exploration of surface metamagnetic transitions in layered antiferromagnets,
but also provides experimental support for realizing quantized transport in
ultra-thin films of MnBi4Te7 and other natural superlattice topological
magnets.
