  The intrinsic magnetic layered topological insulator MnBi2Te4 with nontrivial
topological properties and magnetic order has become a promising system for
exploring exotic quantum phenomena such as quantum anomalous Hall effect.
However, the layer-dependent magnetism of MnBi2Te4, which is fundamental and
crucial for further exploration of quantum phenomena in this system, remains
elusive. Here, we use polar reflective magnetic circular dichroism
spectroscopy, combined with theoretical calculations, to obtain an in-depth
understanding of the layer-dependent magnetic properties in MnBi2Te4. The
magnetic behavior of MnBi2Te4 exhibits evident odd-even layer-number effect,
i.e. the oscillations of the coercivity of the hysteresis loop (at {\mu}0Hc)
and the spin-flop transition (at {\mu}0H1), concerning the Zeeman energy and
magnetic anisotropy energy. In the even-number septuple layers, an anomalous
magnetic hysteresis loop is observed, which is attributed to the
thickness-independent surface-related magnetization. Through the linear-chain
model, we can clarify the odd-even effect of the spin-flop field and determine
the evolution of magnetic states under the external magnetic field. The
mean-field method also allows us to trace the experimentally observed magnetic
phase diagrams to the magnetic fields, layer numbers and especially,
temperature. Overall, by harnessing the unusual layer-dependent magnetic
properties, our work paves the way for further study of quantum properties of
MnBi2Te4.

Materials Science