Magnons and magnetic fluctuations in atomically thin MnBi2Te4
Electron band topology is combined with intrinsic magnetic orders in MnBi2Te4, leading to novel quantum phases. Here we investigate collective spin excitations (i.e. magnons) and spin ﬂuctuations in atomically thin MnBi2Te4 ﬂakes using Raman spectroscopy. In a two- septuple layer with non-trivial topology, magnon characteristics evolve as an external mag- netic ﬁeld tunes the ground state through three ordered phases: antiferromagnet, canted antiferromagnet, and ferromagnet. The Raman selection rules are determined by both the crystal symmetry and magnetic order while the magnon energy is determined by different interaction terms. Using non-interacting spin-wave theory, we extract the spin-wave gap at zero magnetic ﬁeld, an anisotropy energy, and interlayer exchange in bilayers. We also ﬁnd magnetic ﬂuctuations increase with reduced thickness, which may contribute to a less robust magnetic order in single layers.