Magnons and magnetic fluctuations in atomically thin MnBi2Te4*

Intrinsic magnetic topological insulators MnBi2Te4 host 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 magnetic field tunes the ground state through three ordered phases: anti-ferromagnet, canted antiferromagnet, and ferromagnet. Raman selection rules are determined by both the crystal symmetry and magnetic order for their respective magnetic phases. 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 from Raman spectra. Magnetic fluctuations increase with reducing layer thickness and may prevent a long-range magnetic order in a single septuple layer.