Magnons and magnetic fluctuations in atomically thin MnBi2Te4
Dr. Shang-Fan Lee and his postdoc L. –J. Chang, in collaboration with Prof. Elaine Li at Univ. Texas, Austin, studied the magnetic topological material MnBi2Te4. They investigate collective spin excitations, 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: 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, they extract the spin-wave gap at zero magnetic field, an anisotropy energy, and interlayer exchange in bilayers. Magnetic fluctuations increase with reduced thickness, which may contribute to a less robust magnetic order in single layers.
