YRLG Workshop: Correlation and Topology in magnetic materials, July 16th - 18th 2024
Xiaoyu Guo
Surface is present in any finite-size material systems, and its impact on phase transitions can be quite unconventional and intriguing. Theories predicted a few decades ago that surface can undergo a phase transition into an ordered state at a higher temperature than bulk does when the interaction strength at the surface exceeds that inside the bulk – named as surface and extraordinary phase transitions for surface and bulk, respectively. However, given the enhanced thermal fluctuation under the reduced dimensionality from three-dimensional (3D) to two-dimensional (2D) at the surface, the transition temperature is supposed to be suppressed. Thus, the surface and bulk phase transitions are counterintuitive and rare, with a clear experimental realization of these predictions still missing.
Van der Waals (vdW) layered magnets provide a unique platform for potentially realizing such a stringent and rare condition, due to their weak interlayer interactions. In this presentation, we leverage the surface sensitivity of electric dipole second harmonic generation (SHG) to resolve surface magnetism, the bulk nature of electric quadrupole SHG to probe bulk spin correlations, and their interference to capture the two magnetic domain states. We report our finding of surface magnetic phase transition in the A-type vdW antiferromagnet CrSBr at 140 K, 8 K higher than its bulk extraordinary phase transition temperature 132 K, corroborated by our magnetization measurements. We further use density functional theory calculations to identify two key factors for the enhanced surface magnetism — the absence of antiferromagnetic intralayer exchange coupling via pathways through neighboring layers and the enhancement of ferromagnetic intralayer exchange coupling via intra-unit cell atomic reconstructions. Our results not only prove vdW magnets as a platform for realizing long-sought-after theoretical models, but also suggest viable ways to enhance magnetic critical temperatures for 2D vdW magnets.