While the surface-bulk correspondence has been ubiquitously shown in topological phases, the relationship between surface and bulk in Landau-like phases is much less explored. Theoretical investigations since 1970s for semi-infinite systems have predicted the possibility of the surface order emerging at a higher temperature than the bulk, clearly illustrating a counterintuitive situation and greatly enriching phase transitions. But experimental realizations of this prediction remain missing. Here, we demonstrate the higher-temperature surface and lower-temperature bulk phase transitions in CrSBr, a van der Waals (vdW) layered antiferromagnet. 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. Our density functional theory calculations show the suppression of ferromagnetic-antiferromagnetic competition at the surface responsible for this enhanced surface magnetism. Our results not only show unexpected, richer phase transitions in vdW magnets, but also provide viable ways to enhance magnetism in their 2D form.
@article{arxiv.2309.01047,
title = {"Extraordinary" Phase Transition Revealed in a van der Waals Antiferromagnet},
author = {Xiaoyu Guo and Wenhao Liu and Jonathan Schwartz and Suk Hyun Sung and Dechen Zhang and Makoto Shimizu and Aswin L. N. Kondusamy and Lu Li and Kai Sun and Hui Deng and Harald O. Jeschke and Igor I. Mazin and Robert Hovden and Bing Lv and Liuyan Zhao},
journal= {arXiv preprint arXiv:2309.01047},
year = {2023}
}