Guo Xiaoyu, Liu Wenhao, Schwartz Jonathan, Sung Suk Hyun, Zhang Dechen, Shimizu Makoto, Kondusamy Aswin L N, Li Lu, Sun Kai, Deng Hui, Jeschke Harald O, Mazin Igor I, Hovden Robert, Lv Bing, Zhao Liuyan
Department of Physics, University of Michigan, Ann Arbor, MI, USA.
Department of Physics, the University of Texas at Dallas, Richardson, TX, USA.
Nat Commun. 2024 Jul 31;15(1):6472. doi: 10.1038/s41467-024-50900-1.
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 is responsible for this enhanced surface magnetism. Our results not only show counterintuitive, richer phase transitions in vdW magnets, but also provide viable ways to enhance magnetism in their 2D form.
虽然表面-体相对应关系在拓扑相中已被广泛证明，但在类朗道相的表面和体相之间的关系却很少被研究。自20世纪70年代以来，对半无限系统的理论研究预测了表面序在高于体相的温度下出现的可能性，清楚地说明了一种违反直觉的情况，并极大地丰富了相变。但这一预测的实验实现仍然缺失。在这里，我们展示了范德华（vdW）层状反铁磁体CrSBr中高温表面和低温体相的相变。我们利用电偶极二次谐波产生（SHG）的表面敏感性来解析表面磁性，利用电四极SHG的体相性质来探测体相自旋相关性，以及它们的干涉来捕获两种磁畴状态。我们的密度泛函理论计算表明，表面铁磁-反铁磁竞争的抑制是这种增强的表面磁性的原因。我们的结果不仅表明vdW磁体中存在违反直觉、更丰富的相变，还提供了在其二维形式中增强磁性的可行方法。
