Department of Physics, University of Michigan, 450 Church Street, Ann Arbor, Michigan, 48109, USA.
Institute for Quantum Computing, Department of Chemistry, and Department of Physics and Astronomy, University of Waterloo, Waterloo, 200 University Ave W, Ontario, N2L 3G1, Canada.
Nat Commun. 2018 Nov 30;9(1):5122. doi: 10.1038/s41467-018-07547-6.
Two-dimensional (2D) magnetism has been long sought-after and only very recently realized in atomic crystals of magnetic van der Waals materials. So far, a comprehensive understanding of the magnetic excitations in such 2D magnets remains missing. Here we report polarized micro-Raman spectroscopy studies on a 2D honeycomb ferromagnet CrI. We show the definitive evidence of two sets of zero-momentum spin waves at frequencies of 2.28 terahertz (THz) and 3.75 THz, respectively, that are three orders of magnitude higher than those of conventional ferromagnets. By tracking the thickness dependence of both spin waves, we reveal that both are surface spin waves with lifetimes an order of magnitude longer than their temporal periods. Our results of two branches of high-frequency, long-lived surface spin waves in 2D CrI demonstrate intriguing spin dynamics and intricate interplay with fluctuations in the 2D limit, thus opening up opportunities for ultrafast spintronics incorporating 2D magnets.
二维（2D）磁性长期以来一直备受关注，直到最近才在磁性范德华材料的原子晶体中实现。到目前为止，对于这种 2D 磁体中的磁激发还缺乏全面的了解。在这里，我们报告了对二维蜂窝状铁磁体 CrI 的极化微拉曼光谱研究。我们分别在 2.28 太赫兹（THz）和 3.75 THz 的频率下显示出两套零动量自旋波的明确证据，其频率比传统铁磁体高出三个数量级。通过跟踪两种自旋波的厚度依赖性，我们揭示了它们都是表面自旋波，其寿命比它们的时间周期长一个数量级。我们在二维 CrI 中观察到的两支高频、长寿命表面自旋波的结果展示了引人入胜的自旋动力学以及与二维极限中的涨落之间的复杂相互作用，从而为结合二维磁体的超快自旋电子学开辟了机会。
