Raman fingerprint of two terahertz spin wave branches in a two-dimensional honeycomb Ising ferromagnet

TLDR

Two‑dimensional magnetism has recently been realized in van der Waals crystals, yet a comprehensive understanding of their magnetic excitations remains lacking. The authors aim to probe spin‑wave excitations in the 2D honeycomb ferromagnet CrI3. They employ polarized micro‑Raman spectroscopy to detect spin waves in CrI3. They discover two zero‑momentum spin‑wave branches at 2.28 THz and 3.75 THz—three orders of magnitude higher than conventional ferromagnets—both surface modes with lifetimes an order of magnitude longer than their periods, revealing high‑frequency, long‑lived surface spin waves that suggest rich spin dynamics and promise for ultrafast 2D spintronics.

Abstract

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 CrI3. 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 CrI3 demonstrate intriguing spin dynamics and intricate interplay with fluctuations in the 2D limit, thus opening up opportunities for ultrafast spintronics incorporating 2D magnets.