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.
Highlights
Two-dimensional (2D) magnetism has been long sought-after and only very recently realized in atomic crystals of magnetic van der Waals materials
The well-established spintronic devices based on Heisenberg ferromagnets have speeds in the gigahertz (GHz) regime due to the weak magnetic anisotropy[19,20], while the speeds of the recently proposed antiferromagnet-based spintronic devices fall into the terahertz (THz) range owing to the exchange interaction between the two sublattices of the antiferromagnets[21,22,23]
JZ>JXY> 0 for Ising ferromagnetism; and 〈i,j〉 denotes the approximation of the nearest-neighbor exchange coupling
Summary
Two-dimensional (2D) magnetism has been long sought-after and only very recently realized in atomic crystals of magnetic van der Waals materials. The recent discovery of two-dimensional (2D) ferromagnetism[1,2] proves that magnetic anisotropy can overcome thermal fluctuations and stabilize long-range magnetic orders in the 2D limit at finite temperatures. The newly discovered 2D Ising honeycomb ferromagnet CrI3 possesses both merits for realizing high-frequency spin waves: the strong magnetic anisotropy from the Ising-type spin interactions[24] and the large exchange coupling between the two Cr3+ sublattices within the honeycomb framework[25]. I. for all optical phonons scale linearly with their sample thickness, as expected for all bulk modes in quasi2D-layered materials This observation on the two spin waves, shows striking analogy to the surface modes whose I. We show that, from more than ten-layer to monolayer CrI3, the spin wave frequencies (2.28 and 3.75 THz) and the onset temperatures (45 K) remain nearly constant, while their lifetimes decrease significantly from 50 and 100 ps to 15 ps, but remain an order of magnitude longer than their corresponding spin wave temporal periods
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