Understanding Spin Waves in 2D Magnets—A Combined Experimental, Computational and Mathematical Theory Approach

Energy consumption in computing is rising sharply due to energy-intensive technologies like AI and cryptocurrency, while efficiency gains from transistor miniaturization are nearing their limits. Computing with spin waves (SW) offers a sustainable alternative by eliminating the resistive heating present in traditional systems. In recent years, 2D magnetic materials have drawn significant attention for their tunability and suitability for simple device architectures. However, systematic understanding of their fundamental properties and how they are determined by material design is essential for next-generation spintronic devices.

Towards this goal, we present a comprehensive approach to understanding SW properties in 2D van der Waals (vdW) systems combining steady-state and time-resolved spectroscopy with electronic structure calculations and modeling of SW propagation. With photoluminescence (PL), we demonstrate magnon-phonon coupling in CrSBr and detect a low-temperature magnetic phase associated with vacancy defects. Exploiting magnon-exciton coupling in CrSBr, we use transient absorption to detect optically generated magnons at 60 GHz and generate spatially resolved images of the transient response, which can be extended to image SW propagation with diffraction-limited spatial and sub-100-fs temporal resolution.

To guide the selection of novel systems to study, we use density-functional theory (DFT) calculations of magnetic couplings to model the impact of proximity effects, dopants and induced defects on the long-range magnetic order in vdW magnets including CrOCl, FePS, and FePSe3. Our specialized code is uniquely effective in handling the large unit cells required to model dopants and defects. This large-scale calculation also enables exploring the impact of spin-phonon coupling mediated by defect-induced phonon modes on SW propagation in the material. These results extend our understanding of the role of defects in magnon-phonon coupling, and guide and complement our experimental efforts.

We then apply our experimental techniques in situ across a wide range of temperatures (1.6-300 K), magnetic fields (0-7 T), and under mechanical strain to gain insights into SW propagation and characteristics, focusing on how proximity effects due to an organic dopant and vacancy defects modulate SW coherence length and group velocity.

Our theoretical work introduces novel and exact non-commutative Fourier transform formulas that efficiently diagonalize the Hamiltonian for the 1D XXZ spin-½ chain. We numerically couple the exact formulas for the 1D spin chains, via a Trotter expansion, to obtain state-of-the-art simulations of layered 2D spin chains, improving on linear or classical approximations in the literature. Using parameters extracted from our computational and experimental study, we run numerical simulations, matching measured frequencies of the SW propagation, and predict effects under external forces. We also investigate gaps in the low-energy spectrum of the system under disorder arising from external magnetic fields and defects to determine the emergence of localized meta-stable states that may be used to carry information for large time periods.

These concerted theoretical, computational, and experimental efforts enable comprehensive understanding of the role defect engineering and proximity effects can play in enhanced SW properties towards effective and precisely controlled magnonic devices, as well as contributing to a broader comprehension of the underlying physics enabling rational design of novel magnetic systems.