Nonreciprocal spin wave in room-temperature van der Waals ferromagnet $(\rm Fe_{0.78}Co_{0.22})_{5}GeTe_{2}$

TL;DR

This paper addresses how spin waves propagate in a room-temperature van der Waals ferromagnet and what mechanisms drive nonreciprocity. It uses Brillouin light scattering on (Fe0.78Co0.22)5GeTe2 flakes of varying thickness to map spin-wave dispersion and quantify bulk DMI from Stokes/anti-Stokes frequency differences, supported by multilayer simulations to separate dipolar and DMI contributions. The key finding is that spin-wave nonreciprocity in thicker flakes originates from bulk DMI, while dynamic dipolar interactions do not account for it; Co doping enhances the DMI relative to undoped Fe5GeTe2, likely via symmetry breaking of Fe split-sites. This work establishes Co-doped Fe5GeTe2 as a robust platform for room-temperature spin-wave transport and for exploring topological spin textures in van der Waals materials, with tunable bulk DMI through doping.

Abstract

Here, we investigate the spin waves in room-temperature van der Waals ferromagnet $(\rm Fe_{0.78}Co_{0.22})_{5}GeTe_{2}$ by utilizing Brillouin light scattering technique. The spin wave dispersion in flakes of different thicknesses shows the key role of dipolar interaction in the spin waves of vdW ferromagnets, and the non-reciprocity of spin wave in thick flakes is observed, which is attributed to the bulk Dzyaloshinskii-Moriya interaction after excluding the influence of dynamic dipolar interaction. The measured bulk DMI parameter D is 0.08 $\rm mJ/m^2$, which is double that of pure $\rm Fe_5GeTe_2$. Our work shows that Co-doped $\rm Fe_5GeTe_2$ is a promising platform for investigating propagating spin wave and topological spin textures at room temperature.

Figures (4)

• Figure 1: (a) Schematic diagram of the FCGT crystal structure, where the Fe1 site is the splite site, and crystal structure was drawn using the VESTAcry1 and a cif file from the Computational 2D Materials Databasecry2cry3. (b) Schematic diagram of the Brillouin light scattering setup. (c) Brillouin light scattering spectrum of a 2 nm thick FCGT flakes under a magnetic field of 366 mT. The green dots are experimental points, and the red solid line is the result of a Lorentzian fit.
• Figure 2: The the spin-wave frequency in FCGT flakes of different thicknesses varies with the magnetic field. The frequency data is obtained by Lorentz fitting the BLS spectrum, and the solid line is obtained by fitting using Eq. (2)
• Figure 3: (a)The the spin-wave frequency in FCGT flakes of different thicknesses varies with the in-plane wave vector. The frequency data is obtained by Lorentz fitting the BLS spectrum, and the solid line is obtained by fitting using Eq. (3). (b)The difference between the spin wave frequencies of the Stokes peak and the anti-Stokes peak varies with the in-plane wave vector. The solid line is the result of linear fitting.
• Figure 4: (a) Frequency difference of the $[\rm CoFeB(7 nm)/SiO_2(4 nm)]_{\it N}$ multilayer films between the spin wave frequencies of the Stokes peak and the anti-Stokes peak varies with the in-plane wave vector. (b)Simulation results of spin wave dispersion in an $N$-layer structure, where each layer has the same thickness of 7 nm and the same saturation magnetization of 950 kA/m, and the spacing between each layer is 4 nm.