Spin-wave quantisation and nonlinear scattering in non-reciprocal materials

• Applicant: Dr. Helmut Schultheiß, since 8/2023
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 464991070

Spin waves are the elementary, collective excitations of the spin system in magnetically ordered materials. The magnetic moments precess around the equilibrium direction which is defined by the total internal magnetic field. The long-range dipolar interaction and short-range exchange interaction lead to the formation of spin waves propagating through the solid over mesoscopic length scales. The precessional motion of each magnetic moment has a well defined sense of rotation given by the gyromagnetic ratio. This sense of rotation is fixed and doesn’t change upon inversion of the spin-wave propagation direction and, therefore, spin waves are chiral excitations for certain configurations, in particular if the wave vector is perpendicular to the equilibrium direction of the magnetization.This proposal aims to explore this chiral nature in ferromagnetic material systems, where it results in a strong non-reciprocity of the spin-wave dispersion, and to elaborate the impact of this non-reciprocal dispersion on quantisation in micro- and nanostructures and nonlinear spin-wave scattering. Non-reciprocity of spin waves was studied extensively in the last years in the context of interfacial Dzyaloshinskii-Moriya (iDMI) interaction in ultra thin films, where the frequency shift upon reversal of the spin-wave wave vector was used as a probe to quantify the strength of this asymmetric exchange interaction. Due to the interfacial nature of iDMI the film thickness was kept in the range of a few nanometers which causes a significant reduction of the spin-wave propagation length. This hampered up to now the experimental investigation of linear and non-linear spin-wave transport and quantisation effects in the presence of a non-reciprocal dispersion relation. In this proposal we will investigate spin waves in coupled ferromagnetic bilayers, where the non-reciprocity originating from the coupling of spin waves between the layers via dynamic dipolar magnetic fields can be two orders of magnitude larger compared to systems based on iDMI. Moreover, the effects of the chiral nature of spin waves do not scale inversely proportional as in the case of iDMI but proportional to the thickness of the magnetic layers. The resulting increase in the spin-wave group velocity and, hence, decreased spatial decay of propagating spin waves will give us experimental access to spin-wave transport phenomena and quantisation effects using Brillouin-light scattering microscopy to shed light on the impact of non-reciprocal dispersion laws.The objectives are: (i) Maximising the non-reciprocity in ferromagnetic bilayers by systematic variation of thickness and material composition; (ii) Quantifying the spectrum and spatial dynamics of quantised spin wave modes in micro- and nanostructures; (iii) Analysing nonlinear scattering of spin waves in a non-reciprocal medium with broken inversion symmetry of the dispersion relation.

• DFG Programme: Research Grants
• Ehemalige Antragstellerin: Dr. Olga Gladii, until 7/2023