Excitation and Detection of Spinwaves with Light

Abstract

Spinwaves (SWs) are a collective excitation of the local spin moments in a magnetic ordered medium. Spinwaves can be excited electrically using microwave sources, direct currents, and by focused optical short pulses on a thin magnetic film. Spinwaves are utilized in many emerging concepts for new technologies. But the SWs in almost all proof-of-principle SW devices are generated using electrical currents, inductive transducers and antenna structures, which severely limit scalability and the operating frequencies. It has recently been shown that it is possible to create and control SWs of high amplitude using a high repetition rate femtosecond pulsed laser (fs-laser). However, the impact of different material parameters is still unknown as the original demonstration of the technique was conducted using a single NiFe (Py) ferromagnetic thin film with a thickness (20 nm) comparable to the laser penetration depth. In this project, we investigate SWs excited by fs-laser pulse trains (laser comb with 1 GHz repetition rate) using Brillouin Light Scattering (BLS) microscopy. We demonstrate that previous results are reproducible using a much thicker 100 nm film. Furthermore, in such thicker films, optical stimulation of SWs is clearly observed to be efficient with harmonic frequencies up to 15 multiples of the repetition rate of the pulsed fs-laser. The SW BLS intensity exhibits a stronger than parabolic dependence on the laser fluence, and the heat-induced demagnetization is found to follow the Bloch T3/2 law. Moreover, we show that the optical excitation of sustained SWs is reproducible over a wide range of film thicknesses (20-100 nm) and the efficiency is inversely proportional to the film thickness. In order to further corroborate our conclusions and to enhance the spin wave excitation efficiency, additional measurements on a gold-doped Py sample were done. They showed the expected enhanced excitation efficiency compared to the pristine film.