MBI Colloquium: Ultrafast quantitative magneto-acoustics at the nano-scale
Mittwoch, 30. November 2022 // 14.00
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie
Prof. Vasily Temnov | CNRS, Ecole Polytechnique, Palaiseau, France
Here I review some of our recent experimental and theoretical results in ultrafast magneto-acoustics in magnetostrictive materials and nanostructures  in the GHz-to-THz frequency range. Ultrafast magneto-acoustics investigates the coupling between elementary excitations of magnetic order with lattice vibrations in the highest experimentally accessible frequency range using ultrashort laser pulses.
The first part reviews magneto-elastic excitations of ferromagnetic resonance (FMR) with quasi-monochromatic GHz-frequency Surface Acoustic Waves (SAWs) optically excited in nickel thin films on glass using the transient grating geometry . The underlying theoretical modeling, starting from the analysis of phenomenological Landau-Lifshitz-Gilbert (LLG) equations, results in a simple equation of an externally and parametrically driven FMR-oscillator. The letter captures the most essential experimental observations such as the resonant enhancement of the FMR amplitude, the linear parametric sum- and difference frequency mixing and generation of fractional parametric frequencies . The intrinsic complexity of fs-laser-excited magneto-elastic dynamics at laterally patterned magnetic metasurfaces is illustrated with 1D-nickel grating experiments.
The second part discusses the generation of nanometer-wavelength perpendicular standing spin wave modes (exchange magnons) in ferromagnetic thin films by ultrashort pulses of longitudinal acoustic phonons propagating across the film. The dominant role of the acoustic bandwidth and the treatment of phonon-magnon resonances via the decomposition in acoustic and magnetic eigenmodes  are established to interpret the results of recent experiments in suspended nickel thin films , targeting novel mechanisms of inertial magnetization dynamics in the THz regime in ultrathin ferromagnetic membranes [6,7]. The fabrication problem of free-standing thin films could be potentially resolved through some recent progress in single-shot fs-laser nanofabrication technologies .
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