We perform a theoretical investigation of the magnetization switching in polycrystalline Ni nanoparticles induced by ultrashort pulses of surface acoustic waves via magnetoelastic interactions. In our numerical simulations, a Ni nanoparticle is modeled as an ellipsoidal disk deposited on a dielectric substrate. The in-plane external magnetic field breaks the symmetry and allows us to adjust the height of the energy barrier between two metastable magnetization states of the free-energy density and dramatically lower the amplitude of elastic strain pulses required for magnetization switching. The switching threshold is shown to depend on the duration of an acoustic pulse, the magnetic shape anisotropy of an elliptical nanoparticle, the amplitude of the external magnetic field, and the magnetostriction coefficient. We introduce the magnetoelastic switching diagram, allowing for the simultaneous visualization of the switching threshold and its characteristic timescale as a function of various physical parameters.

We analyze dispersion relations of magnons in ferromagnetic nanostructures with uniaxial anisotropy taking into account inertial terms, i.e., magnetic nutation. Inertial effects are parametrized by the damping-independent parameter $\beta$, which allows for an unambiguous discrimination of inertial effects from Gilbert damping parameter $\alpha$. The analysis of magnon dispersion relation shows its two branches are modified by the inertial effect, albeit in different ways. The upper nutation branch starts at $\omega =1/\beta$, the lower branch coincides with ferromagnetic resonance (FMR) in the long-wavelength limit and deviates from the zero-inertia parabolic dependence $\simeq {\omega }_{\text{FMR}}+D{k}^2$ of the exchange magnon. Taking a realistic experimental geometry of magnetic thin films, nanowires, and nanodiscs, magnon eigenfrequencies, eigenvectors, and $Q$-factors are found to depend on the shape anisotropy. The possibility of phase-matched magnetoelastic excitation of nutation magnons is discussed and the condition was found to depend on $\beta$, exchange stiffness $D$, and the acoustic velocity.

We revisit the quantitative analysis of the ultrafast magnetoacoustic experiment in a freestanding nickel thin film by Kim and Bigot [J.-W. Kim and J.-Y. Bigot, Phys. Rev. B 95, 144422 (2017)] by applying our recently proposed approach of magnetic and acoustic eigenmode decomposition. We show that the application of our modeling to the analysis of time-resolved reflectivity measurements allows for the determination of amplitudes and lifetimes of standing perpendicular acoustic phonon resonances with unprecedented accuracy. The acoustic damping is found to scale as $\propto {\omega }^2$ for frequencies up to 80 GHz, and the peak amplitudes reach ${10}^{-3}$. The experimentally measured magnetization dynamics for different orientations of an external magnetic field agrees well with numerical solutions of magnetoelastically driven magnon harmonic oscillators. Symmetry-based selection rules for magnon-phonon interactions predicted by our modeling approach allow for the unambiguous discrimination between spatially uniform and nonuniform modes, as confirmed by comparing the resonantly enhanced magnetoelastic dynamics simultaneously measured on opposite sides of the film. Moreover, the separation of timescales for (early) rising and (late) decreasing precession amplitudes provide access to magnetic (Gilbert) and acoustic damping parameters in a single measurement.

Nonlinear acoustic techniques are established tools for the characterization of micro-inhomogeneous materials with higher sensitivity, compared to linear ultrasonic techniques. In particular, the evaluation of material elastic quadratic nonlinearity via the detection of the second harmonic generation by acoustic waves is known to provide an assessment of the state variation of heat treated micro-structured materials. We report on the first application for non-destructive diagnostics of material thermal aging of finite-amplitude longitudinal acoustic pulses generated and detected by lasers. Finite-amplitude longitudinal pulses were launched in aluminum alloy samples by deposited liquid-suspended carbon particles layer irradiated by a nanosecond laser source. An out-of-plane displacement at the epicenter of the opposite sample surface was measured by an interferometer. This laser ultrasonic technique provided an opportunity to study the propagation in aluminum alloys of finite-amplitude acoustic pulses with a strain up to 5 × 10⁻³. The experiments revealed a signature of the hysteretic quadratic nonlinearity of micro-structured material manifested in an increase of the duration of detected acoustic pulses with an increase of their amplitude. The parameter of the hysteretic quadratic nonlinearity of the aluminum alloy (Al6061) was found to be of the order of 100 and to exhibit more than 50% variations in the process of the alloy thermal aging. By comparing the measured parameter of the hysteretic quadratic nonlinearity in aluminum alloys that were subjected to heat-treatment at 220 °C for different times (0 min, 20 min, 40 min, 1 h, 2 h, 10 h, 100 h, and 1000 h), with measurements of yield strength in same samples, it was established that the extrema in the dependence of the hysteretic nonlinearity and of the yield strength of this alloy on heat treatment time are correlated. This experimental observation provides the background for future research with the application goal of suggested nonlinear laser ultrasonic techniques for non-destructive evaluation of alloys’ strength and rigidity in the process of their heat treatment.