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We show that the resonant coupling of phonons and magnons can be exploited to generate spin currents at room temperature. Surface acoustic wave (SAW) pulses with a frequency of 1.55 GHz and duration of 300 ns provide coherent elastic waves in a ferromagnetic thin film/normal metal (Co/Pt) bilayer. We use the inverse spin Hall voltage in the Pt as a measure for the spin current and record its evolution as a function of time and external magnetic field magnitude and orientation. Our experiments show that a spin current is generated in the exclusive presence of a resonant elastic excitation. This establishes acoustic spin pumping as a resonant analogue to the spin Seebeck effect.
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Using spin-wave tomography (SWaT), we have investigated the excitation and the propagation dynamics of optically-excited magnetoelastic waves, i.e. hybridized modes of spin waves and elastic waves, in a garnet film. By using time-resolved SWaT, we reveal the excitation dynamics of magnetoelastic waves through coherent-energy transfer between optically-excited pure-elastic waves and spin waves via magnetoelastic coupling. This process realizes frequency and wavenumber selective excitation of spin waves at the crossing of the dispersion relations of spin waves and elastic waves. Finally, we demonstrate that the excitation mechanism of the optically-excited pure-elastic waves, which are the source of the observed magnetoelastic waves, is dissipative in nature.
The magnetization dynamics induced by standing elastic waves excited in a thin ferromagnetic film is described with the aid of micromagnetic simulations taking into account the magnetoelastic coupling between spins and lattice strains. The simulations have been performed for the 2 nm thick Fe81Ga19 film dynamically strained by longitudinal and transverse standing waves with various frequencies, which span a wide range around the resonance frequency nu_res of coherent magnetization precession in unstrained Fe81Ga19 film. It is found that standing elastic waves give rise to complex local magnetization dynamics and spatially inhomogeneous dynamic magnetic patterns. The spatio-temporal distributions of the magnetization oscillations in standing elastic waves have the form of standing spin waves with the same wavelength. Remarkably, the amplitude of magnetization precession does not go to zero at the nodes of these spin waves, which cannot be precisely described by simple analytical formulae. In the steady-state regime, the magnetization oscillates with the frequency of elastic wave, except for the case of longitudinal waves with frequencies well below nu_res, where the magnetization precesses with a variable frequency strongly exceeding the wave frequency. The precession amplitude at the antinodes of standing spin waves strongly increases when the frequency of elastic wave becomes close to nu_res. The results obtained for the magnetization dynamics driven by elastic waves are used to calculate the spin current pumped from the dynamically strained ferromagnet into adjacent paramagnetic metal. Importantly, the transverse charge current created by the spin current via the inverse spin Hall effect is high enough to be measured experimentally.
The authors have investigated the contribution of the surface spin waves to spin pumping. A Pt/NiFe bilayer has been used for measuring spin waves and spin pumping signals simultaneously. The theoretical framework of spin pumping resulting from ferromagnetic resonance has been extended to incorporate spin pumping due to spin waves. Equations for the effective area of spin pumping due to spin waves have been derived. The amplitude of the spin pumping signal resulting from travelling waves is shown to decrease more rapidly with precession frequency than that resulting from standing waves and show good agreement with the experimental data.
Effective spin mixing conductance (ESMC) across the nonmagnetic metal (NM)/ferromagnet interface, spin Hall conductivity (SHC) and spin diffusion length (SDL) in the NM layer govern the functionality and performance of pure spin current devices with spin pumping technique. We show that all three parameters can be tuned significantly by the spin orbit coupling (SOC) strength of the NM layer in systems consisting of ferromagnetic insulating Y3Fe5O12 layer and metallic Pd1-xPtx layer. Surprisingly, the ESMC is observed to increase significantly with x changing from 0 to 1.0. The SHC in PdPt alloys, dominated by the intrinsic term, is enhanced notably with increasing x. Meanwhile, the SDL is found to decrease when Pd atoms are replaced by heavier Pt atoms, validating the SOC induced spin flip scattering model in polyvalent PdPt alloys. The capabilities of both spin current generation and spin charge conversion are largely heightened via the SOC. These findings highlight the multifold tuning effects of the SOC in developing the new generation of spintronic devices.
We study the interaction of highly nonlinear solitary waves in granular crystals, with an adjacent linear elastic medium. We investigate the effects of interface dynamics on the reflection of incident waves and on the formation of primary and secondary reflected waves. Experimental tests are performed to correlate the linear medium geometry, materials, and mass with the formation and propagation of the reflected waves. We compare the experimental results with theoretical analysis based on the long-wavelength approximation and with numerical predictions obtained from discrete particle models. Studying variations of the reflected waves velocity and amplitude, we describe how the propagation of primary and secondary reflected waves responds sensitively to the states of the adjacent linear media. Experimental results are found to be in agreement with the theoretical analysis and numerical simulation. This preliminary study establishes the foundation for utilizing reflected solitary waves as novel information carriers in nondestructive evaluation of elastic material systems.
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