Joan Manel Hernàndez

The interaction between surface acoustic waves and magnetization offers an efficient route for electrically controlling magnetic states. Here, we demonstrate the excitation of magnetoacoustic waves in galfenol thin films with a 28 at. % gallium composition, corresponding to the second magnetostrictive peak in bulk samples. We quantify the amplitude of the induced magnetization oscillations using magnetic imaging in an x-ray photoelectron microscope and estimate the dynamic magnetoelastic constants through micromagnetic simulations. Our findings demonstrate the potential of galfenol thin films for magnonic applications and reveal that, despite strong magnetoelastic coupling, magnetic interactions and spin-wave dispersion relations significantly influence the overall amplitude of magnetoacoustic waves.

We use surface acoustic waves of 1 and 3 GHz in hybrid piezoelectric-magnetic systems with either nickel or cobalt as a magnetic layer to generate magnetoacoustic waves and directly image them using stroboscopic x-ray magnetic circular dichroism imaging. Our measurements visualize and quantify the amplitudes of both acoustic and magnetic components of the magnetoacoustic waves, which are generated in the ferromagnetic layer and can propagate over millimeter distances. Additionally, we quantified the magnetoelastic strain component for nickel and cobalt through micromagnetic simulations. Published by the American Physical Society 2024

Microwave power absorption by a two‐layer system deposited on a metallic surface is studied in the experimental setup emulating the response to a radar signal. Layers containing hexaferrite and iron powder in a dried paint of thickness under 1 mm are used. The data is analyzed within a theoretical model derived for a bilayer system from the transmission line theory. A good agreement between experimental and theoretical results is found. The advantage of using a bilayer system over a single‐layer system is demonstrated. How the maximum microwave absorption (minimum reflection) can be achieved through the optimization of the filling factors and thicknesses of the two layers is shown.

Ferromagnetic resonance (FMR) spectroscopy measurements were performed on NbRe/Co/NbRe trilayers in order to probe spin pumping across the superconductor/ferromagnet interface and to detect the possible presence of spin-triplet pairing in the superconducting NbRe layer. FMR spectra were acquired as a function of frequency, magnetic field, and temperature, and reveal that the Gilbert damping parameter associated with spin pumping remains almost constant as temperature goes down through the superconducting transition. Additionally, the dependence of the Gilbert damping parameter on the thickness of the NbRe layer in trilayers is used to determine the values of the spin mixing conductance at the interface ( 18 − 21 nm⁻²) and the spin diffusion length ( 7.1 − 12.5 nm) in the NbRe layer. These findings may suggest that spin pumping would still be effective even though NbRe becomes superconducting, which would indicate that the spin-triplet would be the dominant pairing mechanism. Future experiments are proposed in light of these results.

Spin-torque nano-oscillators (STNO) are microwave auto-oscillators based on magnetic resonances having a nonlinear response with the oscillating amplitude, which provides them with a large frequency tunability including the possibility of mutual synchronization. The magnetization dynamics in STNO are induced by spin-transfer torque (STT) from spin currents and can be detected by changes in electrical resistance due to giant magnetoresistance or tunneling magnetoresistance. The STT effect is usually treated as a dampinglike term that reduces magnetic dissipation and promotes excitation of magnetic modes. However, an additional term, known as Zhang-Li term, has an effect on magnetization gradients such as domain walls, and could have an effect on localized magnetic modes in STNO. Here we study the effect of Zhang-Li torques in magnetic excitations produced in STNO with a nanocontact geometry. Using micromagnetic simulations we find that Zhang-Li torques modify threshold currents of magnetic modes and their effective sizes. Additionally, we show that effects can be controlled by changing the ratio between nanocontact size and layer thickness.

Using hybrid piezoelectric-magnetic systems we have generated large amplitude magnetization waves mediated by magnetoelasticity with up to 25 degrees variation in the magnetization orientation. We present direct imaging and quantification of both standing and propagating acoustomagnetic waves with different wavelengths, over large distances up to several millimeters in a nickel thin film.