arXiv subjects

Joan Manel Hernàndez

Publications and source records attributed to Joan Manel Hernàndez.

13 recordsLinked to original sources

arXiv2025

Radar shielding experiments by composite bilayer magnetic systems

An experimental strong increase of the reflection loss (from 25 up to 35 dBs) and an extension of the absorption bandwidth up to 20% is measured in a set of novel functional bilayer systems. We focus our work on studying the samples in an anechoic chamber under far-field real radar conditions. Each layer consists of a composite material, typically a dielectric matrix filled with random anisotropy hexaferrite and soft metallic materials (powder or wires). Combining the two types of materials into a submillimetric bilayer structure has shown unprecedented improvements in microwave absorption capacities compared to the former absorption of each layer. The capacity to improve the shielding behavior is strongly related to each layer permittivity, permeability, and thickness leading, therefore, to a strong control over the design of novel materials for stealth applications.

cond-mat.mtrl-sci↗

arXiv2024

Microwave absorption in barium hexaferrite nanocomposites with random anisotropy

This work reports experimental evidence of random magnetic behavior observed in modified barium hexagonal ferrites. We observe a significant transition in the magnetic properties of this system when divalent cations (Ni2+, Cu2+, Mn2+) are incorporated into the structure and give rise to a magnetic nanocomposite. Such introduction randomly occurs throughout each sample and creates conditions for such materials to behave as random anisotropy magnets. We verify the occurrence of such behavior in our samples by fitting the magnetization in approaching saturation to the corresponding theoretical model. We therefore analyze the microwave absorption capacities of random anisotropy magnets in the GHz range and predict large and broad absorption signals under certain conditions. The findings presented here postulate, for the first time, ceramic materials as promising random anisotropy magnets and underline their potential as microwave absorbers, in good agreement with recent theoretical models.

cond-mat.mtrl-sci↗

arXiv2024

Magnetoacoustic waves in a highly magnetostrictive FeGa thin film

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, a highly magnetostrictive alloy made of iron (72%) and gallium (28%). 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 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.

cond-mat.mtrl-sci↗

arXiv2023

Study of the magnetoelastic effect in nickel and cobalt thin films at GHz range using X-ray microscopy

We use surface acoustic waves of 1 and 3 GHz in hybrid piezoelectric-magnetic systems with either cobalt or nickel 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 quantifiedy the magnetoelastic strain component for cobalt and nickel through micromagnetic simulations. We obtained a drop in the magnetoacoustic signal at 3 GHz suggesting a speed limit for the efficient magnetoelastic coupling in our hybrid devices.

cond-mat.mes-hall↗

arXiv2023

Magneto-Acoustic Waves in antiferromagnetic CuMnAs excited by Surface Acoustic Waves

Magnetoelastic effects in antiferromagnetic CuMnAs are investigated by applying dynamic strain in the 0.01% range through surface acoustic waves in the GaAs substrate. The magnetic state of the CuMnAs/GaAs is characterized by a multitude of submicron-sized domains which we image by x-ray magnetic linear dichroism combined with photoemission electron microscopy. Within the explored strain range, CuMnAs shows magnetoelastic effects in the form of Néel vector waves with micrometer wavelength, which corresponds to an averaged overall spin-axis rotation up to 2.4 deg driven by the time-dependent strain from the surface acoustic wave. Measurements at different temperatures indicate a reduction of the wave amplitude when lowering the temperature. However, no domain wall motion has been detected on the nanosecond timescale

cond-mat.mes-hall↗

arXiv2022

Resonant and off-resonant magnetoacoustic waves in epitaxial Fe$_3$Si/GaAs hybrid structures

Surface acoustic waves (SAWs) provide an efficient dynamical coupling between strain and magnetization in micro/nano-metric devices. Using a hybrid device composed of a piezoelectric, GaAs, and a ferromagnetic Heusler alloy thin film, Fe$_3$Si, we are able to quantify the amplitude of magnetoacoustic waves generated with SAWs via magnetic imaging in an X-ray photoelectron microscope. The cubic anisotropy of the sample together with a low damping coefficient allows for the observation of resonant and non-resonant magnetoelastic coupling. Additionally, via micromagnetic simulation, we verify the experimental behavior and quantify the magnetoelastic shear strain component in Fe$_3$Si that appears to be very large ($b_2=14\times 10^6$ J/m$^3$), much larger than the one found in Nickel.

cond-mat.mtrl-sci↗

arXiv2020

Effects of the Zhang-Li Torque on Spin Torque nano Oscillators

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 damping-like 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 torque 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.

cond-mat.mes-hall↗

arXiv2019

Generation and imaging of magnetoacoustic waves over millimetre distances

Using hybrid piezoelectric/magnetic systems we have generated large amplitude magnetization waves mediated by magneto-elasticity 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.

physics.app-ph↗

arXiv2018

Generation and stability of dynamical skyrmions and droplet solitons

A spin-polarized current in a nanocontact to a magnetic film can create collective magnetic oscillations by compensating the magnetic damping. In particular, in materials with uniaxial magnetic anisotropy, droplet solitons have been observed a self-localized excitation consisting of partially reversed magnetization that precesses coherently in the nanocontact region. It is also possible to generate topological droplet solitons, known as \emph{dynamical skyrmions}. Here we study the conditions that promote either droplet or dynamical skyrmion formation and describe their stability in magnetic films without Dzyaloshinskii-Moriya interactions. We show that Oersted fields from the applied current as well as the initial magnetization state can determine whether a droplet or dynamical skyrmion forms. Dynamical skyrmions are found to be more stable than droplets. We also discuss electrical characteristics that can be used distinguish these magnetic objects.

cond-mat.mes-hall↗

arXiv2016

Simultaneous imaging of strain waves and induced magnetization dynamics at the nanometer scale

Changes in strain can be used to modify electronic and magnetic properties in crystal structures, to manipulate nanoparticles and cells, or to control chemical reactions. The magneto-elastic (ME) effect--the change of magnetic properties caused by the elastic deformation (strain) of a magnetic material--has been proposed as an alternative approach to magnetic fields for the low power control of magnetization states of nanoelements since it avoids charge currents, which entail ohmic losses. Multiferroic heterostructures \cite{Zheng2004} and nanocomposites have exploited this effect in search of electric control of magnetic states, mostly in the static regime. Quantitative studies combining strain and magnetization dynamics are needed for practical applications and so far, a high resolution technique for this has been lacking. Here, we have studied the effect of the dynamic strain accompanying a surface acoustic wave on magnetic nanostructures. We have simultaneously imaged the temporal evolution of both strain waves and magnetization dynamics of nanostructures at the picosecond timescale. The newly developed experimental technique, based on X-ray microscopy, is versatile and provides a pathway to the study of strain-induced effects at the nanoscale. Our results provide fundamental insight in the coupling between strain and magnetization in nanostructures at the picosecond timescale, having implications in the design of strain-controlled magnetostrictive nano-devices.

cond-mat.mes-hall↗

arXiv2016

Effect of temperature on spin-transfer torque induced magnetic solitons

Spin-transfer torques in a nanocontact to an extended magnetic film can create spin waves that condense to form dissipative droplet solitons. Here we report an experimental study of the temperature dependence of the current and applied field thresholds for droplet soliton formation, as well as the nanocontact's electrical characteristics associated with droplet dynamics. Nucleation of droplet solitons requires higher current densities at higher temperatures, in contrast to typical spin-transfer torque induced switching between static magnetic states. Magnetoresistance and electrical noise measurements show that soliton instabilities become more pronounced with increasing temperature. These results are of fundamental interest in understanding the influence of thermal noise on droplet solitons, and in controlling their dynamics.

cond-mat.mes-hall↗

arXiv2016

Thickness and temperature dependence of the magnetodynamic damping of pulsed laser deposited $\text{La}_{0.7}\text{Sr}_{0.3}\text{MnO}_3$ on (111)-oriented SrTi$\text{O}_3$

We have investigated the magnetodynamic properties of $\text{La}_{0.7}\text{Sr}_{0.3}\text{MnO}_3$ (LSMO) films of thickness 10, 15 and 30 nm grown on (111)-oriented SrTi$\text{O}_3$ (STO) substrates by pulsed laser deposition. Ferromagnetic resonance (FMR) experiments were performed in the temperature range 100--300 K, and the magnetodynamic damping parameter $α$ was extracted as a function of both film thickness and temperature. We found that the damping is lowest for the intermediate film thickness of 15 nm with $α\approx 2 \cdot 10^{-3}$, where $α$ is relatively constant as a function of temperature well below the Curie temperature of the respective films.

cond-mat.mes-hall↗

arXiv2015

Tailoring the magnetodynamic properties of nanomagnets using magnetocrystalline and shape anisotropies

Magnetodynamical properties of nanomagnets are affected by the demagnetizing fields created by the same nanoelements. In addition, magnetocrystalline anisotropy produces an effective field that also contributes to the spin dynamics. In this article we show how the dimensions of magnetic elements can be used to balance crystalline and shape anisotropies, and that this can be used to tailor the magnetodynamic properties. We study ferromagnetic ellipses patterned from a 10 nm thick epitaxial Fe film with dimensions ranging from 50 x 150 nm to 150 x 450 nm. The study combines ferromagnetic resonance (FMR) spectroscopy with analytical calculations and micromagnetic simulations, and proves that the dynamical properties can be effectively controlled by changing the size of the nanomagnets. We also show how edge defects in the samples influence the magnetization dynamics. Dynamical edge modes localized along the sample edges are strongly influenced by edge defects, and this needs to be taken into account in understanding the full FMR spectrum

cond-mat.mes-hall↗