Adrián Begué

In this work, we demonstrate the voltage control of magnetic anisotropy in a strain-mediated Ni ₉₀ Fe ₁₀ /BaTiO ₃ (001) heterostructure.

The study investigates the manipulation of the magnetic anisotropy in a thick (1 μm) Ni₉₀Fe₁₀ layer electrodeposited on a ferroelectric BaTiO₃(001) substrate, using a combination of Magneto-optical Kerr Effect, Photoemission Electron Microscopy with x-ray circular magnetic dichroism and x-ray diffraction. In the as-grown state, the system shows weak perpendicular magnetic anisotropy and characteristic stripe domains. Upon out-of-plane electrical poling of the BaTiO₃ substrate, the magnetic anisotropy switches to in-plane with a strong uniaxial behavior. The perpendicular magnetic anisotropy can be recovered by mild thermal annealing above the BaTiO₃ tetragonal to cubic phase transition and can be cycled by repeated electrical poling/thermal annealing. This method opens the path to a reversible control of the magnetic anisotropy in hybrid lead-free magnetoelectric Ni₉₀Fe₁₀/BaTiO₃ heterostructures from perpendicular to in-plane.

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.

This study focuses on hybrid magnetoelectric Ni ₉₀ Fe ₁₀ /BaTiO ₃ (011) heterostructures, which enable the control of the in-plane magnetization of the magnetostrictive layer through electric voltage.

2D materials possess exceptional mechanical properties making them promising candidates for protecting nanostructures. However, the magnetic field screening properties of 2D materials are largely unexplored. Here it is used Magnetic Force Microscopy (MFM) to unveil the effects on the magnetic field of magnetic nanostructures when 2D materials are placed on top of them. It is demonstrated that while graphene exhibits a weak diamagnetic response due to its unique electronic structure around the Dirac point, the overall screening effect remains minimal (≈0.5% per layer). Conversely, graphene oxide (GO) and MoS₂ show negligible response to the magnetic field, making them ideal for applications where preserving the original magnetic properties is crucial. These findings suggest that 2D materials can offer effective protection while minimally affecting the underlying magnetic functionalities, important for data storage technologies and spintronics.

Agreement between experimental results and a theoretical model. Crystalline structure, elasticity, shape and pairs contribute to the magnetic anisotropy of Ni _{100− x} Fe _x alloys.

This study investigates the out-of-plane magnetization component of electrodeposited Ni90Fe10 thin films grown under different applied magnetic field conditions. The formation of stripe domains is gradual, as there is a thickness range in which the transcritical shape appears in the hysteresis loops, while only magnetic ripples are measured in the magnetic force microscopy images. For instance, samples deposited under the residual magnetic field generated by the switched-off magnetic stirrer exhibit the transcritical shape in the in-plane hysteresis loops at a thickness of 400 nm, even though corresponding magnetic force microscopy images do not reveal the presence of stripe domains. When a perpendicular magnetic field of 100 Oe is applied during growth, stripe domains become visible in microscopy images, along with the transcritical shape in the hysteresis loop at 400 nm. This implies that the critical thickness for stripe formation can be reduced by applying a perpendicular magnetic field during electrodeposition. We have experimentally confirmed that the larger the magnetic field applied in the out-of-plane direction during growth, the smaller the critical thickness. These results underscore the importance of controlling the external magnetic field during electrodeposition for more effective tuning of the magnetic textures in electrodeposited Ni90Fe10 films.

A magnetic surface acoustic wave (SAW) sensor is built by growing a 100 nm galfenol (Fe72Ga28) film by sputtering between the interdigitated transducers of a SAW delay line. Love waves are produced when the shear waves excited on the piezoelectric substrate are guided by a 3.1 μm layer of amorphous SiO2. Due to the magnetostrictive nature of galfenol deposited on top, the application of magnetic fields modulates the propagation of the mechanical excitations along the sensor by the strain coupling. By introducing the delay line in a feedback loop circuit, these changes are studied as resonant frequency variations. Magnetic field cycles of ±40 mT are applied to the sample and the resonant frequency shift is tracked simultaneously. The sensor exhibited hysteretic frequency behavior that depends on the orientation of the applied magnetic field relative to the direction of Love wave propagation. In the configuration in which the wave vector and the applied field form an angle of 45°, the resonant frequency seems to increase with the magnetization induced by the external field. When the wave vector propagation is parallel to the field, two positive peaks appear close to the coercive field of the film, which has not been reported before. This is probably due to a more complex relationship between the acoustic wave and the magnetic state of the film which could be exploited to give rise to new models of magnetic sensors.