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.

The large magnetostriction in FeGa alloys is relevant for manifold applications, but for thin films, it can play a prominent role in controlling the strength of the magnetic anisotropy. Bulk samples show values depending on the extensive preparation procedure compendium, which is limited in its temperature range for high-quality thin-film synthesis. Here, we present a study of the magnetoelastic coupling coefficients B1 and B2 in epitaxial FeGa thin films below 50 nm deposited on the MgO(001) surface at 150 degrees C by the cantilever method. Series of films with 22, 28, and 33 at. % Ga do not show thickness-dependent variations for B1 and B2, but-B1 for the 22 at. % Ga composition is 10 MPa, roughly 2 times the bulk value and smaller than the bulk-like value of-B1=12.1 MPa obtained for a film with 17 at. % Ga. This enhancement is correlated with the A2 crystal structure for the film rather than the coexistence with D03 or other ordered nanometric precipitates proposed for bulk samples. Synchrotron diffraction excludes the formation of long-range L60, or D03 precipitates in samples with (001)A2 peaks at concentrations around 25 at. % Ga, which implies partial chemical disorder. The analysis of extended x-ray absorption fine structure measurements points to a D03 local order with a residual number of Ga-Ga pairs. Considering that the substrate quenches the movable strain in the A2 phase described in dual-phase structures, our results point to the important role of the electronic structure of the iron atoms modified by the presence of Ga in the alloy. This effect enlarges B1 in films with the A2 phase, stabilized using epitaxial growth.

The study investigates the manipulation of the magnetic anisotropy in a thick (1 μm) Ni 90 Fe 10 layer electrodeposited on a ferroelectric BaTiO 3 (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 BaTiO 3 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 Ni 90 Fe 10 /BaTiO 3 heterostructures from perpendicular to in-plane.

This study focuses on hybrid magnetoelectric Ni90Fe10/BaTiO3(011) heterostructures, which enable the control of the in-plane magnetization of the magnetostrictive layer through electric voltage. The heterostructure is both Pb- and rare-earth-free,...

The study investigates the manipulation of the magnetic anisotropy in a thick (1 μm) Ni_90Fe_10 layer electrodeposited on a ferroelectric BaTiO_3(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 BaTiO_3 substrate, the magnetic anisotropy switches to in-plane with a strong uniaxial behavior. This change is ascribed to the magnetoelastic effect due to the switching of the BaTiO_3 ferroelectric [001] axis into the sample plane, as evidenced by XRD. The strong mechanical interaction with the thick Ni_90Fe_10 overlayer prevents the full inversion of the substrate. The perpendicular magnetic anisotropy can be recovered by a mild thermal annealing above the BaTiO_3 tetrahedral 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 Ni_90Fe_10/BaTiO_3 heterostructures from perpendicular to in-plane.

Electrodeposition is a commonly employed technique for synthesizing magnetic thin films, serving both fundamental research and practical applications. Of particular interest are Fe-based alloys like Ni x Fe 1-x and Fe 1-x Ga x , as fundamental properties of ferromagnets as saturation magnetization (M sat ), Curie temperature (T C ), magnetic anisotropy (K) and magnetostriction constant (λ) strongly depend on the composition [1-2]. This work presents our latest findings on Fe-based thin films and multilayers, exploring diverse phenomena ranging from controlling magnetization switching in magnetoelastic structures to fine-tuning the out-of-plane (OOP) component of magnetization [3-4]. Materials exhibiting moderate perpendicular magnetic anisotropy can develop stripe domains above a critical thickness (t cr ). In addition to the interest in the basic understanding of this magnetic texture, nowadays, there is an increasing attention in this type of domains since they can be used for example for spin waves generation. The Ni 80 Fe 20 alloy known as permalloy has been widely investigated because of its low coercivity and null magnetostriction. Sputtered Ni 80 Fe 20 layers has been reported to have t cr around 300 nm [5], and its formation is generally believed to be due to columnar growth. However, for applications like magnetoimpedance, these stripes can be undesirable, hindering the growth of sufficiently thick layers to fabricate reliable sensors. Therefore, it is of interest the control of stripe domains in Ni x Fe 1-x films and whether the electrodeposition technique can also be used to tailor the appearance of this magnetic texture. Initially, we have examined the formation of stripe domains in the Ni 90 Fe 10 alloy that is also of great interest because of its low coercivity and negative magnetrostriction (λ ~ -20 ppm). Subsequently, we have analyzed the magnetic behavior of electrodeposited Ni x Fe 1-x films in which the Fe content has been increased up to 30 at. %. Experimental results show that for Ni 90 Fe 10 films magnetic stirring during growth suppresses the presence of stripes even for a thickness exceeding 1 µm. On the other hand, when growing with unstirred electrolyte, t cr can be reduced if a perpendicular magnetic field of 100 Oe is applied during samples growth. Eventually, it has been observed that t cr depends on the Fe content in the Ni x Fe 1-x and no stripes are observed for a content above 13 at.% when no perpendicular magnetic field is applied during growth. We have also investigated a bilayer structure comprised of two magnetic layers with opposite signs for the magnetostriction constant, Ni 90 Fe 10 (λ ~ -20 ppm) and Fe 70 Ga 30 (λ~ 70 ppm) to analyze the effect of magnetoelasticity on the magnetization reversal process. The exchange correlation length is a key parameter that determines the transition region between two opposite directions of the magnetization that depends on characteristics such as the magnetic anisotropy and the exchange stiffness constant. It is also used to quantify the domain wall thickness (δ) that can be considered as the distance over which the magnetic moments are correlated by exchange interactions, and that plays a fundamental role on magnetic systems relying on interfacial interactions as spring magnets or exchange-biased systems. Since the thickness of the layers cannot be higher than δ to avoid the magnetic switching of uncoupled regions, this implies that the thickness of layers is intrinsically limited in systems with interfacial coupling. We have studied Ni 90 Fe 10 /Fe 70 Ga 30 bilayers with thicknesses exceeding δ to analyze the magnetic switching in these new magnetoelastic bilayer structures. 500 nm-thick Ni 90 Fe 10 and Fe 70 Ga 30 single layers have coercive fields of 22 Oe and 42 Oe, respectively, which guarantees that independent magnetic switchings from each layer can be experimentally observed.This has been confirmed by micromagnetic simulations performed by OOMF in which magnetoelasticity has not been included. However, when measuring the hysteresis loops of different NiFe/FeGa samples with thicknesses exceeding δ it is observed an in-unison magnetization reversal in all cases. Since magnetoelasticity is not an interfacial effect, it enables to control the magnetization reversal process of the whole bilayer regardless of the thickness being possible to promote this simultaneous reversal process regardless of the layer thickness (Fig.1). In conclusion, these results highlight the capability of electrodeposition to achieve new magnetic systems with enhanced functionalities. [1] J. M. D. Coey. Magnetism and Magnetic Materials. Cambridge University Press (2010) [2] Q. Xing, Y. Du, R. J. McQueeney, T. A. Lograsso. Acta Mater. 56 (2008) 4536−4546. [3] N. Coton, J. P. Andres, A. Cabrera, M. Maicas, R. Ranchal. J. Appl. Phys. 134 (2023) 103904. [4] N. Coton, J.P. Andres, E. Molina, M. Jaafar, R. Ranchal. J. Magn. Magn. Mater. 565 (2023) 170246. [5] M. Romera, R. Ranchal, D. Ciudad, M. Maicas, C. Aroca, J. Appl. Phys. 110 (2011) 083910 Figure 1

Abstract2D 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.

Multiferroic heterostructures based on the strain-mediated mechanism present ultralow heat dissipation and large magnetoelectric coupling coefficient, two conditions that require endless improvement for the design of fast nonvolatile random access memories with reduced power consumption. This work shows that a structure consisting of a [Pb(Mg1/3Nb2/3)O3]0.7-[PbTiO3]0.3 (001) substrate on which a crystalline FeGa(001)/MgO(001) bilayer is deposited exhibits a giant magnetoelectric coupling coefficient of order 15 × 10–6 s m–1 at room temperature. That result is a 2-fold increment over the previous highest value. The spatial orientation of the magnetization vector in the epitaxial FeGa film is switched 90° with the application of electric field. The symmetry of the magnetic anisotropy is studied by the angular dependence of the remanent magnetization, demonstrating that poling the sample generates a switchable uniaxial magnetoelastic anisotropy in the film that overcomes the native low 4-fold magnetocrystalline anisotropy energy. Magnetic force microscopy shows that the switch of the easy axis activates the displacement of domain walls and the domain structures remain stable after that point. This result highlights the interest in single-crystalline structures including materials with large magnetoelastic coupling and small magnetocrystalline anisotropy for low-energy-consuming spintronic applications.

The aim of this work is to clarify how in-plane magnetic anisotropy and magnetoelasticity depend on the thickness of Ga-rich FeGa layers. Samples with an Fe72Ga28 composition were grown by sputtering in the ballistic regime in oblique incidence. Although for these growth conditions uniaxial magnetic anisotropy could be expected, in-plane anisotropy is only present when the sample thickness is above 100 nm. By means of differential X-ray absorption spectroscopy, we have determined the influence of both Ga pairs and tetragonal cell distortion on the evolution of the magnetic anisotropy with the increase of FeGa thickness. On the other hand, we have used the cantilever beam technique with capacitive detection to also determine the evolution of the magnetoelastic parameters with the thickness increase. In this case, experimental results can be understood considering the grain distribution. Therefore, the different physical origins for anisotropy and magnetoelasticity open up the possibility to independently tune these two characteristics in Ga-rich FeGa films.

The magnetic domain structure is studied in epitaxial Fe100-xGax/MgO(001) films with 0 < x < 30 and thicknesses below 60 nm by magnetic force microscopy. For low gallium content, domains with the magnetization lying in the film plane and domain walls separating micrometric areas are observed. Above x approximate to 20, the magnetic contrast shows a fine corrugation, ranging from 300 to 900 nm, suggesting a ripple substructure with a periodic oscillation of the magnetization. We discuss the presence of a random magnetic anisotropy contribution, that superimposed to the cubic coherent anisotropy, is able to break the uniform orientation of the magnetization. The origin of that random anisotropy is attributed to several factors: coexistence of crystal phases in the films, inhomogeneous distribution of both internal strain and Ga-Ga next nearest neighbor pairs and interface magnetic anisotropy due to the Fe-O bond.

In this work we present a study of the structural properties of Fe100-xGax(x < 30) films grown by Molecular Beam Epitaxy on Mg0(100). We combine long range and local/chemically selective X-ray probes (X-ray Diffraction and X-ray absorption spectroscopy) together with real space imaging by means of Transmission Electron Microscopy and surface sensitive in situ Reflected High Energy Electron Diffraction. For substrate temperature T-s below 400 degrees C we obtain bcc films while, for x approximate to 24 and T-s >= 400 degrees C the nucleation of the fcc phase is observed. For both systems a Ga anticlustering or local range ordering phenomenon appears. The Ga/Fe composition in the first and second coordination shells of the bcc films is different from that expected for a random Ga distribution and is close to a D0(3) phase, leading to a minimization of the number Ga-Ga pairs. On the other side, a long-range D0(3) phase is not observed indicating that atomic ordering only occurs at a local scale. Overall, the epitaxial growth procedure presented in this work, first, avoids the formation of a long range ordered D0(3) phase, which is known to be detrimental for magnetostrictive properties, and second, demonstrates the possibility of growing fcc films at temperatures much lower than those required to obtain bulk fcc samples. (C) 2018 Elsevier B.V. All rights reserved.