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.

In this work we investigate the strain state in Ni films belonging to [Ni(tNi)/Cu(tCu)] × N superlattices (with tNi = 2 nm, 3 nm, tCu from 0 to 3 nm and N between 1 and 6) by Extended X-ray Absorption Fine Structure at the Ni K-edge. The strain of these Ni blocks can be up to 80% larger than that obtained for single thin films with the same content of Ni. The dependence of the strain on N indicates that its relaxation is not progressive with N but gets blocked for certain N values. Also, the replacement of a single Ni layer by Cu, at the middle of the Ni block, results in a significant increment of the strain of the nickel layers, which manifests the relevant effect of the introduction of copper. The large strain values observed suggest the presence of blocking mechanisms acting on the propagation of dislocations. We argue that this observation is related to repulsive interactions between dislocations enhanced by nonhomogeneos stress due to presence of the copper layers separating the Ni blocks.

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.

STS of Tm adatoms deposited on insulating Cu2N nanoislands and DFT calculations allow distinguishing the 4f occupancy of the adatoms.

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.

Resumen del trabajo presentado al 9th Joint European Magnetic Symposia (JEMS), celebrado en Mainz (Alemania) del 3 al 7 de septiembre de 2018.

Exploring bottom-up procedures to achieve island and particles with a defined size can open opportunities in many applications. This contribution focuses on the growth of epitaxial Tm islands, below the monolayer range, on the W(110) surface by studying in situ the diffusion process at high temperature, between 700 and 1200 K, by means of scanning tunnel microscopy (STM) to determine the topography of the Tm deposits as a function of the coverage and thermal treatments of an initial room temperature deposit. Samples subject to a prolonged heating process, spending several hours at temperatures below 700 K, show that the average Tm islands size remains constant at higher temperatures, in contrast with samples subject to a faster heating. It is observed that the presence of carbon strongly limits the diffusion of Tm, thus leading to the formation of pseudomorphic nanometric islands instead of a full monolayer.

Resumen del trabajo presentado a la 10th Conferencia Fuerzas y Tunel, celebrada en Girona (Espana) del 5 al 7 de septiembre de 2016.

High-density magnetic storage or quantum computing could be achieved using small magnets with large magnetic anisotropy, a requirement that rare-earth iron alloys fulfill in bulk. This compelling property demands a thorough investigation of the magnetism in low dimensional rare-earth iron structures. Here, we report on the magnetic coupling between 4f single atoms and a 3d magnetic nanoisland. Thulium and lutetium adatoms deposited on iron monolayer islands pseudomorphically grown on W(110) have been investigated at low temperature with scanning tunneling microscopy and spectroscopy. The spin-polarized current indicates that both kind of adatoms have in-plane magnetic moments, which couple antiferromagnetically with their underlying iron islands. Our first-principles calculations explain the observed behavior, predicting an antiparallel coupling of the induced 5d electrons magnetic moment of the lanthanides with the 3d magnetic moment of iron, as well as their in-plane orientation, and pointing to a non-contribution of 4f electrons to the spin-polarized tunneling processes in rare earths.

We have investigated the noncentrosymmetric tetragonal heavy fermion compound CeAuAl3 using muon spin rotation (mu SR), neutron diffraction (ND), and inelastic neutron scattering (INS) measurements. We have also revisited the magnetic, transport, and thermal properties. Themagnetic susceptibility reveals an antiferromagnetic transition at 1.1 K with, possibly, another magnetic transition near 0.18 K. The heat capacity shows a sharp lambda-type anomaly at 1.1 K in zero field, which broadens and moves to a higher temperature in an applied magnetic field. Our zero-field mu SR and ND measurements confirm the existence of a long-range magnetic ground state below 1.2 K. Further, the ND study reveals an incommensurate magnetic order with a magnetic propagation vector k = (0,0,0.52(1)) and a spiral structure of Ce moments coupled ferromagnetically within the ab plane. Our INS study reveals the presence of two well-defined crystal electric field (CEF) excitations at 5.1 and 24.6 meV in the paramagnetic phase of CeAuAl3 that can be explained on the basis of the CEF theory and the Kramer's theorem for a Ce ion having a 4f(1) electronic state. Furthermore, low energy quasielastic excitations show a Gaussian line shape below 30 K compared to a Lorentzian line shape above 30 K, indicating a slowdown of spin fluctuations below 30 K. We have estimated a Kondo temperature of T-K = 3.5 K from the quasielastic linewidth, which is in good agreement with that estimated from the heat capacity. This study also indicates the absence of any CEF-phonon coupling unlike that observed in isostructural CeCuAl3 The CEF parameters, energy level scheme, and their wave functions obtained from the analysis of INS data explain satisfactorily the single crystal susceptibility in the presence of two-ion anisotropic exchange interaction in CeAuAl3.

e pattern due to the overlap with the rectangular W(110) substrate. Monolayer as well as isolated Tm adatoms on W present a trivalent ground-state electronic configuration, contrary to divalent gas phase Tm and weakly coordinated atoms in quench-condensed Tm films. Ligand field multiplet simulations of the x-ray absorption spectra further show that Tm has a |J = 6,Jz =± 5� electronic ground state separated by a few meV from the next lowest substates |J = 6,Jz =± 4� and |J = 6,Jz =± 6� . Accordingly, both the Tm atoms and monolayer films exhibit large spin and orbital moments with out-of-plane uniaxial magnetic anisotropy. X-ray magnetic dichroism measurements as a function of temperature show that the Tm monolayers develop antiferromagnetic correlations at about 50 K. The triangular structure of the Tm lattice suggests the presence of significant magnetic frustration in this system, which may lead to either a noncollinear staggered spin structure or intrinsic disorder.

Understanding and controlling macroscopic quantities directly associated with a random field, such as the coercive field H-c and dislocations in magnetic materials, is important for many applications that include films with perpendicular magnetic anisotropy. Here, using a model system with perpendicular magnetic anisotropy, Cu/Ni/Cu, we show that H-c in double 4 nm thick Ni films is about 0.65 times the value obtained for the single Ni film, a fact that can be understood if the propagation of misfit dislocations is not transmitted to the second magnetic block, implying that its statistical distribution remains unchanged whereas the magnetic driving force increases with the nickel thickness. This interpretation is based on the direct measurement of the in-plane and out-of-plane lattice parameters of the Ni blocks by the extended x-ray absorption fine structure method, a chemically selective technique tuned to probe exclusively the environment of the Ni atoms. With this finding and applying the rigid domain walls model, the H-c ratio between double and single Ni films is calculated, yielding a value of 0.71.

The in-plane orientation of the magnetization vector M in bcc-like Fe(110) films grown on Cu(001) is determined by means of scanning electron microscopy with polarization analysis. For thicknesses of 2 nm, slightly above the fcc/bcc phase transition, it is found that M is oriented along the < 110 > directions of the Cu(001) substrate. Following the Pitsch orientational relationship these correspond to magnetically hard < 1 (1) over bar1 > and < 1 (1) over bar2 > axes of bulk iron. This finding is in strong contrast to the behavior reported for thicker films (above 3 nm) of bcc Fe/Cu(001), where the < 100 > directions of the substrate are preferred. The role of strain in the iron film is discussed, inferring that the presence of a shear strain is mandatory to explain the spin reorientation via the magnetoelastic contribution to the magnetic anisotropy energy.

The micromagnetic structure in epitaxial (001)-oriented Cu/Ni(14 nm)/Cu rings fabricated by electron beam and focused ion beam lithographies with external diameter of 3 μm and linewidths between 100 and 500 nm is presented. We found that a state with radial orientation of the magnetization prevails at remanence. The evaluation of the magnetoelastic, magnetocrystalline and magnetostatic energies shows that a value as low as 1.5 × 10-3 for the anisotropic relaxation of the in-plane strain components is enough to induce an effective radial easy magnetization direction.

The role of the strain state in epitaxial (001)-oriented Cu/Ni(14 nm)/Cu rings is investigated using a combination of magnetic force microscopy and finite-element calculations. Rings with an external diameter of 3 and 2 mu m and linewidth W larger than 400 nm show two different structures: domains with magnetization oriented in the radial direction exist at the inner and outer radius, separated by an area in the interior of the ring consisting of stripe domains with perpendicular magnetization. The former is the sole magnetic structure observed for W < 400 nm. Micromagnetic calculations on narrow-linewidth structures indicate that the radial domain-wall structure consists of elliptical Bloch lines with a shorter and longer length along the tangential and radial directions, respectively. Finite-element calculations show that the anisotropic relaxation of the in-plane strain is larger at the ring inner and outer edges than in the interior part of the ring and accounts for the reorientation of the magnetization direction.

Understanding of the relationship between stress and magnetic properties in nanostructures is of both fundamental and practical interest. In the present paper, we illustrate this statement with some recent research results. First, we will see how the magnetoelastic interaction in Dy films controls the magnetic structure at the nanoscale due to the presence of the structural defects and their associated strain fields. Then, it will be shown how the magnetoelastic contribution can dominate the total anisotropy in epitaxial (100) oriented Cu/Ni/Cu nanowires, where the film patterning process performed to produce the nanowires induces strain changes large enough to favor a net in-plane anisotropy transverse to the lines.

We present epitaxial structures made of twin nickel blocks with perpendicular magnetic anisotropy separated by a copper layer which, for some values of this interleaving layer, show domain structures with four levels of contrast in magnetic force microscopy images. This manifold domain structure implies that the magnetization in the Ni blocks, in addition to the parallel orientation, undergoes a non-collinear configuration with respect to each other. To explain this result we consider a magnetoelastic domain structure with M in the plane that can elude the clamping done by the substrate with an average strain of −42 × 10−6 (≈70% of the bulk value). Thus, the out-of-plane anisotropy is balanced and a biquadratic exchange coupling can stabilize the non-collinear domain configurations between the Ni blocks.

The effective magnetic anisotropy K-eff has been studied at room temperature in Cu/Ni/Cu/Ni/Cu(001) epitaxial films with perpendicular magnetization as a function of the interleaving Cu block thickness t(Cu) ranging from 0 nm to 6 nm and being the Ni blocks 3 nm thick. We have found that the value of K-eff decreases when comparing the structures with t(Cu) = 0 and 0.2 nm, although the strain increases. For larger values of t(Cu), K-eff increases, although above t(Cu) = 1.8 nm, K-eff is nearly independent of t(Cu). These results can be explained using the measured in-plane strain epsilon and a surface anisotropy constant, K-s = -0.4 mJ/m(2).http://iopscience.iop.org/article/10.1088/1742-6596/200/7/072019