arXiv subjects

J. E. Prieto

Publications and source records attributed to J. E. Prieto.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic domains reconfiguration on the Fe3O4(110) surface across the Verwey transition by Spin-Polarized Low-Energy Electron Microscopy

We have studied the (110) surface of Fe$_3$O$_4$ single crystals by means of spin-polarized low-energy electron microscopy (SPLEEM). After preparation by sputtering and annealing a well defined reconstructed surface was achieved, composed of rows aligned in the [010] direction. By acquiring SPLEEM images along different spin directions the vector magnetization was mapped on the surface, both at room temperature and at a temperature well below the Verwey transition. At room temperature, domains were observed with their magnetization aligned along the two <111> bulk easy axes which are in the (110) surface plane. They presented 180$^\circ$, 71$^\circ$ and 109$^\circ$ Néel-type domain walls. Below the Verwey transition, the magnetization directions changed to regions where the magnetization was oriented along the in-plane [100] and [001] directions. Those observations can be interpreted as the presence of magnetized regions on the surface where the monoclinic $c$ axis is in-plane in the former, and regions where the $c$ is out-of-plane in an oblique direction in the latter. However, the magnetization was at all times within the surface plane, with no out-of-plane component detected.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic Circular Dichroism at the Oxygen K edge in Microcrystals of Spinels Grown on Ru(0001)

We have measured the circular magnetic dichroism in the x-ray absorption at the K-edge of oxygen in microcrystals of different spinel oxides. The microcrystals are islands of micrometric size and nanometric thickness, grown on Ru(0001) substrates using high-temperature oxygen-assisted molecular beam epitaxy. The domains observed in the oxygen K-edge dichroism have the same distribution and orientation as those observed in x-ray magnetic circular dichroism at the L$_{3}$ edge of the octahedral cations. Integrating the area from a single domain, x-ray magnetic circular dichroic spectra of oxygen were measured and, by the application of the K-edge sum rule, non vanishing orbital magnetic moments aligned with the octahedral cations were found. Density functional theory calculations, which did not show any orbital moment at the oxygen anions, indicate that the energy ranges where oxygen dichroism is observed correspond to those with significant hybridization with the cations d bands. They also show a correlation between the magnitude of the measured value of the oxygen orbital moment and the theoretical one for the cations, and demonstrate that this trend is preserved in the presence of Fe excess in the samples. Our experimental XMCD suggest, following the DFT calculations, that the origin of the oxygen magnetic moment lies in the hybridization of the oxygen unoccupied p-derived bands with the cation bands, mostly with the d-derived ones.

cond-mat.mtrl-sci↗

arXiv2023

Quantifying Li-content for compositional tailoring of lithium ferrite ceramics

Owing to their multiple applications, lithium ferrites are relevant materials for several emerging technologies. For instance, LiFeO2 has been spotted as an alternative cathode material in Li-ion batteries, while LiFe5O8 is the lowest damping ferrite, holding promise in the field of spintronics. The Li-content in lithium ferrites has been shown to greatly affect the physical properties, and in turn, the performance of functional devices based on these materials. Despite this, lithium content is rarely accurately quantified, as a result of the low number of electrons in Li hindering its identification by means of routine materials characterization methods. In the present work, magnetic lithium ferrite powders with Li:Fe ratios of 1:1, 1:3 and 1:5 have been synthesized, successfully obtaining phase-pure materials (LiFeO2 and LiFe5O8), as well as a controlled mixture of both phases. The powders have been compacted and subsequently sintered by thermal treatment (Tmax = 1100 °C) to fabricate dense pellets which preserve the original Li:Fe ratios. Li-content on both powders and pellets has been determined by two independent methods: (i) Rutherford backscattering spectroscopy combined with nuclear reaction analysis and (ii) Rietveld analysis of powder X-ray diffraction data. With good agreement between both techniques, it has been confirmed that the Li:Fe ratios employed in the synthesis are maintained in the sintered ceramics. The same conclusion is drawn from spatially-resolved confocal Raman microscopy experiments on regions of a few microns. Field emission scanning electron microscopy has evidenced the substantial grain growth taking place during the sintering process - mean particle sizes rise from about 600 nm in the powders up to 3.8(6) um for dense LiFeO2 and 10(2) um for LiFe5O8 ceramics.

cond-mat.mtrl-sci↗

arXiv2022

Effect of annealing in the formation of well crystallized and textured SrFe$_{12}$O$_{19}$ films grown by RF magnetron sputtering

We have studied the influence of annealing treatment on the crystalline growth of SrFe_12_O_19 previously deposited on Si (100) substrates using radio frequency (RF) magnetron sputtering. For this goal, two grown films, with and without \textit{ex-situ} heating step, have been analysed and compared to determine the differences in their structural, compositional and magnetic properties. The results obtained by the different analysis techniques, in particular Mössbauer spectroscopy together with EXAFS and XANES data, suggest that the as-grown film is composed of nanocrystalline maghemite nanoparticles and amorphous strontium oxide. A strontium hexaferrite canonical structure with c-axis orientation in the sample plane was found for the annealed film.

cond-mat.mtrl-sci↗

arXiv2022

Growth and characterization of ultrathin cobalt ferrite films on Pt(111)

CoFe2O4 thin films (5 nm and 20 nm thick) were grown by oxygen assisted molecular beam epitaxy on Pt(111) at 523~K and subsequently annealed at 773 K in vacuum or oxygen. They were characterized in-situ using Auger Electron Spectroscopy, Low-Energy Electron Diffraction, Scanning Tunneling Microscopy and Conversion Electron Mössbauer Spectroscopy. The as-grown films were composed of small, nanometric grains. Annealing of the films produced an increase in the grain size and gave rise to magnetic order at room temperature, although with a fraction of the films remaining in the paramagnetic state. Annealing also induced cobalt segregation to the surface of the thicker films. The measured Mössbauer spectra at low temperature were indicative of cobalt ferrite, the both films showing very similar hyperfine patterns. Annealing in oxygen or vacuum affected the cationic distribution, which was closer to that expected for an inverse spinel in the case of annealing in an oxygen atmosphere.

cond-mat.mtrl-sci↗

arXiv2021

Dynamics of Li deposition on epitaxial graphene/Ru(0001) islands

Li metal has been deposited on the surface of a Ru(0001) single crystal containing patches of monolayer-thick epitaxial graphene islands. The use of low-energy electron microscopy and diffraction allowed us to {\em in situ} monitor the process by measuring the local work function as well as to study the system in real and reciprocal space, comparing the changes taking place on the graphene with those on the bare Ru(0001) surface. It is found that Li deposition decreases the work function of the graphene islands but to a much smaller degree than of the Ru(0001) surface, as corresponds to its intercalation below the graphene overlayer. Finally, the diffusion process of Li out of the graphene islands has been monitored by photoelectron microscopy using a visible-light laser.

cond-mat.mtrl-sci↗

arXiv2021

RBS/Channeling characterization of Ru(0001) and thin epitaxial Ru/Al$_2$O$_3$(0001) films

Thin epitaxial films of metals on insulating substrates are essential for many applications, as conducting layers, in magnetic devices or as templates for further growth. In this work, we report on the growth of epitaxial Ru films on single-crystalline Al$_2$O$_3$(0001) substrates by magnetron sputtering and their subsequent systematic characterization using Rutherford backscattering spectrometry of He ions both in random and in channeling conditions. We include results of a Ru(0001) single crystal for comparison. Analysis of channeling shows that films thicker than 35 nm grow with (0001) orientation, a well-defined epitaxial relation with the substrate and a high degree of crystal quality, comparable to the Ru(0001) single crystal. Thinner films of down to 7 nm in thickness, for which relaxation of epitaxial strain is not complete, produce a similar degree of dechanneling. The surface of the films can be prepared in a clean and ordered state in order to allow further epitaxial growth on top.

cond-mat.mtrl-sci↗

arXiv2020

Influence of the growth conditions on the magnetism of SrFe$_{12}$O$_{19}$ thin films and the behavior of Co / SrFe$_{12}$O$_{19}$ bilayers

SrFe$_{12}$O$_{19}$ (SFO) films grown on Si (100) substrates by radio-frequency magnetron sputtering have been characterized in terms of composition, structural and magnetic properties by a combination of microscopy, diffraction and spectroscopy techniques. Mössbauer spectroscopy was used to determine the orientation of the films magnetization, which was found to be controlled by both the sputtering power and the thickness of the films. Additionally, the coupling between the SFO films and a deposited cobalt overlayer was studied by means of synchrotron-based spectromicroscopy techniques. A structural coupling at the SFO/Co interface is suggested to account for the expetimental observations. Micromagnetic simulations were performed in order to reproduce the experimental behaviour of the system.

cond-mat.mtrl-sci↗

arXiv2017

Cross sections of X-ray production induced on Ti, Fe, Zn, Nb and Ta by O, Cl, Cu and Br ions with energies between 4 MeV and 40 MeV

Differential cross section of X-ray production induced by O, Cl, Cu and Br ions with energies between 4 MeV and 40 MeV have been measured for thin targets of Ti, Fe, Zn, Nb and Ta in a direct way. A fully characterized silicon drift diode was used as X-rays detector. Beam currents have been measured by a system of two Faraday cups. Corrections for target thickness effects have been applied to the raw data. Experimental cross sections are compared both with theory and with previously published results. Experimental results from other authors are in reasonable agreement with ours over a wide energy range. Theory produces consistent results in the case of oxygen ions but gives cross sections even orders of magnitude below the experimental ones for heavier ions (ECPSSR-UA) or contrasting results (PWBA) depending on the ion-target combination.

physics.atom-ph↗

arXiv2017

Stranski-Krastanov mechanism of growth and the effect of misfit sign on quantum dots nucleation

The thermodynamics of the Stranski-Krastanov mode of epitaxial growth and the effect of the sign of the lattice misfit are discussed. The Stranski-Krastanov mode of growth represents a sequence of layer-by-layer or Frank-van der Merwe growth followed by the formation of three-dimensional (3D) islands or Volmer-Weber growth. The occurrence of both growth modes mentioned above is in compliance with the wettability criterion of Bauer. The positive wetting function required for the occurrence of the Volmer-Weber growth is originated by the vertical displacements of the atoms close to the edges of the two-dimensional (2D) islands as a result of the relaxation of the lattice misfit. The monolayer high islands become unstable against bilayer islands, bilayer islands in turn become unstable against trilayer islands, etc. beyond some critical islands sizes. Monolayer islands appear as necessary precursors of three-dimensional (3D) islands. The critical island size for mono-bilayer transformation increases steeply with decreasing lattice misfit and diverges at a critical value of the misfit. This value divides the regions of Frank-van der Merwe and Stranski-Krastanov modes in a phase diagram of coordinates wetting-misfit. The transformation of monolayer to multilayer islands takes place either by consecutive nucleation and growth of 2D islands (layer-by-layer transformation), or by nucleation and lateral (2D) growth of multilayer islands (multilayer 2D transformation). The former occurs in the case of "stiff" overlayer materials and mostly in compressed overlayers. The latter takes place in the case of "soft" materials like Pb and In, mostly in tensile overlayers. Tensile films show non-nucleation transformation compared with the nucleation-like behavior of compressed films.

cond-mat.mtrl-sci↗

arXiv2017

A procedure to correct for target thickness effects in heavy-ion PIXE at MeV energies

We describe a novel procedure for the calculation of correction factors for taking into account the effect of target thickness to be applied to the determination of cross sections of X-ray emission induced by heavy ions at MeV energies. We discuss the origin of the correction and describe the calculations, based on simple polynomial fits of both the theoretical cross sections and the ion energy losses. The procedure can be easily implemented. We show several examples for a set of targets specifically produced for cross section measurements and for various combinations of ion type and energy.

physics.atom-ph↗

arXiv2016

Magnetism of epitaxial Tb films on W(110) studied by spin-polarized low-energy electron microscopy

Thin epitaxial films of Tb metal were grown on a clean W(110) substrate in ultra-high vacuum and studied in-situ by low-energy electron microscopy. Annealed films present magnetic contrast in spin-polarized low-energy electron microscopy. The energy dependence of the electron reflectivity was determined and a maximum value of its spin asymmetry of about 1\% was measured. The magnetization direction of the Tb films is in-plane. Upon raising the temperature, no change in the domain distribution is observed, while the asymmetry in the electron reflectivity decreases when approaching the critical temperature, following a power law $ \sim (1 - T/T_C)^β$ with a critical exponent $β$ of 0.39.

cond-mat.mtrl-sci↗

arXiv2016

Cross sections of X-ray production induced by C and Si ions with energies up to 1 MeV/u on Ti, Fe, Zn, Nb, Ru and Ta

X-ray production differential cross sections induced by C and Si ions with energies from 1 MeV/u down to 0.25 MeV/u, produced by the CMAM 5 MV tandem accelerator, have been measured for thin targets of Ti, Fe, Zn, Nb, Ru and Ta in a direct way. X-rays have been detected by a fully characterized silicon drift diode and beam currents have been measured by a system of two Faraday cups. Measured cross sections agree in general with previously published results. The ECPSSR theory with the united atoms correction gives absolute values close to the experimental ones for all the studied elements excited by C ions and for Ta, Nb and Ru excited by Si ions. For Ti, Fe and Zn excited by Si, the basic ECPSSR theory gives better agreement, although on absolute values the gap for Ti is still large.

physics.atom-ph↗

arXiv2014

Step energies and equilibrium shape of strained monolayer islands

Using a simple atomistic model of anharmonic nearest-neighbors interaction, we have calculated the step energies of strained hexagonal monolayer islands. These have been found to decrease with the absolute value of the misfit due to the strain relaxation at steps. The effect is significantly more pronounced in the case of positive misfit owing to the stronger repulsive interatomic forces. Furthermore, (111)-faceted steps are favored at positive misfit (compressed islands) and, to a lesser extent, (100)-faceted steps at negative misfits (tensile islands). The result is rationalized in terms of the different bonding geometries at step edges and a comparison with experiments is included. Thus, the equilibrium shape transforms from regular hexagons at zero misfit to threefold symmetric hexagons with increasing misfit.

cond-mat.mtrl-sci↗

arXiv2011

Second-layer nucleation in coherent Stranski-Krastanov growth of quantum dots

We have studied the monolayer-bilayer transformation in the case of the coherent Stranski-Krastanov growth. We have found that the energy of formation of a second layer nucleus is largest at the center of the first-layer island and smallest on its corners. Thus nucleation is expected to take place at the corners (or the edges) rather than at the center of the islands as in the case of homoepitaxy. The critical nuclei have one atom in addition to a compact shape, which is either a square of i*i or a rectangle of i*(i-1) atoms, with i>1 an integer. When the edge of the initial monolayer island is much larger than the critical nucleus size, the latter is always a rectangle plus an additional atom, adsorbed at the longer edge, which gives rise to a new atomic row in order to transform the rectangle into the equilibrium square shape.

cond-mat.mtrl-sci↗

arXiv2011

Shower approach in the simulation of ion scattering from solids

An efficient approach for the simulation of ion scattering from solids is proposed. For every encountered atom, we take multiple samples of its thermal displacements among those which result in scattering with high probability to finally reach the detector. As a result, the detector is illuminated by intensive "showers", where each event of detection must be weighted according to the actual probability of the atom displacement. The computational cost of such simulation is orders of magnitude lower than in the direct approach and a comprehensive analysis of multiple and plural scattering effects becomes possible. We use the new method for two purposes. First, the accuracy of the approximate approaches, developed mainly for ion-beam structural analysis, is verified. Second, the possibility to reproduce a wide class of experimental conditions is used to analyze some basic features of ion-solid collisions: the role of double violent collisions in low-energy ion scattering; the origin of the "surface peak" in scattering from amorphous samples; the low-energy tail in the energy spectra of scattered medium-energy ions due to plural scattering; the degradation of blocking patterns in 2D angular distributions with increasing depth of scattering. As an example of simulation for ions of MeV energies, we verify the time-reversibility for channeling/blocking of 1 MeV protons in a W crystal. The possibilities of analysis that our approach offers may be very useful for various applications in particular for structural analysis with atomic resolution.

cond-mat.mtrl-sci↗

arXiv2010

Effect of the lattice misfit on the equilibrium shape of strained islands in Volmer-Weber growth

We have studied the effect of the misfit on the equilibrium shape of three-dimensional pyramidal islands grown on a foreign substrate in the case of incomplete wetting (Volmer-Weber mode of growth). We have found that tensile islands have smaller aspect ratios compared with compressed islands owing to its better adhesion to the substrate. The average strains of consecutive layers decrease faster with thickness in compressed than in tensile islands. The strains decrease rapidly with thickness, with the consequence that above a certain height, the upper layers of the pyramid become practically unstrained and does not contribute to a further reduction in the upper base. As a result, the truncated pyramids are not expected to transform into full pyramids. Our results are in good agreement with experimental observations in different systems.

cond-mat.mtrl-sci↗

arXiv2008

Temperature-induced reversal of magnetic interlayer exchange coupling

For epitaxial trilayers of the magnetic rare-earth metals Gd and Tb, exchange coupled through a non-magnetic Y spacer layer, element-specific hysteresis loops were recorded by the x-ray magneto-optical Kerr effect at the rare-earth $M_5$ thresholds. This allowed us to quantitatively determine the strength of interlayer exchange coupling (IEC). In addition to the expected oscillatory behavior as a function of spacer-layer thickness $d_Y$, a temperature-induced sign reversal of IEC was observed for constant $d_Y$, arising from magnetization-dependent electron reflectivities at the magnetic interfaces.

cond-mat.mtrl-sci↗