Growth of barium tungstate nanostructures by MBE, depositing only Ba on W(110) in an oxygen atmosphere. The tungsten atoms are incorporated into the structure through the interaction between the substrate and the oxygen at high temperature.

We report on the growth of nanowires of ferberite (FeWO4) by high-temperature oxygen-assisted molecular beam epitaxy on W(110). This multifunctional material has promising applications in different fields. The wires extend for several millimeters in length, with widths in the hundreds and heights in the tens of nanometers. We have monitored the growth process by real-time low-energy electron microscopy and characterized the wires in-situ by low-energy electron microscopy and laterally-resolved X-ray absorption and photoelectron spectroscopies. Further analysis was performed ex-situ by atomic force and optical microscopies as well as by Raman spectroscopy. The growth of ferberite on W(110) was possible by dosing iron in a molecular oxygen atmosphere likely due to the formation of highly mobile WOx units that can be incorporated into the anisotropic wolframite structure, which in turn is responsible for the highly anisotropic growth. We propose that the same method may be used for the growth of other tungstate or related compounds.

This work investigates the structural and magnetic properties of the off-stoichiometric Heusler alloy Fe₁₁Mn₂Ga₃. X-ray diffraction confirms a face-centered cubic L2₁ structure with significant chemical disorder. Magnetic characterization reveals a complex phase diagram with two distinct transitions: a Curie temperature at Tm2 =309 K and a magnetic reorientation at Tm1 =167 K, below which antiferromagnetic interactions dominate, evidenced by a reentrant decrease in magnetization. This behavior yields a bifunctional magnetocaloric response, with a direct effect near Tm1 and an inverse effect near Tm2. Mössbauer spectroscopy confirms a ferrimagnetic state at room temperature and provides microscopic evidence for the development of non-collinear magnetism at low temperatures. The novelty of these results lies in the consequences of the strong Fe-rich off-stoichiometry, which stabilizes chemical disorder and shifts the balance beteween competing ferromagnetic and antiferromagnetic interactions to unusually high temperatures. The results demonstrate that strategic off-stoichiometry is a powerful tool for stabilizing disordered states and engineering novel magnetic functionality in Heusler alloys.

We describe the growth of synthetic h & uuml;bnerite (MnWO4) by high-temperature oxygen-assisted molecular beam epitaxy on W(110). The h & uuml;bnerite nanowires have widths of hundreds of nanometers, heights of tens of nanometers and lengths in the range of millimeters. The growth was followed in real time by low-energy electron microscopy (LEEM). The nanowires were characterized in situ by low-energy electron microscopy, X-ray absorption and X-ray photoelectron spectroscopy in photoemission microscopy, as well as ex situ by atomic force microscopy, optical microscopy and Raman spectroscopy. H & uuml;bnerite can be grown on W(110) by dosing only manganese in a molecular oxygen environment, likely due to the formation of highly mobile WOx species with diffusion lengths of the order of hundreds of micrometers. These species can react with the deposited Mn and be efficiently incorporated into the wolframite structure of h & uuml;bnerite. The strongly anisotropic growth observed may stem from the inherent anisotropy of the wolframite lattice. We propose that this method may be applicable to the growth of other tungstates as well.

We investigate the interfacial spin structure of prototypical room-temperature antiferromagnet NiO, epitaxially grown on Fe3O4(111)/Ru(0001). The heterostructures were fabricated using high-temperature oxygen-assisted molecular beam epitaxy on a Ru single-crystal substrate. A comprehensive structural characterization was carried out by low-energy electron microscopy and diffraction, while chemical and magnetic properties were probed using photoemission electron microscopy combined with synchrotron radiation. Specifically, X-ray absorption spectroscopy and X-ray magnetic circular/linear dichroism enabled nanoscale analysis of the magnetic ordering. Although bulk NiO is antiferromagnetic and exhibits no net magnetization, hence showing no circular dichroism, interfacial interactions at the Fe3O4/NiO boundary can significantly influence the magnetic domain configuration in NiO, leading to emergent interfacial magnetic behavior. By resolving the spin axis orientation of NiO with nanometer precision and correlating it with the ferrimagnetic domain structure of Fe3O4, we provide new insight into the interfacial coupling mechanisms.

BaFe12O19 (BFO) thin films have been grown on Si(100) substrates by magnetron sputtering from previously synthesized ceramic BFO targets and have been compositionally and structurally characterized. Films grow with the c-axis orientation and magnetization direction parallel to the sample plane. In addition, the magnetic coupling between the BFO film and a deposited cobalt overlayer was studied. Images of X-ray magnetic circular dichroism in photoemission microscopy show magnetic regions in the BFO layer with domain sizes of several micrometers and others without magnetic contrast, the latter attributed to the presence of hematite. Magnetic domains in the Co overlayer show no significant correlations with those in the BFO film, pointing to a negligible magnetic coupling. (c) 2025 The Author(s). Published by Elsevier Espana, S.L.U. on behalf of SECV. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/).

We present the in situ modification of the magnetocrystalline anisotropy of iron-rich cobalt-ferrite epitaxial islands by changing their composition. Crystalline cobalt-ferrite spinel islands have been grown by oxygen-assisted molecular beam epitaxy on a Ru(0001) single crystal. The micro-crystals are several micrometers wide with heights of tens of nanometers. Their spatially-resolved vector magnetization is mapped by x-ray magnetic circular dichroism in photoemission microscopy at the L3 absorption edges of Co and Fe. Thick islands present a closure-like magnetic domain structure with the magnetization directions predominantly aligned along the projections of the easy magnetic axes onto the (111) surface plane. Thinner islands are more affected by growth defects and present a more complex domain structure. Upon deposition of additional Fe in an oxygen background pressure, Co is found to segregate out of the spinel islands, while their Fe content increases. This produces a reduction in the magnetocrystalline anisotropy, which manifests as a reorientation of the magnetization directions towards the edges of the islands, reflecting an increased contribution of the shape anisotropy.

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 L3edge 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 density functional theory 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 and does not correspond to a localized orbital magnetic moment in the oxygen in spinels.

We report here on the ICEMS characterization of nickel ferrite (NixFe3−xO4) thin films having different nickel contents grown on alumina substrates by Ion Beam Sputtering. The spectra corresponding to the films with nominal x = 0.7, 1.0 and 1.2 are characteristic of compounds crystallizing in a spinel-related structure showing two different magnetic sextets associated with Fe3+ located in the tetrahedral and octahedral sites of such structure. The spectra show an additional broad third sextet with a large isomer shift which suggests the occurrence of electron hopping between Fe2+ and Fe3+ ions sitting in the octahedral sites. With increasing nickel content, the linewidth of the sextets increases and their corresponding hyperfine magnetic fields decrease. This is an indication of an increase in structural disorder in the deposited films as their nickel concentrations increase. The cation distribution of the iron ions over the tetrahedral and octahedral sites appears also to depend on the nickel content. The film with x = 1.2 shows a significant increase in the fraction of octahedral iron ions as compared with the expected nominal value suggesting that, for this composition, some Ni2+ could also occupy tetrahedral sites. The Mössbauer spectrum corresponding to the film with x = 1.7 shows a magnetic pattern with very broad lines similar to those shown by amorphous or disordered materials. The average isomer shift is quite high (around 0.40 mms− 1) and characteristic of Fe3+ in octahedral oxygen coordination. This indicates that for the largest nickel content studied (x = 1.7), the film does not contain Fe3+ in tetrahedral environments suggesting that the spinel structure is no longer present. This correlates well with the X-Ray Diffraction data which indicate a structural change from spinel to a disordered rock-salt structure for this particular film with high nickel content.

Thin films of lithium spinel ferrite, LiFe5O8, have attracted much scientific attention because of their potential for efficient excitation, the manipulation and propagation of spin currents due to their insulating character, high-saturation magnetization, and Curie temperature, as well as their ultra-low damping value. In addition, LiFe5O8 is currently one of the most interesting materials in terms of developing spintronic devices based on the ionic control of magnetism, for which it is crucial to control the lithium’s atomic content. In this work, we demonstrate that dual ion beam sputtering is a suitable technique to tailor the lithium content of thin films of lithium ferrite (LFO) by using the different energies of the assisting ion beam formed by Ar+ and O2+ ions during the growth process. Without assistance, a disordered rock-salt LFO phase (i.e., LiFeO2) can be identified as the principal phase. Under beam assistance, highly out-of-plane-oriented (111) thin LFO films have been obtained on (0001) Al2O3 substrates with a disordered spinel structure as the main phase and with lithium concentrations higher and lower than the stoichiometric spinel phase, i.e., LiFe5O8. After post-annealing of the films at 1025 K, a highly ordered ferromagnetic spinel LFO phase was found when the lithium concentration was higher than the stoichiometric value. With lower lithium contents, the antiferromagnetic hematite (α-Fe2O3) phase emerged and coexisted in films with the ferromagnetic LixFe6-xO8. These results open up the possibility of controlling the properties of thin lithium ferrite-based films to enable their use in advanced spintronic devices.

We have grown high-quality magnetite micrometric islands on ruthenium stripes on sapphire through a combination of magnetron sputtering (Ru film), high-temperature molecular beam epitaxy (oxide islands), and optical lithography. The samples have been characterized by atomic force microscopy, Raman spectroscopy, X-ray absorption and magnetic circular dichroism in a photoemission microscope. The magnetic domains on the magnetite islands can be modified by the application of current pulses through the Ru stripes in combination with magnetic fields. The modification of the magnetic domains is explained by the Oersted field generated by the electrical current flowing through the stripes underneath the magnetite nanostructures. The fabrication method is applicable to a wide variety of rock salt and spinel oxides.

We present a spatially resolved X-ray magnetic linear dichroism study of high quality micron-sized mixed nickel-cobalt oxide (NCO) crystals. NixCo1-xO was prepared in-situ by high-temperature oxygen-assisted molecular beam epitaxy on a Ru(0001) single crystal substrate. To check the effect of incorporating Ni into the cobalt oxide films, three different compositions were prepared. The element-specific XMLD measurements reveal strong antiferromagnetic contrast at room temperature and magnetic domains up to one micron in size, reflecting the high structural quality of the NCO islands. By means of vectorial magnetometry, the antiferromagnetic spin axis orientation of the domains was determined with nanometer spatial resolution, and found to depend on the stoichiometry of the prepared crystals.

We employ hierarchy of models (HoM) with 2, 3 and 4 parameters to reanalyze the nucleation kinetics in the electrodeposition as quantified by n(t) - “the number of nuclei vs. time” data obtained at different overvoltages from precise experiments on electrodeposition of Hg on two different types of Pt-cathodes by Ivan V. Markov and Evgenia Stoycheva [1]. The obtained two scales, nmax and , are used to rescale the original data mapping it on a master curve in coordinates . Our main result is obtained by studying further the dependence of these two scales on the overvoltage (analogue of supersaturation). Surprisingly, for one of the two cathode types - the “hemispherical single-crystal electrode” (as defined by the authors), there is discontinuity in the “ vs. overvoltage” dependence – an almost horizontal jump from 85 to 86 mV accompanied by a change in the slope of the dependence - it is ~ -1 and ~ -0.5 after. For the other cathode - “plane structureless platinum electrode”,  decreases smoothly. Combined, these two behaviors point at the so called “cusp catastrophe”. We compare our findings with published results on protein nucleation.

We present a spatially resolved X-ray magnetic linear dichroism study of high quality micron-sized mixed nickel-cobalt oxide (NCO) crystals. Ni x Co 1−x O were prepared in-situ by high-temperature oxygen-assisted molecular beam epitaxy on Ru(0001) single crystal. To check the effect of incorporating Ni in the cobalt oxide films, three different composition were prepared. The element specific XMLD measurements reveal strong antiferromagnetic contrast at room temperature and magnetic domains up to one micron wide, reflecting the high structural quality of the islands. By means of vectorial magnetometry, the spin axis orientation of the domains was determined with nanometer spatial resolution, and found to depend on the stoichiometry of prepared crystals.

Using a hierarchy of three sigmoid growth models with increasing complexity, i.e., number of parameters, we reanalyzed kinetic data for heterogeneous nucleation—the number of nuclei N(t) vs. time t—from archetypical experiments on the electrodeposition of mercury on platinum by I. Markov and E. Stoycheva, to obtain two scales: Nmax and τ. The universal character of the studied phenomenon was revealed when replotting the original data as α ≡ N(t)/Nmax vs. t/τ. Yet the simplest model, the recently introduced α21 model which is aimed to describe diffusion-limited growth in 2D, α21 = tanh2(2t/τ21), fits all datasets with an R2 ≥ 0.989. This can be rationalized by attracting the non-classical notion of two-step nucleation—the nuclei form in a metastable phase which, in this case, grows on the electrode surface. Beyond the universality, we find the dependence of the two obtained scales on the overvoltage, which is increased systematically from 83 to 88 mV to generate the six N(t) datasets for each of the two electrode types—planar and hemispherical. Surprisingly, for one of them, the planar electrode, there is a discontinuity in the dependence—an almost horizontal jump from 85 to 86 mV, while for the hemispherical electrode, τ decreases smoothly.

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 degrees C) to fabricate dense pellets which preserve the original Li:Fe ratios. Li-content on both powders and pellets has been deter-mined 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 tech-niques, 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 approximate to 600 nm in the powders up to 3.8(6) mu m for dense LiFeO2 and 10(2) mu m for LiFe5O8 ceramics. Additionally, microstructural analysis has revealed trapped pores inside the grains of the sintered ceramics, suggesting that grain boundary mobility is governed by surface diffusion. Vibrating sample magnetometry on the ceramic samples has confirmed the ex-pected soft ferrimagnetic behavior of LiFe5O8 (with Ms = 61.5(1) Am2/kg) and the paramagnetic character of LiFeO2 at room temperature. A density of 92.7(6)% is measured for the ceramics, ensuring the mechanical integrity required for both their direct utilization in bulk shape and their use as targets for thin-film deposition.

We report on the response of magnetic domains in thin magnetite microstructures to weak external magnetic fields. Magnetite islands were grown by high-temperature oxygen-assisted molecular beam epitaxy on Ru(0001). The islands, micrometric wide and tens of nanometers thick are of high structural quality, each having been grown from a single nucleation center. Their magnetic domain structure is dominated by shape anisotropy, i.e., they present Landau flux-closure domain configurations. The magnetic domains of the in situ grown microstructures have been imaged directly by means of x-ray magnetic circular dichroism in photoemission electron microscopy while applying external, in-plane magnetic fields along different directions. Upon application of an external field the Landau state vortex core experiences a displacement along a direction perpendicular to the excitation field. The behavior of the Landau state under the applied magnetic field is quantified and compared with micromagnetic simulations. The results highlight the bulk-like magnetic properties of the nanometer-thick microstructures, opening the way to their possible use in technological applications.

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 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.

We have studied the influence of the 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 ex situ heating step, have been analyzed 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. Specifically, Mössbauer spectroscopy results pointed out the presence of Fe 3+ cations occupying octahedral and tetrahedral sites with hyperfine magnetic fields 49.3 T and 44.2 T, respectively, characteristic of a spinel-related structure. A strontium hexaferrite canonical structure with a c-axis orientation in the sample plane was found for the annealed film. Graphical abstract

We have monitored the Verwey transition in micrometer-wide, nanometer-thick magnetite islands on epitaxial Ru films on Al2O3(0001) using Raman spectroscopy. The islands have been grown by high-temperature oxygen-assisted molecular beam epitaxy. Below 100 K and for thicknesses above 20 nm, the Raman spectra correspond to those observed in bulk crystals and high-quality thin films for the sub-Verwey magnetite structure. At room temperature, the width of the cubic phase modes is similar to the best reported for bulk crystals, indicating a similar strength of electron-phonon interaction. The evolution of the Raman spectra upon cooling suggests that for islands thicker than 20 nm, structural changes appear first at temperatures starting at 150 K while the Verwey transition itself takes place at around 115 K. However, islands thinner than 20 nm show very different Raman spectra, indicating that while a transition takes place, the charge order of the ultrathin islands differs markedly from their thicker counterparts.