Félix Jiménez-Villacorta

Room-temperature ferromagnetism in the mixtures of the TiO2 and Co3O4 powders

We report here the observation of ferromagnetism (FM) at 300 K in mixtures of TiO₂ and Co₃O₄ powders despite the antiferromagnetic and diamagnetic characters of both oxides, respectively. The ferromagnetic behavior is found in the early stages of reaction and only for TiO₂ in anatase structure; no FM is found for identical samples prepared with rutile-TiO². Optical spectroscopy and x-ray absorption spectra confirm a surface reduction of octahedral Co^(+3) -> Co^(+2) in the mixtures which is in the origin of the observed magnetism.

Predicting magnetostructural trends in FeRh-based ternary systems

Correlations between magnetic transition temperatures and the average weighted valence band electron concentration ((s + d) electrons/atom) have led to the development of a phenomenological model that predicts the influence of elemental substitution on the magnetostructural response of bulk B2-ordered Fe(Rh1−xMx) or (Fe1−xMx)Rh alloys (M = transition elements; x < 6 at. %). Validation of this model is provided through synthesis and characterization of FeRh with Cu and Au additions. The data and associated trends indicate that the lattice and electronic free energies are both equally important in driving the magnetostructural transition in the bulk FeRh system.

X-ray diffraction and extended x-ray absorption fine-structure characterization of nonspherical crystallographic grains in iron thin films

The characterization of iron thin films grown at different substrate temperatures has been performed by x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS). The film growing of iron at low temperatures provides an excellent system to test the results obtained from both techniques because the crystallographic grains present a variation of size and shape as a function of the growing temperature. In both cases, the shape of the particle must be taken into account to calculate their size. The comparison gives a very good agreement when appropriate models are used, showing the reasons for possible differences between the results obtained from a more simple XRD and EXAFS analysis applied to columnar growth systems.

Surface magnetism in ZnO/Co3O4 mixtures

We recently reported the observation of room temperature ferromagnetism in mixtures of ZnO and Co3O4 despite the diamagnetic and antiferromagnetic character of these oxides, respectively. Here, we present a detailed study on the electronic structure of this material in order to account for the unexpected ferromagnetism. Electrostatic interactions between both oxides lead to a dispersion of Co3O4 particles over the surface of ZnO larger ones. As a consequence, the reduction Co+3→Co2+ at the particle surface takes place as evidenced by x-ray absorption spectroscopy measurements and optical spectroscopy. This reduction allows explaining the observed ferromagnetic signal within the well established theories of magnetism in oxides.

Synthesis and Characterization of a Coupled Binuclear CuI/CuIII Complex

The synthesis through reaction of a C(alpha),C(ortho) dilithiated phosphazene with CuBr and structural characterization of the first example of a binuclear mixed valence [Cu(I)(N(2))/Cu(III)(C(4))] complex showing a metal-metal bond, as well as its applications in cyclopropanation and oxidation reactions, are described.

Towards tailoring the magnetocaloric response in FeRh-based ternary compounds

In this work, we demonstrate that the magnetocaloric response of FeRh-based compounds may be tailored for potential magnetic refrigeration applications by chemical modification of the FeRh lattice. Alloys of composition Fe(Rh1−xAx) or (Fe1−xBx)Rh (A = Cu, Pd; B = Ni; 0 < x < 0.06) were synthesized via arc-melting and subsequent annealing in vacuum at 1000 °C for 48 h. The magnetocaloric properties of the FeRh-based systems were determined using isothermal M(H) curves measured in the vicinity of the magnetostructural temperature (Tt). It is found that the FeRh working temperature range (δTFWHM) may be chemically tuned over a wide temperature range, 100 K ≤ T ≤ 400 K. While elemental substitution consistently decreases the magnetic entropy change (ΔSmag) of the FeRh-based ternary alloys from that of the parent FeRh compound (ΔSmag,FeRh ∼ 17 J/kg K; ΔSmag,FeRh-ternary = 7–14 J/kg K at Happ = 2 T), the net refrigeration capacity (RC), defined as the amount of heat that can be transferred during one magnetic r...

Chemical and structural characterization of copper adsorbed on mosses (Bryophyta).

The adsorption of copper on passive biomonitors (devitalized mosses Hypnum sp., Sphagnum denticulatum, Pseudoscleropodium purum and Brachythecium rutabulum) was studied under different experimental conditions such as a function of pH and Cu concentration in solution. Cu assimilation by living Physcomitrella patents was also investigated. Molecular structure of surface adsorbed and incorporated Cu was studied by X-ray Absorption Spectroscopy (XAS). Devitalized mosses exhibited the universal adsorption pattern of Cu as a function of pH, with a total binding sites number 0.05-0.06 mmolg(dry)(-1) and a maximal adsorption capacity of 0.93-1.25 mmolg(dry)(-1) for these devitalized species. The Extended X-ray Absorption Fine Structure (EXAFS) fit of the first neighbor demonstrated that for all studied mosses there are ∼4.5 O/N atoms around Cu at ∼1.95 Å likely in a pseudo-square geometry. The X-ray Absorption Near Edge Structure (XANES) analysis demonstrated that Cu(II)-cellulose (representing carboxylate groups) and Cu(II)-phosphate are the main moss surface binding moieties, and the percentage of these sites varies as a function of solution pH. P. patens exposed during one month to Cu(2+) yielded ∼20% of Cu(I) in the form of Cu-S(CN) complexes, suggesting metabolically-controlled reduction of adsorbed and assimilated Cu(2+).

Quality of graphene on sapphire: long-range order from helium diffraction versus lattice defects from Raman spectroscopy

We report a new method to produce high-quality, transparent graphene/sapphire samples, using Cu as a catalyst. The starting point is a high-quality graphene layer prepared by CVD on Cu(111)/Al2O3. Graphene on sapphire is obtained in situ by evaporation of the Cu film in UHV. He-diffraction, atomic force microscopy (AFM), Raman spectroscopy and optical transmission have been used to assess the quality of graphene in a metal free area. We used helium atom scattering as a sensitive probe of the crystallinity of the graphene on sapphire. The observation of high reflectivity and clear diffraction peaks demonstrates the presence of flat and homogeneous graphene domains over lateral scales of microns, consistent with the AFM results. Surprisingly, putting graphene on sapphire improves the quality of the He-diffraction spectra. Graphene forms a moire pattern with a (11 × 11) periodicity, aligned with the (1 × 1) sapphire unit cell. The lattice constant of graphene on sapphire is a = (2.44 ± 0.02) A. The phonon dispersion of the graphene flexural mode has been measured. This allowed the determination of the bending rigidity k = 0.61 ± 0.15 eV, and the graphene–sapphire coupling strength g = (5.8 ± 0.4) × 1019 N m−3. The uniformity of the graphene has also been investigated by Raman mapping. Judging by the ratio of the 2D to G peaks, the quality of the graphene is not degraded by Cu removal. The high transparency (80%) measured in the visible range makes this system suitable for many applications that require hybrid properties commonly associated with metals (conductivity) and insulators (transparency). Our study shows that He-diffraction and Raman provide crucial information on quite different, complementary aspects of the same samples.

Mn local order in room temperature ferromagnetic Mn/ZnO multilayers

The local environment of Mn atoms in (ZnO30 A/Mnt)n multilayers (ML) has been studied by means of x-ray absorption spectroscopy at the Mn K edge. A series of ML has been prepared by sputtering with approximately constant total amount of Mn (300 A) while the Mn layer thickness (t) has been varied from 60 to 1 A (modifying consequently the number of Mn–ZnO interfaces) in order to study the Mn–Zn–O system. Variation of the Mn layer thickness (t) has been directly associated with changes in the Mn oxidation states within the sample series. Absorption spectroscopy results for films with t≤15 A indicate mostly MnO2 formation and no signal of Mn in the ZnO lattice is found. Conversely, films with t≥30 A present two different Mn environments, that have been related to the coexistence of rocksalt and wurtzite phases, formed by Zn1−xMnxO mixed solid solutions. The magnetic behaviour of samples is correlated with structural properties. That correlation strongly points towards substitutional Mn in the wurtzite phase as the origin of the observed ferromagnetism above room temperature.

Tuning the magnetostructural phase transition in FeRh nanocomposites

Effects of nanostructuring on the magnetostructural response of the near-equiatomic FeRh phase were investigated in nanocomposite materials synthesized by rapid solidification and subsequent annealing of an alloy of nominal atomic composition (FeRh)5Cu95. Transmission electron microscopy studies confirm attainment of a phase-separated system of nanoscaled (∼10–15 nm diameter) precipitates, consistent with FeRh embedded in a Cu matrix. These nanoprecipitates are crystallographically aligned with the coarse-grained Cu matrix and possess an L10-type (CuAu 1) structure, in contrast to the B2 (CsCl)-type structure of bulk FeRh. It is proposed that the face-centered cubic crystal structure of the Cu matrix serves as a template for the formation and stabilization of the L10 structure in the FeRh nanoprecipitates. Magnetic measurements highlight the existence of multiple magnetic phases in the material exhibiting spin-glass (T ≤ 15 K), ferromagnetic and paramagnetic (T > 20 K) behavior. A thermally hysteretic mag...