Felix Jimenez-Villacorta is Research Associate of the Electronic and Magnetic Materials and Heterostructures Group, starting early 2014.
His research lines cover fundamental aspects of nanomagnetism, which include the study and development of advanced magnetic materials with potential immediate implementation in permanent magnets or tunable magnetostructural-based sensors, as well as fundamental analysis to gain insight into the magnetic interactions and magnetic phenomena at the atomic and nanoscopic scale.
At the present, he is involved in the development of graphene-nanoparticle hybrid materials with controllable magnetic, optical and transport properties. Also, he is very active in the study of multifunctional magnetic nanostructures and heterostructures for advanced applications in information technologies.
Keywords: nanomagnetism, materials for information technologies, graphene-based hybrid materials, magnetostructural functional nanomaterials, nanostructured permanent magnetic materials, x-ray absorption spectroscopy (EXAFS, XANES, XMCD).
Invited speaker: “Nanoscale strategies towards development of advanced Mn-based permanent magnets”
Our recent work highlighted in the ICMM website:
R. Ramírez-Jíménez, L. Álvarez-Fraga, F. Jimenez-Villacorta, E. Climent-Pascual, C. Prieto and A. de Andrés, “Increasing the Interference Enhanced Raman Effect for sensing: learning from graphene bubbles”, Carbon105, 556 (2016)
F. Jiménez-Villacorta, E. Climent-Pascual, R. Ramírez-Jiménez, J. Sánchez-Marcos, C. Prieto and A. de Andrés, “Graphene – ultrasmall silver nanoparticle interactions and their effect on electronic transport and Raman enhancement”, Carbon 101, 305 (2016).
Graphene-nanostructure hybrid materials for advanced sensing applications
Combination of metallic nanostructures with graphene is envisioned to modify with accuracy and to confer extra functionality to the already extraordinary properties of graphene. Physical vapor deposition methods, such as magnetron sputtering or gas-phase aggregation techniques, are reliable methods to incorporate functional nanostructures. The gradual manipulation of the optical and electric properties in these hybrid materials is anticipated to give insight into the development of tunable touch, chemical and biosensors and other advanced electronic devices.
Mn-based rare-earth-free permanent magnetic nanostructured alloys
The so-called “rare-earth crisis” in the 2010’s re-ignited investigation in the search for new concepts in permanent magnetic materials design. The key factor is that advanced fabrication and analysis methods with precision down to the nanoscale that combine composition and crystal structure control and optimization of the microstructure to manipulate the intrinsic magnetic properties of magnets (magnetization, exchange and magnetocrystalline anisotropy) or to enhance extrinsic magnetic features (remanence and coercivity) are now accessible. Thus, current strategies involve nanostructuring and control of crystal structure and composition to the nanoscale through metallurgical non-equilibrium processing techniques convey optimization of the magnetic properties or advantageous modification of the fabrication process of new magnets.
Magnetic materials presenting magnetostructural transitions are subject of intense study in the current days, due to their more than interesting properties, as well as to their potential applications in magnetic memory, caloric nd sensor devices. A very intereseting research pathway is the study of the magnetostructural transition of FeRh (AF-FM with simultaneous lattice expansion at around ambient temperatures) subjected to atomic substitution and nanostructuring effects. Results are providing exciting hints in order to develop nanomagnetic materials with tunable properties.
Graphene - ultrasmall silver nanoparticle interactions and their effect on electronic transport and Raman enhancement
F. Jimenez-Villacorta, E. Climent-Pascual, R. Ramirez-Jimenez, J. Sanchez-Marcos, C. Prieto, A. de Andres,
Carbon 101,305 (2016).
Abstract
The modulation of electrical and Raman properties of ultrafine (~4 nm) Ag nanoparticle/graphene/SiO2 hybrid material at low coverage is evaluated with gradual nanoparticle incorporation by the gas aggregation deposition technique. The different contributing factors, such as doping, impurity scattering or strain, are assessed. Incorporation of Ag nanoparticles produce a very efficient n-type doping of graphene (~7.5 e- per particle) maintaining a constant mobility for particle coverage below ~0.3 monolayers. Doping efficiency is determined by the probability for nanoparticles to be deposited in contact with graphene. The Fermi level upshift is modeled within the charged impurity scattering mechanism in the whole coverage range. A crossover to the regime where impurity scattering dominates is evidenced at large particle concentration. Surface-enhanced Raman scattering is detected in graphene phonons in the limit of vanishing plasmon resonance and very low coverage (~0.08) that correspond to ~1500 nanoparticles at the laser spot. Small distortions of the graphene lattice (~0.012 %) induced by the nanoparticles overcome the predicted changes in Raman phonons related to carrier doping and originate I2D/IG damping. This evolution of physical properties with gradual incorporation of Ag nanoparticles is anticipated to provide valuable hints to tune the optical and electronic performance of these graphene-based hybrid systems.
Interference Enhanced Raman Effect in graphene bubbles
Carbon 105,556 (2016).
Abstract
Stable Graphene/gas/Cu bubbles are formed by the spontaneous oxidation at room conditions of the Cu substrates used as the catalyst agent in chemical vapor deposition growth of graphene. The non homogeneous copper oxidation produces discontinuous Cu2O films where atmospheric molecules get trapped by the impermeable graphene layer forming bubbles. Raman Interference effects are calculated both for graphene and Cu2O phonons in graphene/Cu2O/Cu system using the transfer matrix method finding that Cu2O thickness is limited to 20 nm and maximum Raman enhancement of graphene is 10. Nevertheless, enhancement values up to 60 are detected for micrometric graphene bubbles on copper which, according to the simulations, are 70 nm height. AFM measurements confirm the bubble dimensions and show that they are mechanically softer than graphene on copper. Consistently, the Raman characteristics of the graphene bubbles coincide with free standing undoped graphene with small strain, in the range 0–0.3%, while graphene on Cu2O presents hole doping and larger strain fields. The simulations indicate that interference Raman enhancement can reach values >104 in graphene/dielectric/Aluminum trilayers (dielectric = gas, oxide or polymer). These systems are therefore interesting platforms for optical sensing where graphene is the adequate bio-compatible layer for metallic nanoparticles and molecules deposition.
Indium-tin oxide thin films deposited at room temperature on glass and PET substrates: Optical and electrical properties variation with the H-2-Ar spu
L. Alvarez-Fraga, F. Jimenez-Villacorta, J. Sanchez-Marcos, A. de Andres, C. Prieto,
Appl Surf Sci 344,217 (2015).
Abstract
The optical and electrical properties of indium tin oxide (ITO) films deposited at room temperature on glass and polyethylene terephthalate (PET) substrates were investigated. A clear evolution of optical transparency and sheet resistance with the content of H-2 in the gas mixture of H-2 and Ar during magnetron sputtering deposition is observed. An optimized performance of the transparent conductive properties ITO films on PET was achieved for samples prepared using H-2/(Ar + H-2) ratio in the range of 0.3-0.6%. Moreover, flexible ITO-PET samples show a better transparent conductive figure of merit, Phi(TC) = T-10/R-S, than their glass counterparts. These results provide valuable insight into the room temperature fabrication and development of transparent conductive ITO-based flexible devices. (C) 2015 Elsevier B.V. All rights reserved.
Elastic constants of graphene oxide few-layer films: correlations with interlayer stacking and bonding
RJJ. Rioboo, E. Climent-Pascual, X. Diez-Betriu, F. Jimenez-Villacorta, C. Prieto, A. de Andres,
J Mater Chem C 3,4868 (2015).
Abstract
We propose a strategy to study the elastic properties of extremely thin graphene oxide (GO) films using Brillouin spectroscopy. The dependence of the surface acoustic wave of a gold capping layer on the structural, chemical and morphological changes occurring to the underneath GO film with temperature is reported and analyzed. At room temperature the shear constant c(44) is similar to 17 GPa and hardens up to 28 GPa at 100 degrees C due to the partial elimination of embedded water layers and to interlayer distance shrinking. At 200 degrees C the almost complete elimination of water induces layer stacking disorder, further GO-GO distance reduction and a significant increase of all elastic constants. The in-plane constants harden due to the partial restoration of the sp(2) C network (c(11): from 268 to 620 GPa) and the out of plane constants harden due to the H bonds that now directly connect the neighbouring GO layers (c(44) approximate to 80 GPa). The obtained Young's moduli are significantly higher than those reported for GO paper because the ultra-thin GO films are highly ordered and there is no macroscopic applied strain during the measurement. The results obtained here are associated with the intrinsic properties of GO as in-plane and inter-layer bonding.
Tailoring exchange coupling and phase separation in Fe-Co-Mn nanocomposites
F. Jimenez-Villacorta, I. McDonald, D. Heiman, LH. Lewis,
J Appl Phys 115,17A729 (2014).
Abstract
An intriguing pathway for the realization of metallic, rare-earth-free magnets is described. Creation of a so-called "exchange-bias" permanent magnet is contemplated that mimics the microstructure of Alnico magnets, comprised of FeCo-based precipitates that are separated by a metallic antiferromagnetic phase that replaces non-magnetic NiAl-rich phase of alnico. In this manner, the existing shape anisotropy is augmented by exchange-bias anisotropy through interphase coupling, providing enhanced coercivities. As a proof of concept, nanocomposite alloys have been fabricated from rapidly solidified Fe34Co33Mn33 ribbons that phase separate into an antiferromagnetic Mn-rich c-phase and ferromagnetic alpha-FeCo nanoprecipitates upon post-solidification processing. A progressive enhancement of coercivity, remanence, and exchange bias at T < T-N in the ferromagnetic-antiferromagnetic nanocomposite is noted upon annealing, with maximum properties realized at T-ann approximate to 673K for 30 min. Optimization of the microstructure of these Fe-Co-Mn-based alloys is anticipated to be accompanied by improved magnetic performance that could contribute to the development of next-generation permanent magnets. (C) 2014 AIP Publishing LLC.
Magnetism-structure correlations during the epsilon-->tau transformation in rapidly-solidified MnAl nanostructured alloys†(invited article)
F. Jimenez-Villacorta, JL. Marion, M. Daniil, MA. Willard, LH. Lewis,
Metals 4,8 (2014).
Abstract
Magnetic and structural aspects of the annealing-induced transformation of rapidly-solidified Mn55Al45 ribbons from the as-quenched metastable antiferromagnetic (AF) ε-phase to the target ferromagnetic (FM) L10 τ-phase are investigated. The as-solidified material exhibits a majority hexagonal ε-MnAl phase revealing a large exchange bias shift below a magnetic blocking temperature TB~95 K (Hex~13 kOe at 10 K), ascribed to the presence of compositional fluctuations in this antiferromagnetic phase. Heat treatment at a relatively low annealing temperature Tanneal ≈ 568 K (295 °C) promotes the nucleation of the metastable L10 τ-MnAl phase at the expense of the parent ε-phase, donating an increasingly hard ferromagnetic character. The onset of the ε→τ transformation occurs at a temperature that is ~100 K lower than that reported in the literature, highlighting the benefits of applying rapid solidification for synthesis of the rapidly-solidified parent alloy.
Permanent, or hard, magnets deliver high magnetic flux combined with high stability against demagnetization. These properties render them essential in modern life: they are key components in efficient motors and electric machines and are integral in energy harvesting and conversion, power generation, sensors and in a plethora of automation-based devices. Permanent magnets are also highlighted as paradigmatic examples of the close relationship between basic research industry and technological implementation, as basic research advances may be directly transitioned to realization of better magnets with enhanced technical performance.
Throughout the 20th century, researchers have achieved advances (sometimes disruptive, sometimes just incremental) in diversification and improvement of candidates for numerous permanent magnetic applications. However, earlier researchers did not have the analytical probes and tools that are available today. In the last few years, recent insights derived from advanced tools, design of nanostructured magnets with controlled average crystallite size and orientation, optimized intergranular phase character and directed engineering of intrinsic features via atomic substitution will foster the realization of new materials that may be processed into the next generation of advanced permanent magnets.
This chapter is organized as follows: in the first section, concepts and tools utilized by researchers to develop, tailor and enhance the performance of permanent magnetic materials are presented; in the second section, an up-to-date report of materials synthesized and processed in diverse manners for development of the next generation of permanent magnets is described, with special attention devoted to technical achievements obtained on nanostructured magnets.
Perspectives on Permanent Magnetic Materials for Energy Conversion and Power Generation (Review Article)
LH. Lewis, F. Jimenez-Villacorta,
Metall. Mater. Trans. A 44A,2 (2013).
Abstract
Permanent magnet development has historically been driven by the need to supply larger magnetic energy in ever smaller volumes for incorporation in an enormous variety of applications that include consumer products, transportation components, military hardware, and clean energy technologies such as wind turbine generators and hybrid vehicle regenerative motors. Since the 1960s, the so-called rare-earth "supermagnets," composed of iron, cobalt, and rare-earth elements such as Nd, Pr, and Sm, have accounted for the majority of global sales of high-energy-product permanent magnets for advanced applications. In rare-earth magnets, the transition-metal components provide high magnetization, and the rare-earth components contribute a very large magnetocrystalline anisotropy that donates high resistance to demagnetization. However, at the end of 2009, geopolitical influences created a worldwide strategic shortage of rare-earth elements that may be addressed, among other actions, through the development of rare-earth-free magnetic materials harnessing sources of magnetic anisotropy other than that provided by the rare-earth components. Materials engineering at the micron scale, nanoscale, and Angstrom scales, accompanied by improvements in the understanding and characterization of nanoscale magnetic phenomena, is anticipated to result in new types of permanent magnetic materials with superior performance. DOI: 10.1007/s11661-012-1278-2 (C) The Minerals, Metals & Materials Society and ASM International 2012
Exchange anisotropy in the nanostructured MnAl system
F. Jimenez-Villacorta, JL. Marion, T. Sepehrifar, M. Daniil, MA. Willard, LH. Lewis,
Appl Phys Lett 100,112408 (2012).
Abstract
In this letter, we report on the achievement of exchange anisotropy magnitude in a nanostructured Mn55Al45 alloy fabricated by rapid solidification with large exchange bias values (H-E approximate to 13 kOe at 10K) and a blocking temperature of T-B similar to 95 K. Field-cooled magnetization loops show a prominent exchange bias for T < T-B signaling the simultaneous presence of antiferromagnetic and ferromagnetic phases at these temperatures. Structural probes confirm a majority presence of the high-temperature metastable hexagonal epsilon-MnAl in the as-solidified state with an intriguing double-Bragg peak structure indicative of phase separation. The observed exchange bias is hypothesized to originate from an intimate mixture of antiferromagnetic and nanoscaled ferromagnetic phases or dual mictomagnetic phases, approximating a cluster glass with well-defined variations in the local Mn concentration of the composition and leading to Mn-rich and Mn-poor regions with antiferromagnetic and ferromagnetic characters, respectively. (C) 2012 American Institute of Physics. [http://dx.org/10.1063/1.3695153]
Nanostructured Fe Rh in metallic and insulating films
B. Kaeswurm, F. Jimenez-Villacorta, SP. Bennett, D. Heiman, LH. Lewis,
J Magn Magn Mater 354,284 (2014).
Abstract
The formation of nanostructured FeRh in nonmagnetic metallic and insulating matrices is investigated and correlated with their structural and magnetic properties, with the goal of clarifying synthesis-structure-property relationships in confined geometries. Films containing FeRh with Al2O3 and Cu matrices were deposited on Si substrates using RF sputtering and then annealed up to 1073 K to form chemically-ordered FeRh particles with the B2 structure. FeRh nanostructures formed within the refractory Al2O3 matrix are rounded in shape and have a smaller average particle size of 60 nm, compared to the more irregular nanostructures obtained in the metallic Cu-FeRh sample with a larger average particle size similar to 150 nm The different morphology of the two samples is explained in terms of the dissimilar properties of the matrices, and opens pathways for potential control of the fabrication of FeRh nanostructures. (C) 2013 Elsevier B.V. All rights reserved
Towards tailoring the magnetocaloric response in FeRh-based ternary compounds
R. Barua, F. Jimenez-Villacorta, LH. Lewis,
J Appl Phys 115,17A903 (2014).
Abstract
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(Rh(1-x)A(x)) 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 degrees 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 (T-t). It is found that the FeRh working temperature range (delta T-FWHM) may be chemically tuned over a wide temperature range, 100 K <= T <= 400 K. While elemental substitution consistently decreases the magnetic entropy change (Delta S-mag) of the FeRh-based ternary alloys from that of the parent FeRh compound (Delta S-mag,(FeRh) similar to 17 J/kg K; Delta S-mag,S-FeRh-ternary = 7-14 J/kgK at H-app = 2 T), the net refrigeration capacity (RC), defined as the amount of heat that can be transferred during one magnetic refrigeration cycle, of the modified systems is significantly higher (RCFeRh similar to 150 J/kg; RCFeRh-ternary = 170-210 J/kg at H-app = 2 T). These results are attributed to stoichiometry-induced changes in the FeRh electronic band structure and beneficial broadening of the magnetostructural transition due to local chemical disorder. (C) 2014 AIP Publishing LLC.
Structural evidence for stabilized ferromagnetism in epitaxial FeRh nanoislands (Editor's selection - Highlights of 2013)
M. Loving, F. Jimenez-Villacorta, B. Kaeswurm, DA. Arena, CH. Marrows, LH. Lewis,
J Phys D Appl Phys 46,162002 (2013).
Abstract
Nanoislands of alpha'-FeRh were formed by the deposition of a 10 nm film onto (0 0 1)-MgO and observed by atomic force microscopy. This island-like architecture results in stabilized ferromagnetic (FM) ordering at low temperatures as noted by asymmetry in the x-ray diffraction peaks, a magnetostructural transition with a large magnetic background signal, and broad thermal hysteresis. The combination of structural and magnetic results suggest that the crystallographic arrangement of the FeRh nanoislands consists of a gradient of lattice parameters where there is a compressed inner region of lower lattice parameter values (attributed to antiferromagnetic ordering) which gradually relaxes outward to a region of larger lattice parameters (attributed to a retained FM ordering). The impact of this configuration on magnetostructural transformation is discussed in the context of classical nucleation theory.
R. Barua, F. Jimenez-Villacorta, JE. Shield, D. Heiman, LH. Lewis,
J Appl Phys 113,17B523 (2013).
Abstract
A granular system of FeRh-based nanoprecipitates (similar to 10-15 nm diameter) embedded in a rapidly solidified copper ribbon matrix was found to transit from a metastable tetragonal L1(0) (AuCu-1-type) structure to a stable B2 (CsCl-type) structure upon annealing-induced coarsening to similar to 94 nm. The hysteretic magnetic transition observed at similar to 100 K develops a gradual broadening that accompanies the L1(0) -> B2 crystal structure transition. It is proposed that the Cu matrix influences the structural and magnetic properties of the FeRh-based nanoparticles through interfacial strain and chemical effects. These results emphasize the sensitivity of the magnetostructural response of FeRh to changes in the nanostructural scale, and provide pathways for tailoring the transition. (C) 2013 AIP Publishing LLC.
Predicting magnetostructural trends in FeRh-based ternary systems
R. Barua, F. Jimenez-Villacorta, LH. Lewis,
Appl Phys Lett 103,102407 (2013).
Abstract
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. (C) 2013 AIP Publishing LLC.
Tailored Exchange in Binary Manganese-Noble Metal Alloys
F. Jimenez-Villacorta, D. Heiman, JL. Marion, LH. Lewis,
Ieee Mag Lett 4,1000204 (2013).
Abstract
A general phenomenological model is presented that predicts the evolution of magnetic order and transition temperatures found in binary noble metal (NM) alloys with high Mn content. The model is derived from magnetic measurements and analysis of rapidly solidified Ag1-xMnx (0.25 <= x <= 0.4) alloys, also describing the magnetic attributes of CuMn and AuMn alloys, as reported in the literature. Mn atomic substitution in the face-centered-cubic noble metal lattice results in a tunable average Mn-Mn characteristic interatomic distance estimated from the average Mn concentration that governs the evolution of the magnetic ordering temperature. This evolution is found to be a consequence of the competition between positive ferromagnetic (FM) and negative antiferromagnetic (AF) interatomic exchange coupling revealing a Bethe-Slater-type behavior. Considering the composition range analyzed in this letter and the previous literature (x = 0.02-0.40), the ordering temperature is maximized at x approximate to 0.17 for Ag-Mn and Cu-Mn alloys, with a progressive decrease at higher Mn concentration that is indicative of an increasing AF character. These predictive results enable the development of potential pathways to manipulate and tailor magnetic attributes in Mn-based alloys that are of current interest for tunable exchange-biased systems.
Decoupling mechanisms and magnetic stability of nanostructured iron chains prepared by sputtering
F. Jimenez-Villacorta, E. Cespedes, C. Ocal, C. Prieto,
Appl Phys Lett 98,102513 (2011).
Abstract
Nanostructured Fe/Fe-oxide systems with a chain morphology formed by interconnected metallic clusters were prepared by sputtering at very low substrate temperatures. The effect of the in situ controlled oxidation at low temperature leads to an effective decoupling of the particles forming the chains, which is attributed to a physical separation of particles through formation of a core-shell metal-oxide structure. In contrast, samples oxidized at room conditions exhibit features of strongly correlated particle systems, in which magnetic decoupling and stabilization can be explained within the framework of the random anisotropy model by considering the effect of the oxide phase. (C) 2011 American Institute of Physics. [doi:10.1063/1.3559918]
X-ray magnetic circular dichroism study of the blocking process in nanostructured iron-iron oxide core-shell systems
F. Jimenez-Villacorta, C. Prieto, Y. Huttel, ND. Telling, G. van der Laan,
Phys Rev B 84,172404 (2011).
Abstract
Experimental evidence for magnetothermal behavior in iron-iron oxide nanostructured systems has been obtained using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Fe L(2,3) edges. The purpose of this study is the determination of the blocked state in these spin-glass-like core-shell systems. A first overview of the magnetic species participating in the magnetic response was obtained by analyzing the XMCD at saturating fields. Also, the XAS revealed the existence of an antiferromagnetic FeO phase, likely located at the interface regions. Finally, measurements were performed at low temperature and intermediate field, where a frozen state below the blocking energy is observed. The results show that the oxide phase spins are oriented at low temperature, while the magnetic spins of the metallic core do not contribute to the XMCD, suggesting that the blocking process mainly involves the magnetic particle superspins rather than the oxide coverage phase.
Effects of interparticle interactions in magnetic Fe/Si3N4 granular systems
F. Jimenez-Villacorta, J. Sanchez-Marcos, E. Cespedes, M. Garcia-Hernandez, C. Prieto,
Phys Rev B 82,134413 (2010).
Abstract
An experimental evidence of the progressive modification in the magnetic behavior of granular Fe/Si3N4 samples due to interaction effects between particles is reported. Microstructural features and local structure were determined by x-ray absorption spectroscopy and transmission electron microscopy to select granular samples with predetermined cluster size. Fe/Si3N4 systems have been characterized by ac- and demagnetization measurements to study the gradual evolution of magnetic properties of granular systems, where three different behaviors have been observed. As-deposited samples with Fe thickness layers of 2.5 nm, present a modified superparamagnetic behavior, due to very weak interactions between very small Fe clusters separated by a nonmagnetic FeN phase. An evolution of the average blocking temperature at intermediate fields (T-B similar to H-3/2) is observed, similar to noninteracting systems, but first signatures of a frozen spin state at low temperatures appear. Annealed samples exhibit a noticeable modification from the multilayer character to a random three-dimensional organization of Fe clusters embedded in a Si3N4 matrix. After annealing, samples with initial Fe layer thickness of 0.7 nm provide iron cluster in the range of 1.3 nm and exhibit a superspin-glass state, with a de Almeida-Thouless evolution of the energy barriers (T-B similar to H-2/3) that is explained in terms of increasing interparticle interactions. Moreover, annealed samples, with initial layer thickness of 1.3 nm, supply iron cluster of near 3 nm that present stronger interactions and yield a superferromagnetic state, likely provided by residual ultrasmall particles between the blocked clusters.
Magnetic properties and interaction mechanisms of iron-based core-shell structures prepared by sputtering at low substrate temperatures
F. Jimenez-Villacorta, C. Prieto,
J Phys: Condens Matter 20,085216 (2008).
Abstract
The magnetic properties of partially oxidized nanocrystalline iron thin films prepared by DC-magnetron sputtering at low substrate temperatures in the 175-300 K range are studied. The preparation method is presented as a simple method for fabricating granular structures. Films prepared at intermediate temperatures exhibit granular magnetic behaviour, in which nanocrystalline grains act as almost decoupled particles, surrounded by an oxide shell, forming exchange bias core-shell systems. The magnetic features of granular systems obtained by this new method are described and the mechanisms of interaction between metallic grains and their oxide shells are explained, as are their effects in the magnetization reversal process.
Strain-driven transition from E-type to A-type magnetic order in YMnO3 epitaxial films
F. Jimenez-Villacorta, JA. Gallastegui, I. Fina, X. Marti, J. Fontcuberta,
Phys Rev B 86,024420 (2012).
Abstract
Single-crystal (100)-oriented YMnO3 thin films grown on (110)-oriented SrTiO3 substrates have been studied by polarized extended x-ray absorption fine structure to determine the Mn-O bond lengths. Using a simple geometrical model and previously reported x-ray diffraction data on the same samples, the Mn-O-Mn bonding angles are calculated. We show that the epitaxy-induced in-plane anisotropic strain has a dramatic impact on the bonding angles, allowing the rationalization of the reported existence of cycloidalmagnetic order and concomitant ferroelectricity in moderately strained films and the gradual suppression by larger strains. We shall argue that epitaxial strain allows shifting YMnO3 from an E-type to A-type antiferromagnetic ground state, crossing a cycloidal magnetic region.
X-ray Absorption Spectroscopy and Mossbauer Spectroscopy Studies of Superparamagnetic ZnFe2O4 Nanoparticles
V. Blanco-Gutierrez, F. Jimenez-Villacorta, P. Bonville, MJ. Torralvo-Fernandez, R. Saez-Puche,
J Phys Chem C 115,1627 (2011).
Abstract
Zn ferrite nanoparticles ranging from 3 to 19 nm have been obtained under solvothermal conditions. In addition, two samples of ZnFe2O4/SiO2 nanocomposite of 11 and 20 nm ferrite particle size have been also synthesized by means of the sol gel method. To evaluate the inversion degree of these ferrite samples with mixed spinel structure ((Zn1-xFex)[ZnxFe2-x]-O-4), X-ray absorption spectroscopy measurements at the Zn and Fe K-edge have been performed, and the obtained spectra along with the Fourier transform of the EXAFS signal have been compared to those corresponding to a ceramic sample with a lower inversion degree. All the samples composed by nanosized particles illustrate superparamagnetic behavior above the blocking temperature as it can be inferred from the high values of ZFC and FC magnetic susceptibility that they present. Mossbauer measurements done to the different nanosized particles obtained by the solvothermal method were useful to study the "frozen" to "superparamagnetic" crossover with the thermal variation and estimate the anisotropy constant that has been found to increase as the particle size decreases.
On the Origin of the Magnetism of Mn-Zn-O Systems: Structural, Electronic, and Magnetic Study of Exotic MnO2-delta/ZnO Thin Films
E. Cespedes, MA. Laguna-Marco, F. Jimenez-Villacorta, J. Chaboy, R. Boada, C. Guglieri, A. de Andres, C. Prieto,
J Phys Chem C 115,24092 (2011).
Abstract
The structural, electronic, and magnetic characterization of sputtered MnO2-delta/ZnO films showing room temperature ferromagnetism has been carried out by a number of techniques. To elucidate the origin of the ferromagnetic order, a novel approach by studying XAS and XMCD signals at both Mn and Zn K edges in combination with the macroscopic magnetization measurements has been performed. The analysis of the XAS profile at the Mn edge indicates the coexistence of Mn4+/Mn3+ with a slightly distorted MnO2-delta environment. The XMCD signal at the Zn K edge indicates the existence of an intrinsic magnetic moment in Zn. The Zn edge dichroic intensity is, however, 1 order of magnitude lower than in the case of Mn, being the magnetic moment of Mn, 2.1-2.4 mu(B) per Mn cation participating in the ferromagnetic phase, the main contribution to the total magnetization. The ferromagnetic phase has been assigned to high-density regions of Mn cations with a MnO2-delta distorted environment, mainly allocated at the nonstoichiometric and highly defective ZnO grain boundaries, with a sufficient extension to overcome the thermal energy over room temperature. The ferromagnetic coupling has been related to modifications of the Mn-O-Mn bonds, distances, and angles, promoted by neighboring ZnO and forming stable magnetic configurations that lead to new forms of exchange interaction: ferromagnetic superexchange or a double-exchange mechanism between the Mn4+ and Mn3+. Highly defective regions and structural distortions, together with complex Mn oxidation states, appear to be key features in the magnetic properties of Mn-Zn-O.
Stabilization and Study of SrFe1-xMnxO2 Oxides with Infinite-Layer Structure
M. Retuerto, F. Jimenez-Villacorta, MJ. Martinez-Lope, MT. Fernandez-Diaz, JA. Alonso,
Inorg Chem 50,10929 (2011).
Abstract
A series of layered oxides of nominal composition SrFe1-xMnxO2 (x = 0, 0.1, 0.2, 0.3) have been prepared by the reduction of three-dimensional perovskites SrFe1-xMnxO3-delta with CaH2 under mild temperature conditions of 583 K for 2 days. The samples with x = 0, 0.1, and 0.2 exhibit an infinite-layer crystal structure where all of the apical O atoms have been selectively removed upon reduction. A selected sample (x = 0.2) has been studied by neutron powder diffraction (NPD) and X-ray absorption spectroscopy. Both techniques indicate that Fe and Mn adopt a divalent oxidation state, although Fe2+ ions are under tensile stress whereas Mn2+ ions undergo compressive stress in the structure. The unit-cell parameters progressively evolve from a = 3.9932(4) angstrom and c = 3.4790(4) angstrom for x = 0 to a = 4.00861(15) angstrom and c = 3.46769(16) angstrom for x = 0.2; the cell volume presents an expansion across the series from V = 55.47(1) to 55.722(4) angstrom(3) for x = 0 and 0.2, respectively, because of the larger effective ionic radius of Mn2+ versus Fe2+ in four-fold coordination. Attempts to prepare Mn-rich compositions beyond x = 0.2 were unsuccessful. For SrFe0.8Mn0.2O2, the magnetic properties indicate a strong magnetic coupling between Fe2+ and Mn2+ magnetic moments, with an antiferromagnetic temperature T-N above room temperature, between 453 and 523 K, according to temperature-dependent NPD data. The NPD data include Bragg reflections of magnetic origin, accounted for with a propagation vector k=(1/2,1/2,1/2). A G-type antiferromagnetic structure was modeled with magnetic moments at the Fe/Mn position. The refined ordered magnetic moment at this position is 1.71(3)mu(B)/f.u. at 295 K. This is an extraordinary example where Mn2+ and Fe2+ ions are stabilized in a square-planar oxygen coordination within an infinite-layer structure. The layered SrFe1-xMnxO2 oxides are kinetically stable at room temperature, but in air at similar to 170 degrees C, they reoxidize and form the perovskites SrFe1-xMnxO3-delta. A cubic phase is obtained upon reoxidation of the layered compound, whereas the starting precursor SrFeO2.875 (Sr8Fe8O23) was a tetragonal superstructure of perovskite.
Room-temperature ferromagnetism in the mixtures of the TiO2 and Co3O4 powders (Editor's suggestion)
A. Serrano, EF. Pinel, A. Quesada, I. Lorite, M. Plaza, L. Perez, F. Jimenez-Villacorta, J. de la Venta, MS. Martin-Gonzalez, JL. Costa-Kramer, JF. Fernandez, J. Llopis, MA. Garcia,
Phys Rev B 79,144405 (2009).
Abstract
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.
Microstructural properties and local order around iron in granular metal-insulator Fe/Si3N4 systems prepared by magnetron sputtering
F. Jimenez-Villacorta, E. Cespedes, M. Vila, A. Munoz-Martin, GR. Castro, C. Prieto,
J Phys D Appl Phys 41,205009 (2008).
Abstract
[Fe/Si(3)N4] multilayers have been prepared by magnetron sputtering with a layer thickness of 3 nm for silicon nitride and the iron layer ranging from 1 to 10 nm. A variety of techniques for determining the microstructure, such as x-ray reflectivity, transmission electron microscopy and Rutherford backscattering spectrometry techniques, have been performed to unveil the microstructure of such samples. Also, an analysis of the local environment around iron, by means of extended x-ray absorption fine structure and x-ray absorption near-edge structure spectroscopies performed at the Fe K-edge, confirms the presence of two phases forming the iron layers, the metallic Fe and the FeN phase. Special emphasis has been placed on the x-ray absorption analysis for multilayers with smaller Fe layer thicknesses, which present features of granular systems.
Chemical and Structural Characterization of Copper Adsorbed on Mosses (Bryophita)
AG. Gonzalez, F. Jimenez-Villacorta, AK. Beike, R. Reski, P. Adamo,OS. Pokrovsky,
J Hazard Mater 308,343 (2016).
Abstract
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 mmolgdry−1 and a maximal adsorption capacity of 0.93–1.25 mmolgdry−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 Cu2+ yielded ∼20% of Cu(I) in the form of Cu-S(CN) complexes, suggesting metabolically-controlled reduction of adsorbed and assimilated Cu2+.
Environmental Influence on Zn-Histidine Complexes under No-Packing Conditions
P. Ferrer, F. Jimenez-Villacorta, J. Rubio-Zuazo, I. da Silva, GR. Castro,
J Phys Chem B 118,2842 (2014).
Abstract
This paper describes a combined structural analysis of the Zn-histidine complex, using two different and complementary techniques, X-ray absorption spectroscopy (XAS) and surface X-ray diffraction, paying special attention to the environmental conditions. The current procedure for investigating macromolecules consists of examining simple molecules that exhibit properties similar to those of the larger ones, whose functionality is totally related to the atomic structure. The detailed study of the bonding structure formed by zinc and histidine amino acids is motivated by the fact that this material serves as a model for metalloproteins, such as in metalloproteinase, acting as active sites in enzymatic or structural functions. For XAS modeling, Zn-histidine complexes were dissolved in several aqueous solutions, over a wide pH range. Correlations among the degree of protonation, the steric impediment, and the multiple combinations of the histidine amino acid have been found. For the diffraction study, high-quality crystals grown by the seeding method in a supersaturated solution have been studied, and the samples for the surface diffraction study were mounted on a cell specially designed for solid-liquid or solid-gas interface analysis. The surface structural model was built from XAS results. In both cases, the obtained structures are compared with the bulk one, showing atomic differences and highlighting the importance of the environment in which the complex is studied.
Iron adsorption onto soil and aquatic bacteria: XAS structural study
AG. Gonzalez, OS. Pokrovsky, F. Jimenez-Villacorta, LS. Shirokova, JM. Santana-Casiano, M. Gonzalez-Davila, EE. Emnova,
Chem Geol 372,32 (2014).
Abstract
Although the interaction between Fe and microorganisms has been extensively studied, the main physicochemical factors controlling the mechanisms of Fe adsorption and precipitation on bacterial cell walls remain poorly understood. In this study, we quantified thermodynamic parameters of the Fe adsorption reaction and characterized the speciation of Fe adsorbed on the surface of cyanobacteria and soil heterotrophic bacteria. For this purpose, the molecular mechanisms of iron interaction with typical aquatic and soil bacteria were investigated by combining batch macroscopic adsorption experiments with atomic-level Fe K-edge X-ray absorption fine structure spectroscopy (XAFS). Three cyanobacteria species (Synechococcus sp., Planktothrix sp. and Gloeocapsa sp.) and aerobic heterotrophic soil rhizobacterium (Pseudomonas aureofaciens) were used for Fe3+ and Fe2+ adsorption experiments. These experiments were carried out for a wide range of initial iron concentration (4.5-57.3 mu M) and pH (2.0-6.5). Surface adsorption data were rationalized using a Linear Programming Model (LPM), which allowed quantification of the surface adsorption constants and the number of binding sites. XAS (XANES and EXAFS) analysis of adsorbed iron demonstrated the predominance of O-coordinated Fe3+ species. Moreover, XANES data treatment using a linear combination fit of reference compounds suggested that the atomic environment of iron adsorbed onto soil bacterial surfaces was dominated by phosphoryl moieties with a lesser amount of carboxylates and some contribution of Fe(III)-oxy(hydr) oxide component. Complete oxidation of Fe(II) to Fe(III) was observed in the solid phase as determined by XANES analysis. Binding of Fe(III) to carboxylate groups was only significant for capsular cyanobacteria (Gloeocapsa sp.). The relative proportions of various Fe species at the cell surface determined by thermodynamic analysis of the macroscopic data and by XAS are in a good agreement. Our results suggest that, in the presence of surface organic ligands, the oxidation of divalent iron does occur, but the polymerization of formed Fe(III) oxy(hydr) oxides is partially inhibited and adsorbed iron in the form of both Fe-O-Fe polymers and individual Fe atoms attached to phosphoryl moieties. The presence of EPS reducesmetal-cell binding capacity and enhances Fe polymerization at the bacterial surface. (C) 2014 Elsevier B.V. All rights reserved.
Structure-Composition-Property Relationships of 6H-BaTi1-yCoyO3-delta (0.1 <= y <= 0.4)
L. Miranda, K. Boulahya, M. Hernando, DC. Sinclair, F. Jimenez-Villacorta, A. Varela, JM. Gonzalez-Calbet, M. Parras,
Chem Mater 23,1050 (2011).
Abstract
A solid solution of 6H-type BaTi1-yCoyO3-delta samples where 0.1 <= y <= 0.4 and delta <= 0.05 has been prepared in air and characterized by a combination of X-ray, neutron, and electron diffraction, X-ray absorption spectroscopy, hydrogen-reduction thermogravimetric analysis, high resolution electron microscopy, magnetic susceptibility measurements, and impedance spectroscopy. For y = 0.1, Co acts as an acceptor dopant and is incorporated as Co-III ions with charge balanced achieved by the formation of O(2) oxygen vacancies in the h-BaO3 layers that separate pairs of face-sharing octahedra. For samples with y > 0.1, Co is present as both Co-III and Co-II ions and in all cases delta similar to 0.03. This indicates a change from primarily aliovalent (Co-III) to isovalent (Co-IV) doping with increasing Co-content with a general formula BaTi1-yIV((Co0.06Coy-0.06IV)-Co-IV)O-2.97 for 0.2 <= y <= 0.4. All samples are semiconducting at room temperature with relative permittivity in the range similar to 20-50. The bulk conductivity increases with y and displays complex, non-Arrhenius-type behavior. All samples show paramagnetic behavior that can be fitted to the Curie-Weiss law from similar to 150 to 300 K with a negative Weiss temperature indicating antiferromagnetic interactions between the magnetic Co ions. X-ray absorption spectroscopy data on a sample of y = 0.1 that retained the 6H-structure after heat treatment under reducing conditions revealed the existence of mixed state Co-II and Co-III ions. A solid solution volume therefore exists for 6H-Ba(Ti,Co)O3-delta where the oxidation state of Co can vary from II to IV depending on the preparative conditions.
Synthesis and Characterization of a Coupled Binuclear Cu-I/Cu-III Complex
J. Garcia-Lopez, V. Yanez-Rodriguez, L. Roces, S. Garcia-Granda, A. Martinez, A. Guevara-Garcia, GR. Castro, F. Jimenez-Villacorta, MJ. Iglesias, FL. Ortiz,
J Am Chem Soc 132,10665 (2010).
Abstract
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.
