Person:
Muñoz Noval, Álvaro

First Name

Álvaro

Last Name

Muñoz Noval

Affiliation

Universidad Complutense de Madrid

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Ciencias Físicas

Department

Física de Materiales

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Now showing 1 - 10 of 16

Crystal defects and optical emissions of pulse electrodeposited ZnO
(Pergamon-Elsevier Science Ltd, 2020-10-10) Manzano, Cristina V.; Serrano, Aída; Muñoz Noval, Álvaro; Fernández, José F.; Martín González, Marisol
ZnO has been widely studied in the last decades as an n-type semiconductor due to its wide application range, for example, in optoelectronics, solar cells, light-emitting diodes, thermoelectrics, amongst others. The material efficiency for certain applications is highly dependent on the presenting film morphology. Electrodeposition is well-known as a technique with precise control over the structural and morphological properties of the obtained materials. When the structural and morphological properties are tuned, it is possible to find a wide variety of defects in the ZnO structure. In this study, ZnO films were grown using pulsed electrodeposition with variation of the reduction potential. The crystal order, structural defects and optical emissions of the films have been analyzed by X-Ray Diffraction (XRD), X-ray Absorption Near-Edge Structure (XANES), Extended X-ray Absorption Fine Structure (EXAFS) and Photoluminescence (PL). ZnO film grown at less negative reduction potential presents a stronger texture along [0001] by XRD, higher crystalline order, and more zinc vacancies by XANES and EXAFS. The films obtained at less negative potential present less OH - trapped in the ZnO structure and a relatively higher level of defects (O_i )^0, (O_Zn)^0, (O_i)^(-/2-) and (O_Zn)^(0/-) than those grown at higher reduction potentials by PL. This will be related to the fact that at less negative potentials there is less concentration of OH- at the film surface than at more negative potentials. The combination of X-ray absorption spectroscopy and photoluminescence reveals the complicated nature of the atomic defect in electrodeposited ZnO films. Allowing to evidence the preferential presence of atomic defect as a function of the reduction potential. In this work, we have also compared those defects with reference compounds such as a Zn foil and ZnO polycrystalline powder.
Correlation between local structure and magnetic behavior in co-sputtered Tb_xFe_(73)Ga_(27-x) (7 ≤ x ≤11) thin films
(Elsevier Science SA, 2017-12-05) Muñoz Noval, Álvaro; Salas Colera, E.; Bartolomé Izquierdo, Pablo; Serrano, A.; Castro, G. R.; Ranchal Sánchez, Rocío
We report on the evolution of the microstructure of Tb-Fe-Ga films deposited by co-sputtering from Tb_(33)Fe_(67) and Fe_(72)Ga_(28) targets. The sputtering power was fixed (90 W) in the Fe_(72)Ga_(28) whereas it was increased from 50 to 90 W in the Tb_(33)Fe_(67) target resulting on Tb_xFe_(73)Ga_(27-x) layers with 7 ≤ x ≤ 11. The local structure was determined by means of x-ray absorption fine structure spectroscopy at Fe-K, Ga-K and Tb-L_(3) edges. The increase of Tb in the alloy promotes the phase segregation that produces a larger amount of the TbFe_2 structural phase. The structural results have been correlated with the magnetic characterization that shows the enhancement of the out-of-plane component of the magnetization.
Local disorder and structure relation induced by magnetic exchange interactions in A_2(Mo_(1-y)Mn_y)_2O_7 pyrochlores
(Elsevier Science SA, 2021-06-05) Castellano, C.; Scavini, M.; Berti, G.; Rubio Marcos, F.; Lamura, G.; Sanna, S; Salas Colera, E.; Muñoz Noval, Álvaro; Cimberle, M. R.; Demartin, F.
We present an extended X-ray absorption fine structure study at the Mo K-edge of A_2(Mo_(1-y)Mn_y)_2O_7 (A = Gd, Ho; y = 0.05 and 0.10) pyrochlores, as a function of temperature and composition, coupled to diffractometric and magnetic characterizations. Extending the study reported in our previous paper on the pristine A_2Mo_2O_7 compounds to these Mo/Mn partially substituted samples, where we hypothesize a competition between double-exchange and superexchange couplings, we aim to check which structure parameters are related to the nature of the nearest-neighbor magnetic interactions, looking for the presence and evolution of order and structure anomalies. Two Ho samples A_2(Mo_(1-y)Mn_y)_2O_7 (y = 0.05 and 0.10) keep the spin-glass nature of the parent compositions with a strong distortion of the MoO octahedron and of the Mo - Ho and Mo - Mo second shell. On the other hand, two Gd_2(Mo_(1-y)Mn_y)_2O_7 samples, ferromagnetic at low temperature in the undoped case, show the appearance on a local scale of a spin-glass transition mirrored by a quite high structure disorder and by a distortion coherent with a lattice frustration after Mn-doping. Therefore, the Gd and Ho Mn-doped samples display a similar frustrated behavior, differently from the corresponding undoped ones.
Unveiling the different physical origins of magnetic anisotropy and magnetoelasticity in ga-rich fega thin films
(American Chemical Society, 2020-02-27) Bartolomé, Pablo; Begué, A; Muñoz Noval, Álvaro; Ciria, M.; Ranchal Sánchez, Rocío
The aim of this work is to clarify how in-plane magnetic anisotropy and magnetoelasticity depend on the thickness of Ga-rich FeGa layers. Samples with an Fe72Ga28 composition were grown by sputtering in the ballistic regime in oblique incidence. Although for these growth conditions uniaxial magnetic anisotropy could be expected, in-plane anisotropy is only present when the sample thickness is above 100 nm. By means of differential X-ray absorption spectroscopy, we have determined the influence of both Ga pairs and tetragonal cell distortion on the evolution of the magnetic anisotropy with the increase of FeGa thickness. On the other hand, we have used the cantilever beam technique with capacitive detection to also determine the evolution of the magnetoelastic parameters with the thickness increase. In this case, experimental results can be understood considering the grain distribution. Therefore, the different physical origins for anisotropy and magnetoelasticity open up the possibility to independently tune these two characteristics in Ga-rich FeGa films.
A combined micro-Raman, X-ray absorption and magnetic study to follow the glycerol-assisted growth of epsilon-iron oxide sol-gel coatings
(Elsevier Science SA, 2022-02-05) López Sánchez, Jesús; Serrano, Aída; Campo, A. del; Muñoz Noval, Álvaro; Salas Colera, Eduardo; Cabero Piris, Mariona; Varela del Arco, María; Abuín, Manuel; Castro, German R.; Rubio Zuazo, Juan; Rodríguez de la Fuente, Óscar; Carmona Tejero, Noemí
Epsilon iron oxide (epsilon-Fe_2O_3) coatings on Si(100) substrates are obtained by an easy one-pot sol-gel recipe assisted by glycerol in an acid medium. Glycerol, given its small dimensions, enables the formation of epsilon-Fe_2O_3 nanoparticles with a size of a few nanometers and the highest purity is reached in coatings after a densification treatment at 960 degrees C. The structural and compositional evolution up to 1200 degrees C is studied by confocal Raman microscopy and X-ray absorption spectroscopy techniques, correlating the existing magnetic properties. We report a novel characterization method, which allows monitoring the evolution of the precursor micelles as well as the intermediate and final phases formed. Furthermore, the inherent industrial technology transfer of the sol-gel process is also demonstrated with the epsilon-Fe_2O_3 polymorph, impelling its application in the coatings form.
Novel perovskite materials for thermal water splitting at moderate temperature
(Wiley-V C H VERLAG GMBH, 2019-07-08) Azcondo, M. Teresa; Orfila, María; Marugán, Javier; Sanz Martín, Raúl; Muñoz Noval, Álvaro; Salas Colera, Eduardo; Ritter, Clemens; García Alvarado, Flaviano; Amador, Ulises
Materials with the formula Sr_2CoNb_1-xTi_xO_(6-delta) (x=1.00, 0.70; delta=number of oxygen vacancies) present a cubic perovskite-like structure. They are easily and reversibly reduced in N_2 or Ar and re-oxidized in air upon heating. Oxidation by water (wet N_2), involving splitting of water at a temperature as low as 700 ºC, produces hydrogen. Both compounds displayed outstanding H_2 production in the first thermochemical cycle, the Sr_2CoNb_(0.30)Ti_(0.70)O_(6-delta) material retaining its outstanding performance upon cycling, whereas the hydrogen yield of the x=1 oxide showed a continuous decay. The retention of the materials' ability to promote water splitting correlated with their structural, chemical, and redox reversibility upon cycling. On reduction/oxidation, Co ions reversibly changed their oxidation state to compensate the release/recovery of oxygen in both compounds. However, in Sr_2CoTiO_(6-delta), two phases with different oxygen contents segregated, whereas in Sr_2CoNb_(0.30)Ti_(0.70)O_(6-delta) this effect was not evident. Therefore, this latter material displayed a hydrogen production as high as 410 mu molH_2/g_(perovskite) after eight thermochemical cycles at 700 ºC, which is among the highest ever reported, making this perovskite a promising candidate for thermosolar water splitting in real devices.
Realization of macroscopic ratchet effect based on nonperiodic and uneven potentials
(Nature publishing group, 2021-08-16) Rollano, V.; Gómez, A.; Muñoz Noval, Álvaro; Ory, M. C. de; Menghini, M.; González Herrera, Elvira María; Vicent López, José Luis
Ratchet devices allow turning an ac input signal into a dc output signal. A ratchet device is set by moving particles driven by zero averages forces on asymmetric potentials. Hybrid nanostructures combining artificially fabricated spin ice nanomagnet arrays with superconducting films have been identified as a good choice to develop ratchet nanodevices. In the current device, the asymmetric potentials are provided by charged Neel walls located in the vertices of spin ice magnetic honeycomb array, whereas the role of moving particles is played by superconducting vortices. We have experimentally obtained ratchet effect for different spin ice I configurations and for vortex lattice moving parallel or perpendicular to magnetic easy axes. Remarkably, the ratchet magnitudes are similar in all the experimental runs; i. e. different spin ice I configurations and in both relevant directions of the vortex lattice motion. We have simulated the interplay between vortex motion directions and a single asymmetric potential. It turns out vortices interact with uneven asymmetric potentials, since they move with trajectories crossing charged Neel walls with different orientations. Moreover, we have found out the asymmetric pair potentials which generate the local ratchet effect. In this rocking ratchet the particles (vortices) on the move are interacting each other (vortex lattice); therefore, the ratchet local effect turns into a global macroscopic effect. In summary, this ratchet device benefits from interacting particles moving in robust and topological protected type I spin ice landscapes.
Vortex dynamics controlled by local superconducting enhancement
(IOP Publishing Ltd., 2019-11-29) Rollano, V.; Muñoz Noval, Álvaro; Valle, J. del; Menghini, M.; Ory, M. C. de; Prieto, J. L.; Navarro Palma, Elena; González Herrera, Elvira María; Vicent López, José Luis
A controlled local enhancement of superconductivity yields unexpected modifications in the vortex dynamics. This local enhancement has been achieved by designing an array of superconducting Nb nanostructures embedded in a V superconducting film. The most remarkable findings are: (i) vanishing of the main commensurability effect between the vortex lattice and the array unit cell, (ii) hysteretic behavior in the vortex dynamics, (iii) broadening of the vortex liquid phase and (iv) strong softening of the vortex lattice. These effects can be controlled and they can be quenched by reducing the Nb array superconducting performance applying an in-plane magnetic field. These results can be explained by taking into account the repulsive potential landscape created by the superconducting Nb nanostructures on which vortices move.
Investigation into the electrodeposition of Ga-Fe-O thin films
(Elsevier Science SA, 2021-08-01) Prados Díaz, Alicia; Muñoz Noval, Álvaro; Ranchal Sánchez, Rocío
In this work we present an exhaustive investigation about the possibility of using the electrodeposition technique to grow Ga-Fe-O thin films. The morphological and structural characterization has been performed by means of scanning electron microscopy, X-ray diffractometry and X-ray absorption near edge structure measurements. These results have been complemented with chronoamperometric measurements recorded during growth. We have explored different electrolytes, growth potentials, and pH being observed that it is possible to tune the Fe/ Ga ratio by means of the potential whereas the surface morphology is closely related to the Na_3-citrate content in the electrolyte. The surface roughness is reduced as the Fe content in the layer is increased upon the raise of citrate in the electrolyte composition. X-ray diffractometry indicates that Ga can become partially oxidized, whereas Fe seems to keep its metallic state in its vast majority since we have not obtained clear evidences of oxidation. Thus, this investigation highlights the inherent difficulties to synthetize Ga-Fe-O compounds by means of the electrodeposition technique.
Unusual magnetic hysteresis and transition between vortex and double pole states arising from interlayer coupling in diamond-shaped nanostructures
(Amer Chemical Soc., 2022-12-05) Parente Campos, Ana; Navarro, H.; Vargas, N. M.; Lapa, P.; Basaran, Ali C.; González Herrera, Elvira María; Redondo, C.; Morales, R.; Muñoz Noval, Álvaro; Schuller, Ivan K.; Vicent López, José Luis
Controlling the magnetic ground states at the nanoscale is a longstanding basic research problem and an important issue in magnetic storage technologies. Here, we designed a nanostructured material that exhibits very unusual hysteresis loops due to a transition between vortex and double pole states. Arrays of 700 nm diamond-shaped nanodots consisting of Py(30 nm)/Ru(tRu)/Py(30 nm) (Py, permalloy (Ni_(80)Fe_(20)) trilayers were fabricated by interference lithography and e-beam evaporation. We show that varying the Ru interlayer spacer thickness (t_(Ru)) governs the interaction between the Py layers. We found this interaction mainly mediated by two mechanisms: magnetostatic interaction that favors antiparallel (antiferromagnetic, AFM) alignment of the Py layers and exchange interaction that oscillates between ferromagnetic (FM) and AFM couplings. For a certain range of Ru thicknesses, FM coupling dominates and forms magnetic vortices in the upper and lower Py layers. For Ru thicknesses at which AFM coupling dominates, the magnetic state in remanence is a double pole structure. Our results showed that the interlayer exchange coupling interaction remains finite even at 4 nm Ru thickness. The magnetic states in remanence, observed by magnetic force microscopy (MFM), are in good agreement with corresponding hysteresis loops obtained by the magneto-optic Kerr effect (MOKE) and micromagnetic simulations.