M. Velez

Publications (21)19.4 Total impact

• Article: Perpendicular magnetic anisotropy in Nd-Co alloy films nanostructured by di-block copolymer templates

  F Valdes-Bango, F J G Alonso, G Rodriguez-Rodriguez, L M Fernandez, A Anillo, L Ruiz-Valdepenas, E Navarro, J L Vicent, M Velez, J I Martin, J M Alameda
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  ABSTRACT: Nd-Co amorphous magnetic films with perpendicular magnetic anisotropy have been grown on nanostructured templates prepared with self-organized di-block poly(styrene)-block-poly(4-vinylpyridine) copolymer layers with a periodic structure of 60 nm spaced pores. These templates modify both the magnetic film topography and mechanical strain on a local scale. The effect of these structural changes is particularly noticeable in the low thickness range of the magnetic films where the transition from in-plane to out-of plane magnetization takes place. The Nd-Co films grown on the copolymer template present lower perpendicular magnetic anisotropy and, also, stronger stripe domain pinning effects in comparison with reference films grown on flat Si substrates. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4759276]
  Journal of Applied Physics. 01/2012; 112(8):6.
• Article: Amorphous and Crystalline Magnetic/Superconducting Hybrids: Interplay Between Periodic Defects and Random Defects

  A. Gomez, D. Perez de Lara, E.M. Gonzalez, A. Alija, J.I. Martin, M. Velez, J.L. Vicent
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  ABSTRACT: Arrays of nanometric Ni dots embedded in amorphous Mo₃Si ( a -Mo₃Si) or crystalline Nb superconducting films modify the vortex lattice dynamics and the dissipation mechanisms, close to critical temperatures. The vortex lattice force vs. velocity characteristics have been measured in both hybrids. The velocities of the ordered and synchronized vortices are the same in both systems (hybrids based on Nb films or a -Mo₃Si films), but remarkably the needed driving forces are one order of magnitude smaller in a -Mo₃Si hybrids than in Nb based hybrids. This experimental behavior points out magnetic/superconducting hybrids, based on amorphous superconductors films, could be a promising material for controlling vortex motion with very low driving forces.
  IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
• Source

  Available from: Juan MR Parrondo

  Article: Crossed-ratchet effects and domain wall geometrical pinning

  V. I. Marconi, A. B. Kolton, J. A. Capitan, J. A. Cuesta, A. Perez-Junquera, M. Velez, J. I. Martin, J. M. R. Parrondo
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  ABSTRACT: The motion of a domain wall in a two dimensional medium is studied taking into account the internal elastic degrees of freedom of the wall and geometrical pinning produced both by holes and sample boundaries. This study is used to analyze the geometrical conditions needed for optimizing crossed ratchet effects in periodic rectangular arrays of asymmetric holes, recently observed experimentally in patterned ferromagnetic films. Geometrical calculations and numerical simulations have been used to obtain the anisotropic critical fields for depinning flat and kinked walls in rectangular arrays of triangles. The aim is to show with a generic elastic model for interfaces how to build a rectifier able to display crossed ratchet effects or effective potential landscapes for controlling the motion of interfaces or invasion fronts.
  12/2010;
• Source

  Available from: arxiv.org

  Article: Vortex ratchet reversal at fractional matching fields in kagomélike array with symmetric pinning centers

  D. Perez de Lara, A. Alija, E. M. Gonzalez, M. Velez, J. I. Martin, J. L. Vicent
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  ABSTRACT: Arrays of Ni nanodots embedded in Nb superconducting films have been fabricated by sputtering and electron-beam lithography techniques. The arrays are periodic triangular lattices of circular Ni dots arranged in a kagomélike pattern with broken reflection symmetry. Relevant behaviors are found in the vortex lattice dynamics: (i) at values lower than the first integer matching field, several fractional matching fields are present when the vortex lattice moves parallel or perpendicular to the reflection symmetry axis of the array showing a clear anisotropic character in the magnetoresistance curves, (ii) injecting an ac perpendicular to the reflection symmetry axis of the array yields an unidirectional motion of the vortex lattice (ratchet effect) as a result of the interaction between the whole vortex lattice and the asymmetric lattice of dots, (iii) increasing the input current amplitudes the ratchet effect changes polarity independently of matching field values. These experimental results can be explained taking into account the vortex lattice density.
  Physical review. B, Condensed matter 10/2010; 82(17):174503.
• Source

  Available from: arxiv.org

  Article: Vortex ratchet reversal at fractional matching fields in kagom\'e-like array with symmetric pinning centers

  D. Perez de Lara, A. Alija, E. M. Gonzalez, M. Velez, J. I. Martin, J. L. Vicent
  [show abstract] [hide abstract]
  ABSTRACT: Arrays of Ni nanodots embedded in Nb superconducting films have been fabricated by sputtering and electron beam lithography techniques. The arrays are periodic triangular lattices of circular Ni dots arranged in a kagom\'e-like pattern with broken reflection symmetry. Relevant behaviors are found in the vortex lattice dynamics : i) At values lower than the first integer matching field, several fractional matching fields are present when the vortex lattice moves parallel or perpendicular to the reflection symmetry axis of the array showing a clear anisotropic character in the magnetoresistance curves, ii) injecting an ac current perpendicular to the reflection symmetry axis of the array yields an unidirectional motion of the vortex lattice (ratchet effect) as a result of the interaction between the whole vortex lattice and the asymmetric lattice of dots, iii) increasing the input current amplitudes the ratchet effect changes polarity independently of matching field values. These experimental results can be explained taking into account the vortex lattice density. Comment: 9 pages, 4 figures, 1 table
  10/2010;
• Source

  Available from: Nuno Miguel Santos

  Article: Magnetic properties of amorphous Co 0.74 Si 0.26 /Si multilayers with different number of periods

  © G N Kakazei, N M Santos, C Quiros, M Velez, J I Martin, J M Alameda, V O Golub, O Y Saliuk, Yu G Pogorelov, M C Carmo, N A Sobolev, J B Sousa
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  ABSTRACT: Two sets of [Co 0.74 Si 0.26 (5 nm)/Si(s)] n amorphous films were prepared by magnetron sputtering: one in the form of multilayers with the Si spacer thickness s fixed at 3 nm, and the number of periods n varying from 1 to 10; and another one with only two periods and s varying from 3 to 24 nm (trilayers). In both series, the Co 0.74 Si 0.26 layer thickness t was fixed at 5 nm. All the samples except the one with s = 24 nm demonstrate anti-ferromagnetic coupling. Their magnetic properties at room temperature were probed by magnetooptical trans-verse Kerr effect (MOTKE) and ferromagnetic resonance (FMR). The relative increase of the saturation magne-tization M s (for trilayers with respect to the one with s = 24 nm; for multilayers with respect to the single layer one) obtained from the FMR measurements was compared with the exchange coupling strength AF J H obtained from the MOTKE studies. AF J H and M s dependencies vs n and s were found to be very similar to each other. Possible mechanisms of this similarity are discussed. PACS: 75.70.Cn Magnetic properties of interfaces; 75.50.Kj Amorphous and quasicrystalline magnetic materials; 76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance; 71.70.Gm Exchange interactions.
  Fizika Nizkikh Temperatur. 01/2010; 36(8-9):1029-1033.
• Article: Magnetic properties of amorphous Co0.74Si0.26 / Si multilayers with different numbers of periods

  G.N. Kakazei, N.M. Santos, C. Quiros, M. Velez, J.I. Martin, J.M. Alameda, V.O. Golub, O.Y. Saliuk, Yu.G. Pogorelov, M.C. Carmo, N.A. Sobolev, and J.B. Sousa
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  ABSTRACT: Two sets of [Co0.74Si0.26(5 nm)/Si(s)]n amorphous films were prepared by magnetron sputtering: one in the form of multilayers with the Si spacer thickness s fixed at 3 nm, and the number of periods n varying from 1 to 10; and another one with only two periods and s varying from 3 to 24 nm (trilayers). In both series, the Co0.74Si0.26 layer thickness t was fixed at 5 nm. All the samples except the one with s = 24 nm demonstrate antiferromagnetic coupling. Their magnetic properties at room temperature were probed by magnetooptical transverse Kerr effect (MOTKE) and ferromagnetic resonance (FMR). The relative increase of the saturation magnetization Ms (for trilayers with respect to the one with s = 24 nm; for multilayers with respect to the single layer one) obtained from the FMR measurements was compared with the exchange coupling strength AF H J obtained from the MOTKE studies. AF H J and Ms dependencies vs n and s were found to be very similar to each other. Possible mechanisms of this similarity are discussed.
  Fizika Nizkikh Temperatur. 01/2010; 36(8/9):1029.
• Source

  Available from: digital.csic.es

  Article: Enhancement of synchronized vortex lattice motion in hybrid magnetic/amorphous superconducting nanostructures

  D. Perez de Lara, A. A. Alija, E. M. Gonzalez, J. I. Martin, M. Velez, J. M. Colino, J. V. Anguita, J. L. Vicent
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  ABSTRACT: Superconducting a-Mo3Si and Nb films have been grown on arrays of Ni nanodots. We have studied the vortex lattice dynamics close to critical temperatures. Different vortex lattice configurations are obtained with the same array unit cell. These different vortex lattices occur at matching conditions between the vortex lattice and the array unit cell. The interplay between the random intrinsic pinning of the superconducting films and the periodic pinning of the array govern the vortex lattice configurations. Different vortex lattice configurations and enhancement of synchronized vortex lattice motion are obtained by increasing the periodic pinning strength and decreasing the random pinning strength.
  Applied Physics Letters 03/2009; 94(12):122506-122506-3. · 3.84 Impact Factor
• Source

  Available from: Yves Huttel

  Article: Magnetic order of Cr thin films in Nb/Cr/Fe-nanoisland hybrid: A comparative study between magnetic and superconducting properties

  E. Navarro, M. Velez, Y. Huttel, A. Perez Junquera, J. I. Martin, O. F. de Lima, A. Cebollada, J. M. Alameda, J. L. Vicent
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  ABSTRACT: Shifted hysteresis loops characteristic of the exchange bias effect between a ferromagnet and an antiferromagnet are demonstrated in structures formed by a 2.5 nm Cr layer deposited on top of an array of Fe nanoislands (Cr/Fe-nanoislands). This effect evidences the persistence of antiferromagnetic (AF) order for Cr layers much thinner than the thickness reported in the literature. The field shift measured is found to increase for the smallest island sizes, which can be related with the enhancement of the Fe-nanoisland surface to volume ratio. The comparative study between superconducting proximity effects in Nb/Cr/Fe-nanoislands and Nb/normal metal/Fe-nanoisland hybrids (where the normal metals used are Al and Pt) confirms the presence of AF order in the 2.5 nm Cr spacer layer. A much shorter penetration depth of the Cooper pairs into the AF Cr layers than in the normal metal Pt and Al spacer layers is deduced.
  Journal of Applied Physics 03/2009; · 2.17 Impact Factor
• Article: Resolving antiferromagnetic states in magnetically coupled amorphous Co-Si-Si multilayers by soft x-ray resonant magnetic scattering

  S M Valvidares, C Quiros, A Mirone, J -M Tonnerre, S Stanescu, P Bencok, Y Souche, L Zarate, J I Martin, M Velez, N B Brookes, J M Alameda
  Physical Review B 08/2008; 78(6). · 3.69 Impact Factor
• Chapter: Phase diagram, vortex dynamics and dissipation in thin films and superlattices of 1:2:3 superconducting cuprates

  M. Velez, J. I. Martin, E. M. Gonzalez, J. L. Vicent
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  ABSTRACT: The mixed state of High Temperature Superconductors (HTS) is a new field that allows us to study very interesting and new phenomena related with glassy behavior. There are many topics that could be addressed, but in this paper we are going to deal with two topics. The first one concerning to the single vortex dynamics, and the second one related with the dissipation mechanisms of the vortices and the so-calle d lock-in transition. The former will be studied using the anomalous behavior of the Hall effect in the mixed state with c-axis oriented 1:2:3 films the latter using as an appropriate tool the critical current, the pinning force and the resistivity (dissipation) in the m ixed state of 1:2:3 a-axis oriented superconductor / non-superconductors multilayers.
  05/2007: pages 316-336;
• Article: Interlayer coupling mechanisms in amorphous CoxSi1-x/Si multilayers

  L Zarate, C Quiros, M Velez, G Rodriguez-Rodriguez, J I Martin, J M Alameda
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  ABSTRACT: Interlayer coupling has been studied in amorphous CoxSi1-x/Si multilayers with a Curie temperature close to room temperature. Results show an antiferromagnetic coupling mechanism that is clearly detectable in the temperature range above 250 K. This antiferromagnetic coupling presents a decrease with increasing Si spacer thickness, but with decay lengths of the order of several nm, longer than the typical values for metallic spacers. Its strength, typically of the order of 10(-5) erg/cm(2), decreases monotonically with increasing temperature, which could be compatible with a mechanism associated with the presence of defects or magnetic impurities in the interlayers. At lower temperatures, due to the increase in saturation magnetization, the magnetic behavior of the multilayers is dominated by other ferromagnetic coupling mechanisms of magnetostatic origin (Neel's "orange peel" and domain-wall coupling) and the antiparallel alignment of the layers is not observed any more.
  Physical Review B. 01/2006; 74(1).
• Article: Magnetization reversal processes in amorphous and polycrystalline Co-Si patterned nanowires

  R. Morales, J.I. Martin, M. Velez, J.M. Alameda, F. Briones, J.L. Vicent
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  ABSTRACT: Arrays of amorphous and polycrystalline Co_xSi_1-x nanowires have been prepared by electron beam lithography and a lift-off technique. The angular dependence of the magnetooptical transverse Kerr-effect hysteresis loops for both kind of samples has been compared in order to analyze the interplay between material microstructure and element shape at the submicrometric scale. The overall magnetic properties of the Co-Si nanowires are found to be dominated by patterning induced shape anisotropy, with the presence of coherent rotations in the magnetization reversal process in a wide angular range around the hard axis direction. It is found that the more homogenous microstructure of the amorphous samples results in a softer magnetic behavior and on a much better definition of the wires uniaxial anisotropy.
  IEEE Transactions on Magnetics 10/2002; · 1.36 Impact Factor
• Article: Structure and magnetic properties of amorphous Co-Si alloy films

  M. Velez, S.M. Valvidares, J. Diaz, R. Morales, J.M. Alameda
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  ABSTRACT: Co_xSi_1-x alloy films have been prepared by sputtering in the composition range 0.65 ≤ x ≤ 1 at room temperature, and their structural and magnetic properties have been characterized by X-ray diffraction, extended X-ray absorption fine structure spectroscopy, and by magnetooptical Kerr effect magnetometry. The samples are polycrystalline with a mixture of fcc and hcp phases down to x = 0.76. For lower Co content, there is a transition to an amorphous structure that can be described by a dense random packing of hard spheres model. For x = 0.65 a change is found in the atomic environment of the Co atoms in the amorphous structure, that becomes more similar to a disordered Co₂Si phase. These structural changes are reflected also on the magnetic properties of the films: essentially isotropic with high coercivity for x ≥ 0.76, with well-defined uniaxial magnetic anisotropy in the 0.75 ≥ x ≥ 0.66 composition range, and non magnetic for x below 0.65.
  IEEE Transactions on Magnetics 10/2002; · 1.36 Impact Factor
• Article: Vortex lattice channeling effects in Nb films induced by anisotropic arrays of mesoscopic pinning centers

  M. Velez, D. Jaque, J. I. Martín, M. I. Montero, Ivan K. Schuller, J. L. Vicent
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  ABSTRACT: Rectangular arrays of Ni dots with small interdot separation have been fabricated in superconducting Nb films. The geometry of these rectangular arrays defines deep pinning potential channels. Two effects have been observed. (i) When the vortices move perpendicular to the channels (i.e., along the long side of the rectangular array) an enhancement of the background pinning is observed, as predicted by theoretical models. This enhancement occurs for all magnetic fields. (ii) A new anisotropic matching effect, which has not been theoretically predicted, has been measured. The position of the matching fields depends on the direction of the vortex flow. When the vortices move parallel to the potential channels (i.e., along the short side of the rectangular array) the vortex lattice matches the rectangular array, but when they move along the long side of the rectangular array only a triangular lattice is observed.
  Phys. Rev. B. 02/2002; 65(10).
• Source

  Available from: arxiv.org

  Article: Order in driven vortex lattices in superconducting Nb films with nanostructured pinning potentials

  M. Velez, D. Jaque, J. I. Martin, F. Guinea, J. L. Vicent
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  ABSTRACT: Driven vortex lattices have been studied in a material with strong pinning, such as Nb films. Samples in which natural random pinning coexists with artificial ordered arrays of defects (submicrometric Ni dots) have been fabricated with different geometries (square, triangular and rectangular). Three different dynamic regimes are found: for low vortex velocities, there is a plastic regime in which random defects frustrate the effect of the ordered array; then, for vortex velocities in the range 1-100 m/s, there is a sudden increase in the interaction between the vortex lattice and the ordered dot array, independent on the geometry. This effect is associated to the onset of quasi long range order in the vortex lattice leading to an increase in the overlap between the vortex lattice and the magnetic dots array. Finally, at larger velocities the ordered array-vortex lattice interaction is suppresed again, in agreement with the behavior found in numerical simulations. Comment: 8 text pages + 4 figures
  04/2001;
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  Available from: ArXiv

  Article: Temperature dependence and mechanisms for vortex pinning by periodic arrays of Ni dots in Nb films

  J. I. Martin, M. Velez, A. Hoffmann, I. K. Schuller, J. L. Vicent
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  ABSTRACT: Pinning interactions between superconducting vortices in Nb and magnetic Ni dots were studied as a function of current and temperature to clarify the nature of pinning mechanisms. A strong current dependence is found for a square array of dots, with a temperature dependent optimum current for the observation of periodic pinning, that decreases with temperature as (1-T/Tc)3/2. This same temperature dependence is found for the critical current at the first matching field with a rectangular array of dots. The analysis of these results allows to narrow the possible pinning mechanisms to a combination of two: the interaction between the vortex and the magnetic moment of the dot and the proximity effect. Moreover, for the rectangular dot array, the temperature dependence of the crossover between the low field regime with a rectangular vortex lattice to the high field regime with a square configuration has been studied. It is found that the crossover field increases with decreasing temperature. This dependence indicates a change in the balance between elastic and pinning energies, associated with dynamical effects of the vortex lattice in the high field range. Comment: 12 text pages (revtex), 6 figures (1st jpeg, 2nd-6th postscript) accepted in Physical Review B
  09/2000;
• Article: Dissipation mechanisms in EuBa2Cu3O7 /SrTiO3 and EuBa2Cu3O7/PrBa2Cu3 O7 multilayers

  M. Velez, E.M. Gonzalez, J.I. Martin, J.L. Vicent
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  ABSTRACT: Superconducting multilayers of c-axis and a-axis oriented EuBa₂Cu₃O₇/PrBa₂Cu₃O₇ and c-axis oriented EuBa₂Cu₃O ₇/SrTiO₃ have been grown by dc and rf sputtering on (100) SrTiO₃ substrates. The critical current densities and the tails of the resistivity transitions (ρ, B) have been used to study the dissipation mechanisms. The activation energy (U) follows a log B law in all the systems and textures. A crossover from U=U₀ B^-α to U=U₀ log B occurs depending on the superconducting layer thickness, The type of multilayer and texture governs the values of the activation energies U₀. The former result seems to be related with the competition between the thickness of the superconducting layer and the vortex correlation length, and the latter one (U₀ values) with the coupling between the superconducting layers
  IEEE Transactions on Appiled Superconductivity 07/1997; · 1.04 Impact Factor
• Source

  Available from: arxiv.org

  Article: Flux flow resistivity and vortex viscosity of high-Tc films

  J. I. Martin, M. Velez, F. Guinea, J. L. Vicent
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  ABSTRACT: The flux flow regime of high-T$_{\rm c}$ samples of different normal state resistivities is studied in the temperature range where the sign of the Hall effect is reversed. The scaling of the vortex viscosity with normal state resistivity is consistent with the Bardeen-Stephen theory. Estimates of the influence of possible mechanisms suggested for the sign reversal of the Hall effect are also given. Comment: 3 pages. 4 figures upon request
  07/1996;
• Article: Critical currents and thermal activation in a-axis oriented EuBa2Cu3O7 thin films

  M. Velez, J.I. Martin, J.L. Vicent
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  ABSTRACT: a-axis oriented EuBa₂Cu₃O₇/SrTiO₃(100) thin films have been grown by DC magnetron sputtering. Critical current and resistivity have been measured as a function of temperature, down to 15 K, and field, up to 9 T. In spite of the existence of 90° grain boundaries, the critical current is not limited by weak links. The lower part of the resistivity transition and the field dependence of the pinning force may be understood in terms of thermally activated flux creep with activation energy U(T,H)=U₀(T)/H and U₀ values of several hundreds of meV.
  IEEE Transactions on Appiled Superconductivity 07/1995; · 1.04 Impact Factor