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We investigated the vortex phase diagram of needle shaped high quality NiBi3 single crystals by transport measurements. The current is applied along the crystalline b-axis of this intermetallic quasi-1D BCS superconductor. The single crystals show a Ginzburg-Levanchuk (Gi) parameter few orders of magnitude larger than other low Tc BCS superconductors. Vortex phase diagram, critical currents and pinning forces have been extracted from the experimental data. The main findings are: 1) Enhancement of the vortex liquid phase in comparison with low Tc superconductors, 2) reentrance of the liquid phase at low fields and 3) deviation of the pinning force vs field from the usual pinning mechanisms. The interplay between weak pinning, due to quenched disorder, and the quasi-1D character of the material could be a hint to explain the lack of a single pinning mechanism., 19 pages, 6 figures

Controlling the magnetic ground states at the nanoscale is a long-standing 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-shape nanodots consisting of Py(30 nm)/Ru(tRu)/Py(30 nm) (Py, permalloy (Ni80Fe20)) trilayers were fabricated by interference lithography and e-beam evaporation. We show that varying the Ru interlayer spacer thickness (tRu) 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 Magneto-Optic Kerr Effect (MOKE) and micromagnetic simulations., Comment: 16 pages, 4 figures, 1table

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 Néel 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 Néel 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., This work was supported by Spanish MICINN grants FIS2016-76058 (AEI/FEDER, UE), EU COST- CA16218. IMDEA Nanociencia acknowledges support from the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (MICINN, Grant SEV-2016-0686). MCO and AG acknowledges financial support from Spanish MICINN Grant ESP2017-86582-C4-1-R and IJCI-2017-33991; AMN acknowledges financial support from Spanish CAM Grant 2018-T1/IND-10360. MV acknowledges financial support from Spanish MICINN Grant PID2019-104604RB/AEI/10.13039/50110001103.

Little–Parks effect names the oscillations in the superconducting critical temperature as a function of the magnetic field. This effect is related to the geometry of the sample. In this work, we show that this effect can be enhanced and manipulated by the inclusion of magnetic nanostructures with perpendicular magnetization. These magnetic nanodots generate stray fields with enough strength to produce superconducting vortex–antivortex pairs. So that, the L–P effect deviation from the usual geometrical constrictions is due to the interplay between local magnetic stray fields and superconducting vortices. Moreover, we compare our results with a low-stray field sample (i.e. with the dots in magnetic vortex state) showing how the enhancement of the L–P effect can be explained by an increment of the effective size of the nanodots., With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

We have designed, fabricated and tested a robust superconducting ratchet device based on topologically frustrated spin ice nanomagnets. The device is made of a magnetic Co honeycomb array embedded in a superconducting Nb film. This device is based on three simple mechanisms: (i) the topology of the Co honeycomb array frustrates in-plane magnetic configurations in the array yielding a distribution of magnetic charges which can be ordered or disordered with in-plane magnetic fields, following spin ice rules; (ii) the local vertex magnetization, which consists of a magnetic half vortex with two charged magnetic Néel walls; (iii) the interaction between superconducting vortices and the asymmetric potentials provided by the Néel walls. The combination of these elements leads to a superconducting ratchet effect. Thus, superconducting vortices driven by alternating forces and moving on magnetic half vortices generate a unidirectional net vortex flow. This ratchet effect is independent of the distribution of magnetic charges in the array., We thank support from Spanish MINECO grants FIS2016-76058 (AEI/FEDER, UE), Spanish CM grant S2013/MIT2850. IMDEA Nanociencia acknowledges support from the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (MINECO, Grant SEV-2016-0686). DG acknowledges RYC-2012-09864, S2013/MIT-3007 and ESP2015-65597-C4-3-R for financial support and AG acknowledges financial support from Spanish MINECO Grant ESP2015-65597-C4-1-R.

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., Comment: 16 pages, 8 figures

With the aim of achieving stable, substantial remanences adequate for exploitation in stray field-based applications, we report on the hysteresis behavior occurring in arrays of single-crystal Fe motifs, a-beam lithographed into prisms with triangular bases and different orientations of their magnetocrystalline axes with respect to the morphological symmetry axes. From both experimental and simulational analyses we recognize the fact that the magnetization reversal processes of our samples were mediated by motif-sized vortices. Their nucleation and annihilation fields and sites within the motifs, and their field-induced displacements, are discussed in terms of the magnetocrystalline and configurational anisotropies and inter-motif dipolar interactions. From our data, we conclude that reduced remanences as large as 0.85 (sufficient for the application requirements), protected by nucleation fields of several tens of Oe, can be produced in arrays where magnetocrystalline easy axes reinforce and partly compensate the easiest and hardest configurational ones, respectively. The angular dependence of the reduced remanence associated with interplay of these anisotropies corresponds to a symmetry reduction from the triaxial one linked to the triangular morphology down to an effective uniaxial one. We also identify, for the particular case of inter-nanoprism distances that are short in comparison with the dimensions of the motif base, a contribution to the remanence enhancement originating from the dipolar interactions.

The development of multilayered materials with engineered magnetic properties compels a deep knowledge of physical properties at the atomic scale. The magnetic anisotropy is a key property in these materials. This work accounts for the magnetic anisotropy energy and its correlation with atomic properties of Co/Pd multilayers with the number of Co/Pd repetitions. Magnetometry measurements confirm stronger perpendicular magnetic anisotropy energies as the number of repetitions increases up to 40. However, the intrinsic anisotropy, related to the Co–Pd orbital hybridization and spin–orbit coupling, saturates at 15 repetitions. This finding is supported by x-ray magnetic circular dichroism analysis that reveals a direct correlation of the atomic Co and Pd orbital magnetic moments and the effective anisotropy of the system. The proximity effect that accounts for the Pd induced magnetization, along with the increasing Co moment, provides a suitable mechanism for the observed anisotropy energy layer dependence., This work received funding from the MINECO-AEI FIS2016-76058, UE FEDER “Una manera de hacer Europa”; the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant No. 734801, and the Basque Country Grant Nos. IT1162-19 and PIBA 2018-11. UPV/EHU authors are thankful for the technical and human support provided by the laser facility, x-ray service, electron microscopy, and magnetic measurement units of SGIker UPV/EHU. We acknowledge the access to BL29 at ALBA synchrotron light facility via official Proposal No. 2015091407. M.V. acknowledges funding via MINECO [Grant No. FIS2016-78591-C3-2-R (AEI/FEDER, UE)]. IMDEA Nanociencia acknowledges the support from the “Severo Ochoa” Program for Centers of Excellence in R&D (MINECO, Grant No. SEV-2016-0686). R.M. acknowledges enlightening discussions with Dr. Arturo Ponce-Pedraza (UTSA) on TEM.

We have studied two nanomagnet systems with strong (Co/Pd multilayers) and weak (NdCo alloy films) stray magnetic fields by probing the out-of-plane magnetic states with superconducting vortices. The hybrid samples are made of array of nanomagnets embedded in superconducting Nb thin films. The vortex motion detects relevant magnetic state features, since superconducting vortices are able to discriminate between different magnetic stray field strengths and directions. The usual matching effect between the superconducting vortex lattice and the periodic pinning array can be quenched by means of disorder magnetic potentials with strong stray fields at random. Ordered stray fields retrieve the matching effect and yield asymmetry and shift in the vortex dissipation signal. Furthermore vortices can discriminate the sizes of the nanomagnet magnetic domains, detecting magnetic domain sizes as small as 70 nm. In addition, we observe that the vortex cores play the crucial role instead of the supercurrents around the vortex., Comment: 22 pages, 8 figures

We present here an experimental set-up system to excite and measure simultaneously surface plasmon resonance (SPR) and magneto-optic signal in hybrid magneto-plasmonic systems using two independent light sources. The system can be used to excite and measure both types of SPR, localized surface plasmons in nanostructures and surface plasmon polaritons in thin films. It also allows measuring SPR in presence of magnetic fields and recording magnetooptical hysteresis loops while exciting SPR.

We present here a study of the possible local heating effects in surface plasmon resonance sensors using low power lasers (∼mW/mm 2 ). By measuring the increase of electrical resistivity in an Au stripe while exciting SPR and performing numerical simulations we determine upper limits to the local heating that result of the order of ∼0.1 K and ∼1 K for laser power densities of 1.5 mW/mm 2 and 15 mW/mm 2 respectively.

Superconducting 2H-NbSe2 single crystals show intrinsic low pinning values. Therefore, they are ideal materials with which to explore fundamental properties of vortices. (V, I) characteristics are the experimental data we have used to investigate the dissipation mechanisms in a rectangular shape 2H-NbSe2 single crystal. Particularly, we have studied dissipation behavior with magnetic fields applied in the plane of the crystal and parallel to the injected currents, i.e. in the force-free state where the vortex helical instability governs the vortex dynamics. In this regime, the data follow the elliptic critical state model and the voltage dissipation shows an exponential dependence.Moreover, this exponential dependence can be observed for in-plane applied magnetic fields up to 40 degrees off the current direction, which implies that the vortex helical instability plays a role in dissipation even out of the force-free configuration., Comment: 13 pages, 6 figures

We have designed and fabricated a nanodevice exhibiting simultaneously ferromagnetic properties of nanostructures with plasmonic properties of continuous films. Our device consists of an array of nanomagnets on top of a continuous plasmonic film. The patterned nanomagnets magnetic state is single domain and well-defined shape anisotropy. Despite the presence of the patterned media on top of the Au film, the system exhibits surface plasmon resonance characteristics of a continuous film, i.e., propagating surface plasmon-polaritons.

Superconducting films in contact with non-superconducting regular arrays can exhibit commensurability effects between the vortex lattice and the unit cell of the pinning array. These matching effects yield a slowdown of the vortex flow and the corresponding dissipation decrease. The superconducting samples are Nb films grown on Si substrates. We have studied these matching effects with the array on top, embedded or threading the Nb superconducting films and using different materials (Si, Cu, Ni, Py dots and dots fabricated with Co/Pd multilayers). These hybrids allow studying the contribution of different pinning potentials to the matching effects. The main findings are: i) Periodic roughness induced in the superconducting film is enough to generate resistivity minima; ii) A minor effect is achieved by magnetic pinning from periodic magnetic field potentials obtained by dots with out of plane magnetization grown on top of the superconducting film, iii) In the case of array of magnetic dots embedded in the films vortex flow probes the magnetic state; i.e. magnetoresistance measurements detect the magnetic state of very small nanomagnets. In addition, we have studied the role played by the local order in the commensurability effects. This was attained using an array that mimics a smectic crystal. We have found that preserving the local order is crucial. If the local order is not retained the magnetoresistance minima vanish., Comment: 11 pages, 7 figures

In order to compare magnetic and non-magnetic pinning, we have nanostructured two superconducting films with the regular arrays of pinning centers: Cu (non-magnetic) dots in one case and Py (magnetic) dots in the other. For low applied magnetic fields, when all the vortices are pinned in the artificial inclusions, the magnetic dots prove to be better pinning centers, as has been generally accepted. Unexpectedly, when the magnetic field is increased and interstitial vortices appear, the results are very different: we show how the stray field generated by the magnetic dots can produce an effective reduction of the penetration length. This results in strong consequences in the transport properties, which, depending on the dot separation, can lead to an enhancement or worsening of the transport characteristics. Therefore, the election of the magnetic or non-magnetic character of the pinning sites for an effective reduction of dissipation will depend on the range of the applied magnetic field. Published by AIP Publishing.

Nb films have been fabricated on top of array of Ni nanodots. The array of periodic pinning potentials modifies the vortex lattice for specific values of the external applied magnetic field. By means of an implemented code developed from scratch, computer simulations based only on the vortex–vortex and the vortex–nanodot interactions provide the total interaction between vortices and pinning sites as well as the position of the vortices in the array unit cell. This simulation approach could be performed on square, rectangular or triangular arrays of nanodefects of different size.

Superconducting rectifiers have been fabricated by electron beam lithography, sputtering and ion etching techniques. The rectifiers are made growing Nb films on top of arrays of submicrometric Ni triangles. In this device, in the superconducting mixed state, injecting an input AC current yields an output DC voltage. This effect is due to the motion of the vortex lattice on periodic asymmetric pinning potentials. The vortex lattice dynamics follows ratchet effect behavior. The device shows two ratchet effects: Longitudinal and transverse. In the longitudinal ratchet effect the DC signal polarity could be switched increasing the applied magnetic field strength. Otherwise, the sign of the transverse effect does not change increasing the applied magnetic field.

This review is dedicated to summarizing the recent research on vortex dynamics and pinning effects in superconducting films with artificial magnetic structures. The fabrication of hybrid superconducting/ magnetic systems is presented together with the wide variety of properties that arise from the interaction between the superconducting vortex lattice and the artificial magnetic nanostructures. Specifically, we review the role that the most important parameters in the vortex dynamics of films with regular array of dots play. In particular, we discuss the phenomena that appear when the symmetry of a regular dot array is distorted from regularity towards complete disorder including rectangular, asymmetric, and aperiodic arrays. The interesting phenomena that appear include vortex-lattice reconfigurations, anisotropic dynamics, channeling, and guided motion as well as ratchet effects. The different regimes are summarized in a phase diagram indicating the transitions that take place as the characteristic distances of the array are modified respect to the superconducting coherence length. Future directions are sketched out indicating the vast open area of research in this field.

We present a simple nanodevice that can operate in two modes: i) three-state memory and ii) reading device. The nanodevice is fabricated with an array of ordered triangular-shaped nanomagnets embedded in a superconducting thin film. The input signal is ac current and the output signal is dc voltage. Vortex ratchet effect in combination with out of plane magnetic anisotropy of the nanomagnets is the background physics which governs the nanodevice performance., Comment: 10 pages, 4 figures

Smectic order has been generated in superconducting Nb films with two-fold symmetry arrays of symmetric pinning centers. Magnetic fields applied perpendicularly to the films develop a vortex matter smectic phase that is easily detected when the vortices commensurate with the pinning center array. The smectic phase can be turned on and off with external parameters., Comment: 15 pages, 5 figures

The temperature and field dependences of the AC and DC magnetic moment of superconducting and ferromagnetic Nb/Ni multilayers were measured using a SQUID magnetometer with magnetic field applied parallel to the multilayer plane. Periodic kinks in the superconducting upper critical field are evidence for nucleation of a hierarchy of Abrikosov vortex lattices aligned parallel to the multilayer. Small cusps in the low-field, isothermal DC magnetization are evidence that supercurrents are sensitive to extremely small changes in the Ni layer magnetization. Smooth ferromagnetic hysteresis is observed in the normal state, but is supplanted below the superconducting transition by two reproducible discontinuities that indicate magnetic switching of the Ni layers is tightly coupled to the supercurrents. The discontinuities are attributed to the non-dipole character of the moment near switching fields and, therefore, cannot be analyzed by standard magnetometer software. Ferromagnetic resonance spectra were measured in parallel and perpendicular DC magnetic fields at room temperature and 4.2 K, and resulting data suggest that Ni layers interact magnetically in the superconducting state.

We study vortex flow resistance (VFR) in films of Pb with holes as periodic pinning centers (PPCs) and of Nb with PPCs in the form of Ni dots, as function of temperature, dc current and constant applied magnetic field. The experimental resolution is better than 10 −5 of the normal state resistance. At high temperatures near to T c and high drive currents the resistance shows local minima both at matching fields and zero field. For lower temperatures, however, in a narrow temperature range before the vortex system becomes completely frozen, we observe suppression of the VFR with increasing magnetic field. In the Pb film with PPCs these phenomena show up as a clear zero field resistance excess, which is gradually suppressed by the applied magnetic field. We attribute this unusual feature in the magnetoresistance to thermally excited vortex–antivortex pairs. In the Nb superconducting film with PPCs we observe a gradual suppression of the VFR near integer matching fields up to n = 3 at the lowest accessible dc currents and relevant temperatures. This anomalous behaviour is followed by the observation of negligible VFL for higher fields ( H 3 H H 4 ) and a strong enhancement of VFR for H > H 4 . We tentatively explain this observation as due to the moving vortex glass.

Nb films have been grown on arrays of asymmetric pinning centers. The lattice vortex dynamics could be modified, almost at will, by periodic pinning potentials. In the case of asymmetric pinning potentials a vortex ratchet effect occurs: the vortex lattice motion is rectified. That is, an injected ac current yields an output dc voltage, which polarity could be tuned. The output signal polarity could be switched with the applied magnetic field and the ac current strength. Ratchet effect occurs when asymmetric potentials induce outward particles flow under external fluctuations in the lack of driven direct outward forces. The output signal is similar using magnetic or non-magnetic submicrometric array of pinning centers. This device works as an adiabatic rocking ratchet. This superconducting ratchet could be a model to study biological motors.

Nb/Cu superconducting superlattices have been fabricated by dc magnetron sputtering. The structural characterization has been made by X-ray diffraction techniques. We have measured upper critical fields both parallel and perpendicular to the layered structure, H c2∥ and H c2⊥ . Also, the critical depinning currents j c have been measured as a function of the angle between the applied magnetic field and Nb/Cu interfaces. The superlattice induced anisotropy of this system has been analyzed. It turns out that, for superlattices with Nb layers decoupled by the Cu spacer, a much higher anisotropy is observed in critical currents than in critical fields.

We have grown a -axis oriented EuBa 2 Cu 3 O 7 thin films on SrTiO 3 (001) substrates using a magnetron sputtering technique. In these films the CuO 2 planes are aligned parallel to the substrates. The film microstructure shows microdomains. The microdomains are grains with a 90° rotation of the c -axis. We have done transport measurements in applied magnetic fields up to 16 T. We have found a reduced quasi-2D dimensionality of the vortex glass transition probably resulting of vortex coherence being limited by disorder at domain boundaries. The activation energy for vortex motion in the liquid state displays a 1/H 0.5 field dependence characteristic of strongly anisotropic superconductor.

Nb films grown on top of an array of asymmetric pinning centers show a vortex ratchet effect. A net flow of vortices is induced when the vortex lattice is driven by fluctuating forces on an array of pinning centers without reflection symmetry. This effect occurs in the adiabatic regime and it could be mimiced only by reversible DC driven forces.

Vortex–lattice dynamics has been studied in superconducting Nb thin films with periodic arrays of magnetic pinning centers. Magnetotransport measurements have been used to study vortex–lattice velocity as a function of the in-plane direction of the driving force. This has been done for samples with arrays of decreasing anisotropy, from rectangular arrays (two-fold symmetry) to square arrays (four-fold symmetry). Strongly guided vortex motion along privileged directions has been found in the former, but not in the latter. The results are discussed in terms of channeling effects.

Magnetron sputtering, electron beam lithography and etching techniques allow us to fabricate Nb thin films on array of periodic nonsuperconducting nanometric triangles. These asymmetric pinning potentials induce a ratchet effect on the vortex lattice motion: injecting an ac current on the sample yields a dc voltage. This rectification effect does not depend on the ac frequency in the kHz range. The effect occurs for magnetic as well as nonmagnetic pinning centers. The rectification is enhanced at lower temperatures.

Fabrication of native Nb oxide barriers and their influence on the tunneling behavior of Nb/Ni planar junctions are investigated. Each junction film electrode was grown by magnetron sputtering technique and two methods were used to obtain the barrier. In the first method, a NbxOy oxide layer was formed at room conditions on top of Nb. In the second one, a saturated water vapor atmosphere is used to obtain the NbxOy layer. Samples prepared on these ways show different tunneling characteristics related with changes in the barrier. Characterization of the barrier was done by low-angle X-ray diffraction, atomic force microscopy and X-ray photoelectron spectroscopy.

The magnetic behaviour of patterned Ni wires has been studied by magnetotransport measurements to analyse the mechanisms that govern the magnetization reversal in these ordered nanostructures as a function of temperature in the 150–350 K range. These results show reversal processes driven by a temperature-dependent combination of two different mechanisms: coherent rotation and a curling-like irreversible process. When the field is decreased from saturation the magnetization rotates reversibly until it reaches the nanowires' direction where it reverses its sign by an irreversible process at low fields. The amplitude in the magnetoresistance of this irreversible process is reduced as the temperature is increased, with a sharp step at room temperature. In fact, the analysis of the angular dependence reveals that it is compatible with an incoherent curling process up to 300 K in the whole angular range. For higher temperatures, a crossover towards coherent rotation is observed in the irreversible contribution for high angles between the applied field and the wire direction.

Nanostructures have interesting properties due to the confinement of electrons in small structures. In addition, since an appreciable fraction of the electronic wave-function resides outside the physical extension of the nanostructure, proximity effects become more important with decreasing size. Moreover, in magnetic nanostructures the magnetic fields also extend a considerable distance outside the physical extent of the nanostructure. Thus, the interplay between size confinement and proximity effects become particularly interesting in magnetic nanostructures. We give two interesting experimental examples of proximity effects with magnetic nanostructures. In one, small magnetic dots radically modify the magnetotransport properties of superconducting films. In the other, the properties of a ferromagnet with nanoscopic antidots are considerably changed because of proximity with an antiferromagnet.

Fabrication and structural characterization of periodic structures in the submicrometric range are ones of the most interesting topics in the nanotechnology field. In this work we report on the fabrication of 2-dimensional periodic nanostructures (superlattices) by magnetron sputtering and 1-dimensional (lines) and 0-dimensional (dots) periodic nanostructures by electron beam lithography and etching techniques. The interplay between the characteristic sample length scale and the length scale which governs the physics, could strongly modify the physical properties of the system. As one typical example we will present the superconducting properties of Nb films in three systems: (a) Nb/Ni multilayers (2D), (b) periodic lines in Nb films (1D) and (c) periodic Ni dots (0D) in Nb films.

Ordered arrays of submicrometric magnetic pinning centers have been fabricated in superconducting Nb films by electron beam lithography. Periodic pinning effects, observed when the vortex lattice matches the array of dots, have been studied as a function of temperature, current and array geometry in order to analyze the physics of the interaction between the vortex lattice and the periodic array of microscopic pinning centers. These results have allowed pointing to the magnetic origin of the relevant pinning mechanism in this system and to dynamic ordering effects of the vortex lattices. The effect of the relative direction of the driving force with respect to the pinning potential is dependent on the anisotropy configuration of the artificial submicrometric array.

To develop a full understanding of interactions in nanomagnet arrays is a persistent challenge, critically impacting their technological acceptance. This paper reports the experimental, numerical and analytical investigation of interactions in arrays of Co nanoellipses using the first-order reversal curve (FORC) technique. A mean-field analysis has revealed the physical mechanisms giving rise to all of the observed features: a shift of the non-interacting FORC-ridge at the low-H$_c$ end off the local coercivity H$_c$ axis; a stretch of the FORC-ridge at the high-H$_c$ end without shifting it off the H$_c$ axis; and a formation of a tilted edge connected to the ridge at the low-H$_c$ end. Changing from flat to Gaussian coercivity distribution produces a negative feature, bends the ridge, and broadens the edge. Finally, nearest neighbor interactions segment the FORC-ridge. These results demonstrate that the FORC approach provides a comprehensive framework to qualitatively and quantitatively decode interactions in nanomagnet arrays., Comment: 19 pages, 4 figures. 9 page supplemental material including 3 figures

Hybrid magnetic arrays embedded in superconducting films are ideal systems to study the competition between different physical (such as the coherence length) and structural length scales such as are available in artificially produced structures. This interplay leads to oscillation in many magnetically dependent superconducting properties such as the critical currents, resistivity and magnetization. These effects are generally analyzed using two distinct models based on vortex pinning or wire network. In this work, we show that for magnetic dot arrays, as opposed to antidot (i.e. holes) arrays, vortex pinning is the main mechanism for field induced oscillations in resistance R(H), critical current Ic(H), magnetization M(H) and ac-susceptibility χ ac(H) in a broad temperature range. Due to the coherence length divergence at Tc, a crossover to wire network behaviour is experimentally found. While pinning occurs in a wide temperature range up to Tc, wire network behaviour is only present in a very narrow temperature window close to Tc. In this temperature interval, contributions from both mechanisms are operational but can be experimentally distinguished. Fil: Gomez, A. Universidad Complutense de Madrid; España Fil: del Valle, J. Universidad Complutense de Madrid; España Fil: Gonzalez, E. M.. Universidad Complutense de Madrid; España. IMDEA. Madrid; España Fil: Chiliotte, Claudio Ezequiel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina Fil: Carreira, Santiago José. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina Fil: Bekeris, Victoria Isabel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina Fil: Prieto, J. L.. Universidad Politécnica de Madrid; España Fil: Schuller, Ivan K.. University of California at San Diego; Estados Unidos Fil: Vicent, J. L.. IMDEA. Madrid; España. Universidad Complutense de Madrid; España

Magnetization reversal processes have been studied in hybrid magnetic/superconducting Nd-Co/Nb bilayers by the comparison of out-of-plane magnetic hysteresis loops and superconducting phase diagrams as a function of magnetic layer thickness and of disorder in the magnetic layer induced by a nanostructured copolymer template. A good correlation is found between the regimes corresponding to percolation effects in the superconductor and to the transition from extended to confined superconductivity with the characteristic fields for reverse domain nucleation and fast domain expansion in the magnetic layer, indicating that superconductivity nucleates on the disordered network imprinted on the superconducting layer by the labyrinth domain structure of the magnetic layer. As disorder increases in the magnetic layer, percolation effects disappear from the superconducting transitions in agreement with a more homogeneous magnetization reversal process. © 2014 AIP Publishing LLC., This work was supported by Spanish MINECO under grants FIS2008-06249 and Consolider CSD2007-00010 and by CAM under grant S2009/MAT-1726.

In this work, we improve the ability to tailor the switching mechanism in nanomagnets by introducing an additional, highly controlled source of anisotropy: magnetocrystalline anisotropy. We analyze the vortex dynamics in single crystal Fe nanotriangles with different orientations of the crystalline axes. By experimental studies and simulation, we show that the angular dependence of the vortex annihilation field springs from the convolution of the crystalline and configurational anisotropies. In contrast, the remanence and the nucleation field present a much simpler behavior controlled by the existence of a single symmetry axis when shape and crystalline orientation are taken into account. © 2014 AIP Publishing LLC., We thank the financial support by Spanish MINECO, Grant Nos. MAT2010-18432, MAT2010-21088-C03-03, FIS2008-06249 (Grupo consolidado), CONSOLIDER CSD2007-00010, FUNCOAT CSD-2008-0023, and CAM Grant No. S2009/MAT1726.

Superconducting and electronic properties of Nd 1.85 Ce 0.15 Cu 1.01 O y ceramic samples have been studied as a function of the oxygen content. It has been observed that values of oxygen content ‘ y ’ above those corresponding to the CuO–Cu 2 O equilibrium partial oxygen pressure destroy superconductivity. On the other hand, for lower oxygen contents, resistivity and magnetic superconducting transitions are present with critical temperature values up to 20 K. The normal state Hall effect has also been measured presenting a negative R H coefficient in all the samples. For superconducting samples, R H is not significantly dependent on the oxygen content value. In addition, the Hall coefficient is almost temperature independent in these samples. It is in contrast with some of the non-superconducting samples where the absolute value of R H increases as T decreases. These results reveal that, for samples obtained by annealing below the CuO–Cu 2 O equilibrium partial oxygen pressure, the enhancement of superconducting properties as oxygen is reduced is not produced by a change in the carrier density, pointing then to a structural ordering effect as an explanation.

Ordered arrays of 250 μm long submicrometric Ni wires have been fabricated on silicon substrates by electron beam lithography. The center-to-center distance between two adjacent parallel wires is 900 nm, while the linewidth is 200 nm. The array is prepared over a total area of 250×250 μm 2 , so that the magnetic behavior can be studied by magnetooptical Kerr effect. The results show an increment in the coercive field H C of these submicrometric patterned structures in comparison with the continuous films. The analysis of the angular dependence of H C reveals that the magnetization reversal process of the Ni wires is well consistent with an incoherent rotation by curling.

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., 12 text pages (revtex), 6 figures (1st jpeg, 2nd-6th postscript) accepted in Physical Review B

a-Axis-oriented superconducting EuBa2Cu3O7 films and EuBa2Cu3O7/PrBa2Cu3O7 superlattices have been grown by sputtering on SrTiO3 (100) substrates. Structural analysis by high resolution transmission electron microscopy and X-ray diffraction has been performed. Both kind of samples are well-oriented with the c-axis parallel to the substrate plane, the structure at the boundaries is highly coherent and, in the superlattices, high angle X-ray satellite peaks are observed. The presence of PrBa2Cu3O7 layers in the superlattices induces a reduction in the superconducting transition temperature that can be attributed to a charge redistribution process in the CuO2 planes, which in this a-axis orientation belong simultaneously to insulating and superconducting layers.

The angular dependence of the in-plane resistivity ρ(T,H, θ) of melt textured YBa_2Cu_3O_7/Y_2BaCuO_5 composites has been measured in a large range of magnetic fields and temperatures and from them, the intrinsic anisotropy of the superconducting state has been verified following the anisotropic Ginzburg-Landau approach. The influence of correlated defects like twin boundaries and quenched disorder generated by Y_2BaCuO_5 precipitates on the pinning behavior of these composites in the liquid vortex state is analyzed, and the corresponding phase diagram is determined and compared to that of twinned single crystals. We show that the irreversibility line displays an upwards shift due to twin boundary pinning enabling to define a ''quenched'' Bose glass transition. A new region in the vortex liquid state is identified where twin boundary pinning defines a partially entangled liquid vortex state characterized by a short-range c-axis vortex coherence. The transition to the entangled liquid phase is experimentally determined. The relevance of this depinning line and its unique position with respect to twinned single crystals is discussed. [S0163- 1829(99)01741-5].

Fe/Cu superlattices exhibit a structural and magnetic transition with increasing Fe layer thickness, thus increasing the Curie temperature. An enhancement in the superconducting transition temperature is found in proximity coupled Nb/[Fe/Cu] layers as the Fe thickness increases. These results indicate a weakening of the magnetic proximity effect for the material with higher Curie temperature, and suggest a dominant role of spin-flip scattering in the pair breaking processes which give rise to the proximity effect. {copyright} {ital 1999} {ital The American Physical Society}

The study of nanomagnets is the subject of increasing scientific effort. The size, the thickness and the geometric shape of the elements determine the magnetic properties and then the domain configuration. In this work, we fabricated by electron-beam lithography the three different arrays of Ni nanostructures keeping the size, the thickness and also the distance constant between the elements but varying the geometry: square, triangular and circular. The domain structure of the nanomagnets is studied by magnetic force microscopy.

We have made Co/Ag multilayers with ten repeats on MgO (1 0 0) single-crystal substrates at room temperature by magnetron sputtering from independent sources and keeping the Ag layers as thick as 40 A. The analysis of the X-ray diffraction indicates a coherent structure of Ag layers with a lattice parameter 2.355 A close to the bulk FCC (1 1 1) spacing and Co layers with 2.13 A spacing corresponding to HCP (1 0 0) planes. Other experiments show the granular in-plane morphology and a large interfacial disorder. These Co layers develop in-plane magnetization and electrical magneto-transport whose properties are thickness dependent.