J. L. Vicent

Ordered magnetic nanostructures: fabrication and properties

The fabrication methods and physical properties of ordered magnetic nanostructures with dimensions on the submicron to nanometer scale are reviewed. First, various types of nanofabrication techniques are described, and their capabilities and limitations in achieving magnetic nanostructures are discussed. Specifically, we address electron beam lithography, X-ray lithography, laser interference lithography, scanning probe lithography, step growth methods, nanoimprint, shadow masks, radiation damage, self-assembled structures, and the use of nanotemplates. Then the magnetic properties of these nanostructures are reviewed, including properties of single dots, magnetic interactions in arrays, dynamic effects, magnetic behavior of nanostructured lines and wires, giant magnetoresistance effect, and properties of films with arrays of holes. Finally, the physical properties in hybrid systems, where the magnetic arrays interact with superconducting and semiconducting layers, are summarized.

Temperature Dependence and Mechanisms of Vortex Pinning by Periodic Arrays of Ni Dots in Nb Films

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, which decreases with temperature as

Directional vortex motion guided by artificially induced mesoscopic potentials

(1\ensuremath{-}{T/T}_{C}{)}^{3/2}.

Flux pinning and weak links in the behavior of the critical current of a‐axis and c‐axis EuBa2Cu3O7 superconducting thin films

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 one 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 the dynamical effects of the vortex lattice in the high-field range.

Epitaxial Fe (001) micro tiling: Size and interaction effects

Rectangular pinning arrays of Ni dots define a potential landscape for vortex motion in Nb films. Magnetotransport experiments in which two in-plane orthogonal electrical currents are injected simultaneously allow one to select the direction and magnitude of the Lorentz force on the vortex lattice, thus providing the angular dependence of the vortex motion. The background dissipation depends on the angle at low magnetic fields, which is progressively smeared out with increasing field. The periodic potential locks in the vortex motion along channeling directions. Because of this, the vortex-lattice direction of motion is up to 85degrees away from the applied Lorentz force direction.

Surface and bulk magnetic anisotropy in amorphous alloys

Pure a‐axis and c‐axis oriented EuBa2Cu3O7 films have been grown by a dc sputtering technique. The a‐axis films show critical current values typically one order of magnitude lower than c‐axis films. The Jc(T) behavior could be governed, in a‐axis films, by the microstructure of these films with small domains and 90° boundaries, but Jc(B) does not seem to be limited by weak links. The magnetic‐field dependence of Jc shows a similar behavior in a‐axis and c‐axis films with a leading role of pinning in controlling the critical current in the high‐field range. Jc(B) is limited by flux creep with a similar distribution of pinning energies in both types of films, in spite of the very different microstructure.

Nanostructures and the proximity effect

The magnetic properties of 200 A epitaxial Fe (001)/MgO (001) tiling are studied as a function of tile size and separation. For edge sizes above ∼3 μm, the individual tiles maintain the single domain behavior while below ∼3 μm the tiles break into domains due to demagnetizing effects. In addition, below an inter tile separation threshold of about 0.9 μm, a single tile magnetization switch provokes a reversal cascade in all the tiles; while, above this threshold, the individual tile’s magnetization switches independently. In this last case, we have experimental access to the distribution of nucleation sites. Thus, we have found a clear signature of the magnetic interaction between patterned epitaxial micro tiles with in-plane magnetization.

Transverse rectification in superconducting thin films with arrays of asymmetric defects

Magnetic measurements of hysteresis loops and transverse susceptibilities have been done on as‐cast iron based amorphous alloys on the surface and in the bulk of the ribbons. Bitter technique has been used to obtain the domain patterns. a‐Fe78B13Si9 shows an anisotropy perpendicular to the ribbon plane with maze domain ‘‘island’’ structures. The surface measurements do not detect any difference in behavior compared to the observed bulk properties. a‐Fe81.5B14.5Si4 shows different behavior in the surface and in the bulk of the ribbon with a perminvar‐type hysteresis loop from the surface and magnetization perpendicular to the ribbon plane with well‐aligned closure domains. The analysis of the different magnetic experimental data and selective changes in the internal stresses produced by different treatments support a model of the layered character of the anisotropies in this sample with tensile stresses and in‐plane anisotropy in the surface and compressive stresses and anisotropy perpendicular to the ribb...

Mixed-state properties of superconducting Nb/Ni superlattices

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.

Anisotropy measurements in mesoscopic magnets by magneto-optical torque

Superconducting Nb films have been grown on top of arrays of Cu nanotriangles. These asymmetric pinning centers strongly modify the vortex lattice dynamics. Two rectification effects have been observed: (i) longitudinal ratchet effect when the input currents are injected perpendicular to the triangle reflection symmetry axis and (ii) transverse rectification effect when the input currents are injected parallel to the triangle reflection symmetry axis and the output voltage drop occurs perpendicular to the triangle reflection symmetry axis. Increasing the applied magnetic field, the former shows a change of the output voltage polarity, the transverse output voltage does not show any polarity reversal.