J. I. Martín

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

Crossed-Ratchet Effects for Magnetic Domain Wall Motion

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

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

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.

Topological defects and misfit strain in magnetic stripe domains of lateral multilayers with perpendicular magnetic anisotropy

We study both experimentally and theoretically the driven motion of domain walls in extended amorphous magnetic films patterned with a periodic array of asymmetric holes. We find two crossed-ratchet effects of opposite sign that change the preferred sense for domain wall propagation, depending on whether a flat or a kinked wall is moving. By solving numerically a simple phi(4) model we show that the essential physical ingredients for this effect are quite generic and could be realized in other experimental systems involving elastic interfaces moving in multidimensional ratchet potentials.

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

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 flow 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 suppressed again, in agreement with the behavior found in numerical simulations.

Nanostructures and the proximity effect

Stripe domains are studied in perpendicular magnetic anisotropy films nanostructured with a periodic thickness modulation that induces the lateral modulation of both stripe periods and in-plane magnetization. The resulting system is the 2D equivalent of a strained superlattice with properties controlled by interfacial misfit strain within the magnetic stripe structure and shape anisotropy. This allows us to observe, experimentally for the first time, the continuous structural transformation of a grain boundary in this 2D magnetic crystal in the whole angular range. The magnetization reversal process can be tailored through the effect of misfit strain due to the coupling between disclinations in the magnetic stripe pattern and domain walls in the in-plane magnetization configuration.

Mixed-state properties of superconducting Nb/Ni superlattices

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.

Anisotropy measurements in mesoscopic magnets by magneto-optical torque

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.

Anisotropic pinning enhancement in Nb films with arrays of submicrometric Ni lines

Superlattices of ðNb10 nm=NitÞ 8 (1 6 t 6 5 nm) have been grown on Si(1 0 0) by magnetron sputtering. Samples show textured growth of Nb(1 1 0) and Ni(1 1 1), with negligible interdiffusion and interface roughness below 0.3 nm. The critical superconducting temperature (Tcs) of these superlattices decreases as the Ni thickness (tNi) increases. Ferromagnetic order in Ni layers is suppressed for tNi < 2:2 nm. We have explored the behavior of these superlattices in the mixed state by measuring the upper critical fields. Critical fields in perpendicular (Hc2?) and parallel (Hc2k) configurations show moderate superlattice-induced anisotropy values, which increase as tNi increases. From Hc2ðT Þ measurements, we have found that superconducting superlattices behave like a three-dimensional system. This dimensional behavior is discussed in terms of coupling of Nb through Ni layers. 2001 Elsevier Science B.V. All rights reserved.