Different approaches to generate matching effects using arrays in contact with superconducting films.

Abstract

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.

Magnetic order and disorder in nanomagnets probed by superconducting vortices

“…Figure 2 shows that this matching effect is absent. The second finding reported in [46] is that preserving the local order is crucial to achieve matching effects. If the local order is not retained the magnetoresistance minima vanish.…”

“…Recently, del Valle et al have reported two interesting findings on commensurability effects in hybrid samples fabricated with arrays in contact with superconducting films [46]. First, periodic roughness in the superconducting film is enough to generate resistivity minima.…”

“…This is achieved with a Co/Pd sample in the as-grown state: there are only one or two domains per triangle with sizes of several hundreds of nm, i.e. comparable to array unit cell (800 nm): Previous works 46 had shown that physical displacement of the pinning sites on a length scale comparable with the array length scale quench periodic pinning and minima in the resistivity disappear. The results for sample A (Figs 1a and 2 ) show an alternative method to create disorder on this same length scale (using the domains within each Co/Pd triangle) but with a clear advantage over the method used in 46 : Magnetic disorder is erasable with a magnetic field (as shown in Fig.…”

“…This is achieved with a Co/Pd sample in the as-grown state: there are only one or two domains per triangle with sizes of several hundreds of nm, i.e. comparable to array unit cell (800 nm): Previous works [46] had shown that physical displacement of the pinning sites on a length scale comparable with the array length scale quench periodic pinning and minima in the resistivity disappear. The results for sample A (Fig.…”

Vortex dynamics controlled by local superconducting enhancement

“…This indicates that the interstitial sites are not energetically favorable when the magnetic field is tilted beyond this angle and usual commensurability for the first matching field arises. In order to understand this effect, it has to be considered that the potential landscape created by the Nb nanotriangles emerges from both the repulsive potential created by the superconducting character of the nanotriangles, and the attractive one created by the periodic corrugation [21]. Therefore, when the parallel component (H || ) of the applied magnetic field increases, the superconducting performance of the Nb nanotriangles is diminished and the antipinning potential is smoothed.…”