Arrays of artif icial magnetic pinning centers for vortex matter

BEKERIS, V.; CHILOTTE, C.; PÉREZ DAROCA, D.; PAZQUINI, G.; OLIVA, M. I.; VILLEGAS, J.E.; SCHULLER, I.K.

Valparaíso, CHILE

Congreso; Encuentro internacional SÓLIDOS'09; 2009

Universidad Técnica Federico Santa María

It is well known that the introduction of periodic arrays of well defined pinning centers in superconducting samples can reduce vortex mobility and give rise to higher critical currents. Basically there are two ways to artificially control vortex pinning: by suppression of the superconducting order parameter thus generating a minimum in the self energy of the vortex line, or by lowering the magnetic energy of the system by modulation of the magnetic field. The progressive development of material deposition techniques and self organization, have made it possible to fabricate superconductor-ferromagnet artificial structures at nanometer scales, which is the scale of the relevant superconducting parameters that intervene in vortex pinning phenomena. In this work we study the pinning effects of magnetic nano particles arrayed periodically and close to a superconducting film. As ferromagnetic materials in close contact to superconducting materials induce proximity effect, pinning effects could be related to suppression of the order parameter and to magnetic field modulation. In order to study the effect of these contributions, we have studied dc and ac magnetic properties of superconducting Nb films either deposited on top of an array of Fe nanoparticles or deposited on top of a SiO2 buffer layer ~100nm thick separating the superconducting film from the nanoparticles, to avoid proximity effects. We present preliminary results on ac susceptibility, c (H,T) and dc magnetization, M(H,T), of Nb films (~100 nm thick), prepared as described elsewhere [1]. M(H) results indicate that the Fe particles ~ 120 nm are in “magnetic vortex” state with remanent magnetization oriented perpendicular to the film surface. FORC diagrams in the normal state indicate that interactions between magnetic dots are weak. Samples where proximity effect is expected to contribute, show wide superconducting transitions and matching field effects (at fields for which there is one flux quantum per unit cell of the array and collective behavior may be expected) are not as sharp as in samples where the effect has been reduced by a separating isolating layer. Additionally, we observe in c (H,T) and in M(H,T) response, a distinct hysteretical behavior for each type of sample, that will be described.