Phase diagram of a superconductor / ferromagnet bilayer

The magnetic field (H) - temperature (T) phase diagram of a superconductor is significantly altered when domains are present in an underlying ferromagnet with perpendicular magnetic anisotropy. When the domains have a band-like shape, the critical temperature Tc of the superconductor in zero field is strongly reduced, and the slope of the upper critical field as a function of T is increased by a factor of 2.4 due to the inhomogeneous stray fields of the domains. Field compensation effects can cause an asymmetric phase boundary with respect to H when the ferromagnet contains bubble domains. For a very inhomogeneous domain structure, Tc~H^2 for low H and Tc~H for higher fields, indicating a dimensional crossover from a one-dimensional network-like to a two-dimensional behavior in the nucleation of superconductivity.Comment: 6 pages, 7 figure

Anisotropic vortex pinning in superconductors with a square array of rectangular submicron holes

We investigate vortex pinning in thin superconducting films with a square array of rectangular submicron holes ("antidots"). Two types of antidots are considered: antidots fully perforating the superconducting film, and "blind antidots", holes that perforate the film only up to a certain depth. In both systems, we observe a distinct anisotropy in the pinning properties, reflected in the critical current Ic, depending on the direction of the applied electrical current: parallel to the long side of the antidots or perpendicular to it. Although the mechanism responsible for the effect is very different in the two systems, they both show a higher critical current and a sharper IV-transition when the current is applied along the long side of the rectangular antidots

Superconducting Vortices and Elliptical Ferromagnetic Textures

In this article an analytical and numerical study of superconducting thin film with ferromagnetic textures of elliptical geometries in close proximity is presented. The screening currents induced in the superconductor due to the magnetic texture are calculated. Close to the superconducting transition temperature $T_c$ the spontaneous creation of superconducting vortices becomes energy favorable depending on the value of the magnetization and the geometrical quantities of the magnetic texture. The creation of vortices by elliptic dots is more energy favorable than those created by circular ones. The superconductor covered by elliptic dots array exhibits anisotropic transport properties.Comment: 4 pages, 5figure

Vortex Pinball Under Crossed AC Drives in Superconductors with Periodic Pinning Arrays

Vortices driven with both a transverse and a longitudinal AC drive which are out of phase are shown to exhibit a novel commensuration-incommensuration effect when interacting with periodic substrates. For different AC driving parameters, the motion of the vortices forms commensurate orbits with the periodicity of the pinning array. When the commensurate orbits are present, there is a finite DC critical depinning threshold, while for the incommensurate phases the vortices are delocalized and the DC depinning threshold is absent.Comment: 4 pages, 4 postscript figure

Superconducting Wigner Vortex Molecule near a Magnetic Disk

Within the non-linear Ginzburg-Landau (GL) theory, we investigate the vortex structure in a superconducting thin film with a ferromagnetic disk on top of it. Antivortices are stabilized in shells around a central core of vortices (or a giant-vortex) with size-magnetization controlled ``magic numbers''. An equilibrium vortex phase diagram is constructed. The transition between the different vortex phases occurs through the creation of a vortex-antivortex pair under the magnetic disk edge.Comment: 4 pages, 4 figures. Submitted to Phys. Rev. Let

Phase-Locking of Vortex Lattices Interacting with Periodic Pinning

We examine Shapiro steps for vortex lattices interacting with periodic pinning arrays driven by AC and DC currents. The vortex flow occurs by the motion of the interstitial vortices through the periodic potential generated by the vortices that remain pinned at the pinning sites. Shapiro steps are observed for fields B_{\phi} < B < 2.25B_{\phi} with the most pronouced steps occuring for fields where the interstitial vortex lattice has a high degree of symmetry. The widths of the phase-locked current steps as a function of the magnitude of the AC driving are found to follow a Bessel function in agreement with theory.Comment: 5 pages 5 postscript figure

Superconducting Transition Temperature in Heterogeneous Ferromagnet-Superconductor Systems

We study the shift of the the superconducting transition temperature $T_c$ in ferromagnetic-superconducting bi-layers and in a superconducting film supplied a square array of ferromagnetic dots. We find that the transition temperature in these two cases change presumably in opposite direction and that its change is not too small. We extend these results to multilayer structures. We predict that rather small external magnetic field $\sim 10$ Oe can change the transition temperature of the bilayer by 10% .Comment: 9 pages, 2 figure

Vortex states in 2D superconductor at high magnetic field in a periodic pinning potential

The effect of a periodic pinning array on the vortex state in a 2D superconductor at low temperatures is studied within the framework of the Ginzburg-Landau approach. It is shown that attractive interaction of vortex cores to a commensurate pin lattice stabilizes vortex solid phases with long range positional order against violent shear fluctuations. Exploiting a simple analytical method, based on the Landau orbitals description, we derive a rather detailed picture of the low temperatures vortex state phase diagram. It is predicted that for sufficiently clean samples application of an artificial periodic pinning array would enable one to directly detect the intrinsic shear stiffness anisotropy characterizing the ideal vortex lattice.Comment: 8 pages, 5 figure

Temperature dependence and mechanisms for 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, 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.Comment: 12 text pages (revtex), 6 figures (1st jpeg, 2nd-6th postscript) accepted in Physical Review

Superconducting transition in Pb/Co nanocomposites: effect of Co volume fraction and external magnetic field

Pb films embedded with homogeneously distributed cobalt (Co) nanoparticles (mean size 4.5 nm) have been prepared. Previous transport investigations have shown that Co particles induce spontaneous vortices below the superconducting transition temperature (T$_{c}$) in zero external magnetic field. In this paper we study in detail the influence of the Co volume franction and an external magnetic field on the superconducting transition in such composites. The large difference in T$_c$-reduction between the as-prepared and annealed samples can be attributed to the different superconducting coherence lengths and the resulting different diameters of the spontaneous vortices in these samples.Comment: 4 pages, 5 figure