Tunable Field Induced Superconductivity

We investigate the transport properties of a thin superconducting Al layer covering a square array of magnetic dots with out-of-plane magnetization. A thorough characterization of the magnetic properties of the dots allowed us to fine-tune their magnetic state at will, hereby changing the influence of the dots on the superconductor in a continuous way. We show that even though the number of vortex-antivortex pairs discretely increases with increasing the magnetization of the dots, no corresponding discontinuity is observed in the resistance of the sample. The evolution of the superconducting phase boundary as the magnetic state of the dots is swept permits one to devise a fully controllable and erasable field induced superconductor

Tunable pinning in superconducting films with magnetic micro-loops

We study the flux pinning properties of superconductor/magnetic micro-rings lattice hybrid structures. The used open triangular micromagnets represent an eight-fold degree of freedom system, with six polarized and two flux-closure possible states. By conveniently choosing the magnetic state of the underlying rings it is possible to induce different pinning potentials. We show that the magnetic vortex state with minimum stray field produces a weaker pinning in comparison with the polarized states

On the origin of the reversed vortex ratchet motion

We experimentally demonstrate that the origin of multiply reversed rectified vortex motion in an asymmetric pinning landscape is a consequence not only of the vortex-vortex interactions but also essentially depends on the ratio between the characteristic interaction distance and the period of the asymmetric pinning potential. Our system consists of an Al film deposited on top of a square array of size-graded magnetic dots with a constant lattice period a=2\mu m. Four samples with different periods of the size gradient d were investigated. For large d the dc voltage Vdc recorded under a sinusoidal ac excitation indicates that the average vortex drift is from bigger to smaller dots for all explored positive fields. As d is reduced a series of sign reversals in the dc response are observed as a function of field. We show that the number of sign reversals increases as d decreases. These findings are in agreement with recent computer simulations and illustrate the relevance of the different characteristic lengths for the vortex rectification effects.Comment: accepted in Phys. Rev. Let

Magnetic dipole induced guided vortex motion

We present evidence of magnetically controlled guided vortex motion in a hybrid superconductor/ferromagnet nanosystem consisting of an Al film on top of a square array of permalloy square rings. When the rings are magnetized with an in-plane external field H, an array of point-like dipoles with moments antiparallel to H, is formed. The resulting magnetic template generates a strongly anisotropic pinning potential landscape for vortices in the superconducting layer. Transport measurements show that this anisotropy is able to confine the flux motion along the high symmetry axes of the square lattice of dipoles. This guided vortex motion can be either re-routed by 90 degrees by simply changing the dipole orientation or even strongly suppressed by inducing a flux-closure magnetic state with very low stray fields in the rings.Comment: 5 pages, 3 figure

Guided nucleation of superconductivity on a graded magnetic substrate

We demonstrate the controlled spatial nucleation of superconductivity in a thin film deposited on periodic arrays of ferromagnetic dots with gradually increasing diameter. The perpendicular magnetization of the dots induces vortex-antivortex molecules in the sample, with the number of (anti)vortices increasing with magnet size. The resulting gradient of antivortex density between the dots predetermines local nucleation of superconductivity in the sample as a function of the applied external field and temperature. In addition, the compensation between the applied magnetic field and the antivortices results in an unprecedented enhancement of the critical temperature

Enhanced pinning and proliferation of matching effects in a superconducting film with a Penrose array of magnetic dots

The vortex dynamics in superconducting films deposited on top of a five-fold Penrose array of magnetic dots is studied by means of transport measurements. We show that in the low pinning regime (demagnetized dots) a few periodic and aperiodic matching features coexist. In the strong pinning regime (magnetized dots) a richer structure of unforeseen periodic and aperiodic vortex patterns appear giving rise to a clear enhancement of the critical current in a broader field range. Possible stable vortex configurations are determined by molecular dynamics simulations

Magnetic confinement of the superconducting condensate in superconductor/ferromagnet hybrid composites

The influence of an inhomogeneous magnetic field on the magnetoresistance of thin Al films, used in different superconductor/ferromagnet hybrids, has been investigated. Two contrasting magnetic textures with out-of-plane magnetization are explored, namely (i) a plain film in a multidomain state and (ii) an array of micro-sized dots. The stray fields of the ferromagnetic structures confine the superconducting condensate and, accordingly, modify the condition for the nucleation of superconductivity. By switching between different magnetic states of the ferromagnet, this confinement can be tuned at will, hereby reversibly changing the dependence of the critical temperature Tc on an external magnetic field H. In particular, the continuous evolution from a conventional linear Tc(H) dependence with a single maximum to a reentrant superconducting phase boundary with multiple Tc peaks has been demonstrated

Localization of superconductivity in superconductor-electromagnet hybrids

We investigate the nucleation of superconductivity in a superconducting Al strip under the influence of the magnetic field generated by a current-carrying Nb wire, perpendicularly oriented and located underneath the strip. The inhomogeneous magnetic field, induced by the Nb wire, produces a spatial modulation of the critical temperature T_c, leading to a controllable localization of the superconducting order parameter (OP) wave function. We demonstrate that close to the phase boundary T_c(B_ext) the localized OP solution can be displaced reversibly by either applying an external perpendicular magnetic field B_ext or by changing the amplitude of the inhomogeneous field.Comment: 10 pages, 6 figure

Dipole-induced vortex ratchets in superconducting films with arrays of micromagnets

We investigate the transport properties of superconducting films with periodic arrays of in-plane magnetized micromagnets. Two different magnetic textures are studied: a square array of magnetic bars and a close-packed array of triangular microrings. As confirmed by MFM imaging, the magnetic state of both systems can be adjusted to produce arrays of almost point-like magnetic dipoles. By carrying out transport measurements with ac drive, we observed experimentally a recently predicted ratchet effect induced by the interaction between superconducting vortices and the magnetic dipoles. Moreover, we find that these magnetic textures produce vortex-antivortex patterns, which have a crucial role on the transport properties of this hybrid system.Comment: 4 pages, 4 figure

Mesoscopic cross-film cryotrons: Vortex trapping and dc-Josephson-like oscillations of the critical current

We investigate theoretically and experimentally the transport properties of a plain Al superconducting strip in the presence of a single straight current-carrying wire, oriented perpendicular to the superconducting strip. It is well known that the critical current of the superconducting strip, Ic, in such cryotron--like system can be tuned by changing the current in the control wire, Iw. We demonstrated that the discrete change in the number of the pinned vortices/antivortices inside the narrow and long strip nearby the current-carrying wire results in a peculiar oscillatory dependence of Ic on Iw.Comment: 8 pages, 8 figure