Influence of artificial pinning on vortex lattice instability in superconducting films

in New Journal of Physics (2012), 14

In superconducting films under an applied dc current, we analyze experimentally and theoretically the influence of engineered pinning on the vortex velocity at which the flux-flow dissipation undergoes an ... [more ▼]

In superconducting films under an applied dc current, we analyze experimentally and theoretically the influence of engineered pinning on the vortex velocity at which the flux-flow dissipation undergoes an abrupt transition from low to high resistance. We argue, based on a nonuniform distribution of vortex velocity in the sample, that in strongly disordered systems the mean critical vortex velocity for flux-flow instability (i) has a nonmonotonic dependence on magnetic field and (ii) decreases as the pinning strength is increased. These findings challenge the generally accepted microscopic model of Larkin and Ovchinnikov (1979 J. Low. Temp. Phys. 34 409) and all subsequent refinements of this model which ignore the presence of pinning centers. [less ▲]

Probing the discrete motion of vortices with rf excitations

in Physica C: Superconductivity (2010), 470(19), 857-859

In this work we present experimental results on the rectification of vortices in a superconductor/ferromagnet system under a high frequency drive. The two-dimensional pinning landscape, induced by the ... [more ▼]

In this work we present experimental results on the rectification of vortices in a superconductor/ferromagnet system under a high frequency drive. The two-dimensional pinning landscape, induced by the stray fields of the ferromagnetic template, has no intrinsic asymmetry. Nevertheless, an asymmetric potential is artificially induced by an applied dc bias. The experimental results unambiguously show a biased, discrete motion of the vortices in the periodic potential at frequencies above 10 MHz. This synchronized motion is very sensitive to the external applied field. Increasing temperature leads to a reduction of the pinning potential, which in turn results in a lower ac power needed to drive the vortex lattice. (C) 2010 Elsevier B.V. All rights reserved. [less ▲]

Pinning effects on the vortex critical velocity in type-II superconducting thin films

in Physica C: Superconductivity (2010), 470(19), 904-906

We study the influence of artificial pinning centers on the vortex critical velocity in Al thin films deposited on top of a periodic array of Permalloy (FeNi) square rings. We demonstrate that the field ... [more ▼]

We study the influence of artificial pinning centers on the vortex critical velocity in Al thin films deposited on top of a periodic array of Permalloy (FeNi) square rings. We demonstrate that the field dependence of the flux flow velocity strongly depends on the particular magnetic state of the rings. In particular, we find that, even when the rings are in a flux closure state, i.e. with little stray field, the vortex critical velocity shows a non-monotonic magnetic field dependence. This behaviour is in sharp contrast with the results obtained in a reference plain film, with no rings underneath. A comparison with the intrinsic strong pinning Nb films previously studied, suggests an interpretation in terms of a channel-like motion of vortices, here induced by the artificial pinning structure. (C) 2010 Elsevier B.V. All rights reserved. [less ▲]

Critical field enhancement in hybrid superconductor/ferromagnet mesoscopic disks

in Journal of Applied Physics (2009), 105(2),

We investigated experimentally the nucleation of superconductivity in a mesoscopic hybrid structure, consisting of a thin superconducting disk covered with a ferromagnetic layer with an in-plane magnetic ... [more ▼]

We investigated experimentally the nucleation of superconductivity in a mesoscopic hybrid structure, consisting of a thin superconducting disk covered with a ferromagnetic layer with an in-plane magnetic moment. By applying a magnetic field in the plane of the structure, the remanent magnetic state of the ferromagnet can be switched from a flux-closure state where field lines are confined inside the ferromagnet to a polarized state with nonzero stray fields at the edges. This change in the magnetic state causes a drastic modification on the superconductor/normal-state phase boundary of the hybrid sample. In the polarized state a re-entrant transition line and a strong broadening of the phase boundary are observed. [less ▲]

Switchable magnetic dipole induced guided vortex motion (vol 93, 022507, 2008)

in Applied Physics Letters (2009), 94(21),

erratum to the Appl. Phys. Lett. 93, 022507 (2008)

Switchable magnetic dipole induced guided vortex motion

in Applied Physics Letters (2008), 93(2),

We present evidence of magnetically controlled vortex motion in an Al film on top of a periodic array of Permalloy square rings. The resulting magnetic template generates a strongly anisotropic pinning ... [more ▼]

We present evidence of magnetically controlled vortex motion in an Al film on top of a periodic array of Permalloy square rings. 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 rerouted by 90 degrees simply changing the dipole orientation or even suppressed by inducing a flux-closure magnetic state with very low stray fields in the rings. (C) 2008 American Institute of Physics. [less ▲]

Rectification effects in superconductors with magnetic pinning centers

in Physica C: Superconductivity (2008), 468(7-10), 563-567

We investigate the dynamics and pinning properties of vortices in superconducting At films deposited on top of a close-packed array of Py microsized. loops by electrical transport measurements. The ... [more ▼]

We investigate the dynamics and pinning properties of vortices in superconducting At films deposited on top of a close-packed array of Py microsized. loops by electrical transport measurements. The micromagnets have an in-plane magnetic moment that can be set in different magnetic states by applying an external field parallel to the plane of the pattern. When the loops are set in the magnetic vortex-state, for which the stray field is the smallest, a weaker pinning in comparison with the polarized states (i.e. strong stray field) is observed. In addition, a clear influence of the chosen magnetic state of the Py rings on the dynamics of the vortex motion under an ac-excitations is obtained. When the magnetic elements are in the as-grown state a rectification signal which reverses sign when the field changes polarity is observed. In contrast to that, when the array of loops is magnetized the observed rectification effect is independent of the field polarity and can be reversed by reorienting the magnetization of the micromagnets. (c) 2008 Elsevier B.V. All rights reserved. [less ▲]

Manipulation of the vortex motion in nanostructured ferromagnetic/superconductor hybrids

in Applied Physics Letters (2007), 90(18),

The authors investigate the rectified motion of vortices in superconducting films deposited on top of a close-packed array of open in-plane magnetized triangular micromagnets. The dc voltage induced by ... [more ▼]

The authors investigate the rectified motion of vortices in superconducting films deposited on top of a close-packed array of open in-plane magnetized triangular micromagnets. The dc voltage induced by the vortex drift under an ac excitation is recorded for three different magnetic configurations of the triangles. When the magnetic elements are in the as-grown state a rectification signal which reverses sign when the field changes polarity is observed. In contrast to that, when the array of triangles is magnetized the observed rectification effect is independent of the field polarity and can be reverted by reorienting the magnetization of the micromagnets. (C) 2007 American Institute of Physics. [less ▲]