Field polarity dependent nucleation of superconductivity in quasi-one-dimensional magnetic templates

in Physica C: Superconductivity (2010), 470(19), 860-862

We investigate the nucleation of superconductivity in an Al/Al(2)O(3)/Py trilayer system by electrical transport measurements. Magnetic force microscopy images taken at room temperature show that the 0.7 ... [more ▼]

We investigate the nucleation of superconductivity in an Al/Al(2)O(3)/Py trilayer system by electrical transport measurements. Magnetic force microscopy images taken at room temperature show that the 0.7 mu m thick Py-film form stripes of magnetic domains with alternating out-of-plane stray field. After applying a strong out of plane magnetic field H the superconductor/normal phase boundary becomes asymmetric with respect to H = 0. This lack of field polarity symmetry results from the unbalanced size distribution of domains with opposite polarity. (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 ▲]

Guided nucleation of superconductivity on a graded magnetic substrate

in Applied Physics Letters (2010), 96(3),

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 ... [more ▼]

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. [less ▲]

Direct visualization of magnetic vortex pinning in superconductors

in Physical Review. B : Condensed Matter (2009), 79(14),

We study the vortex structure in a Pb film deposited on top of a periodic array of ferromagnetic square microrings by combining two high-resolution imaging techniques: Bitter decoration and scanning Hall ... [more ▼]

We study the vortex structure in a Pb film deposited on top of a periodic array of ferromagnetic square microrings by combining two high-resolution imaging techniques: Bitter decoration and scanning Hall-probe microscopy (SHPM). The periodicity and strength of the magnetic pinning potential generated by the square microrings are controlled by the magnetic history of the template. When the square rings are in the magnetized dipolar state, known as the onion state, the strong stray field generated at the domain walls prevents the decoration of vortices. SHPM images show that the stray field generated by the dipoles is much stronger than the vortex field, in agreement with the results of simulations. Real-space vortex imaging has revealed that in the onion state, the corners of the square rings act as effective pinning centers for vortices. [less ▲]

Symmetry-Induced Giant Vortex State in a Superconducting Pb Film with a Fivefold Penrose Array of Magnetic Pinning Centers

in Physical Review Letters (2009), 103(6),

A direct visualization of the flux distribution in a Pb film covering a fivefold Penrose array of Co dots is obtained by mapping the local field distribution with a scanning Hall probe microscope. We ... [more ▼]

A direct visualization of the flux distribution in a Pb film covering a fivefold Penrose array of Co dots is obtained by mapping the local field distribution with a scanning Hall probe microscope. We demonstrate that stable vortex configurations can be found for fields H similar to 0.8H(1), H(1), and 1.6H(1), where H(1) corresponds to one flux quantum per pinning site. The vortex pattern at 0.8H(1) corresponds to one vacancy in one of the vertices of the thin tiles, whereas at 1.6H(1) the vortex structure can be associated with one interstitial vortex inside each thick tile. Strikingly, for H=1.6H(1), interstitial and pinned vortices arrange themselves in ringlike structures ("vortex corrals") which favor the formation of a giant vortex state at their center. [less ▲]

Transition from turbulent to nearly laminar vortex flow in superconductors with periodic pinning

in Physical Review. B : Condensed Matter (2009), 80(14),

We revisit the vortex dynamics in Al thin films containing an artificial periodic array of antidots by means of electrical transport measurements. We clearly identify a turbulent to laminarlike vortex ... [more ▼]

We revisit the vortex dynamics in Al thin films containing an artificial periodic array of antidots by means of electrical transport measurements. We clearly identify a turbulent to laminarlike vortex flow transition which manifests itself as a negative differential resistivity. This transition is accompanied by a strong irreversibility in the voltage-current characteristics. The dynamical phase diagrams obtained as a function of commensurability, temperature, and driving force are in good agreement with the early predictions by Reichhardt et al. [Phys. Rev. Lett. 78, 2648 (1997)] based on molecular dynamic simulations. [less ▲]

A Study of Chirality in Gold Nanostructures with Second Harmonic Generation

in Acta Physica Polonica A (2009), 116(4), 498-500

We present the second harmonic generation - circular dichroism response from C-shaped gold nanostructures for P-polarized incoming light. By eliminating the contributions of anisotropy to the signal, an ... [more ▼]

We present the second harmonic generation - circular dichroism response from C-shaped gold nanostructures for P-polarized incoming light. By eliminating the contributions of anisotropy to the signal, an absolute measure of the second harmonic generation - circular dichroism is presented. The system of planar nanostructures is an excellent model for studying the correlation between optical properties and chiral morphology at the nanoscale. Samples with both enantiomorphs of the chiral pattern were prepared and, demonstrating the unambiguous presence of chirality, the second harmonic generation - circular dichroism response from two such samples was fitted simultaneously with a single set of fitting parameters. [less ▲]

Magnetically controlled superconducting weak links

in Applied Physics Letters (2009), 95(3),

We have designed and characterized a magnetic template which can be switched between chains of parallel and antiparallel field distribution by applying an in-plane magnetic field. The parallel field ... [more ▼]

We have designed and characterized a magnetic template which can be switched between chains of parallel and antiparallel field distribution by applying an in-plane magnetic field. The parallel field profile creates highly mobile vortex channels in a superconducting film deposited on top, reproducing the behavior of a weak link as evidenced by the presence of Shapiro steps in the current-voltage characteristics under rf excitation. The Josephson coupling can be fully suppressed by changing the field distribution to the antiparallel state. As a result, a reversible ON/OFF switch for magnetically induced weak links has been demonstrated. [less ▲]

Nucleation of superconductivity and vortex matter in superconductor-ferromagnet hybrids

in Superconductor Science and Technology (2009), 22(5),

The theoretical and experimental results concerning the thermodynamical and low-frequency transport properties of hybrid structures, consisting of spatially separated conventional low-temperature ... [more ▼]

The theoretical and experimental results concerning the thermodynamical and low-frequency transport properties of hybrid structures, consisting of spatially separated conventional low-temperature superconductors (S) and ferromagnets (F), are reviewed. Since the superconducting and ferromagnetic parts are assumed to be electrically insulated, no proximity effect is present and thus the interaction between both subsystems is through their respective magnetic stray fields. Depending on the temperature range and the value of the external field H(ext), different behavior of such S/F hybrids is anticipated. Rather close to the superconducting phase transition line, when the superconducting state is only weakly developed, the magnetization of the ferromagnet is solely determined by the magnetic history of the system and it is not influenced by the field generated by the supercurrents. In contrast to that, the nonuniform magnetic field pattern, induced by the ferromagnet, strongly affects the nucleation of superconductivity, leading to an exotic dependence of the critical temperature T(c) on H(ext). Deeper in the superconducting state the effect of the screening currents cannot be neglected anymore. In this region of the phase diagram T-H(ext) various aspects of the interaction between vortices and magnetic inhomogeneities are discussed. In the last section we briefly summarize the physics of S/F hybrids when the magnetization of the ferromagnet is no longer fixed but can change under the influence of the superconducting currents. As a consequence, the superconductor and ferromagnet become truly coupled and the equilibrium configuration of this 'soft' S/F hybrid requires rearrangements of both superconducting and ferromagnetic characteristics, as compared with 'hard' S/F structures. [less ▲]

Self-organized mode-locking effect in superconductor/ferromagnet hybrids

in Physical Review. B : Condensed Matter (2009), 79(5),

The vortex dynamics in a low-temperature superconductor deposited on top of a rectangular array of micrometer size permalloy triangles is investigated experimentally. The rectangular unit cell is such ... [more ▼]

The vortex dynamics in a low-temperature superconductor deposited on top of a rectangular array of micrometer size permalloy triangles is investigated experimentally. The rectangular unit cell is such that neighboring triangles physically touch each other along one direction. This design stabilizes remanent states which differ from the magnetic vortex state typical of individual noninteracting triangles. Magnetic force microscopy images have revealed that the magnetic landscape of the template can be switched to an ordered configuration after magnetizing the sample with an in-plane field. The ordered phase exhibits a broad flux-flow regime with relatively low critical current and a highly anisotropic response. This behavior is caused by the spontaneous formation of two separated rows of vortices and antivortices along each line of connected triangles. The existence of a clear flux-flow regime at zero external field supports this interpretation. The density of induced vortex-antivortex pairs is directly obtained using a high-frequency measurement technique which allows us to resolve the discrete motion of vortices. Strikingly, the presence of vortex-antivortex rows gives rise to a self-organized synchronized motion of vortices which manifests itself as field independent Shapiro steps in the current-voltage characteristics. [less ▲]