References of "Moshchalkov, V. V."
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See detailPinning effects on the vortex critical velocity in type-II superconducting thin films
Leo, A.; Grimaldi, G.; Nigro, A. et al

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 ▲]

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See detailLinearly polarized second harmonic generation microscopy reveals chirality
Valev, V. K.; Silhanek, Alejandro ULg; Smisdom, N. et al

in Optics Express (2010), 18(8), 8286-8293

In optics, chirality is typically associated with circularly polarized light. Here we present a novel way to detect the handedness of chiral materials with linearly polarized light. We performed Second ... [more ▼]

In optics, chirality is typically associated with circularly polarized light. Here we present a novel way to detect the handedness of chiral materials with linearly polarized light. We performed Second Harmonic Generation (SHG) microscopy on G-shaped planar chiral nanostructures made of gold. The SHG response originates in distinctive hotspots, whose arrangement is dependent of the handedness. These results uncover new directions for studying chirality in artificial materials. (C) 2010 Optical Society of America [less ▲]

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See detailGuided nucleation of superconductivity on a graded magnetic substrate
Milosevic, M. V.; Gillijns, W.; Silhanek, Alejandro ULg et al

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 ▲]

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See detailDirect visualization of the vortex distributions in a superconducting film with a Penrose array of magnetic pinning centers: Symmetry induced giant vortex state
Kramer, R. B. G.; Silhanek, Alejandro ULg; Van de Vondel, J. et al

in Physica C: Superconductivity (2010), 470(19), 758-761

Using scanning Hall probe microscopy a direct visualization of the flux distribution in a Pb film covering a fivefold Penrose array of Co dots is obtained. We demonstrate that stable vortex configurations ... [more ▼]

Using scanning Hall probe microscopy a direct visualization of the flux distribution in a Pb film covering a fivefold Penrose array of Co dots is obtained. 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 ring-like structures ("vortex corrals") which favor the formation of a giant vortex state at their center. (C) 2010 Published by Elsevier B.V. [less ▲]

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See detailDirect visualization of magnetic vortex pinning in superconductors
Menghini, Mariela; Kramer, R. B. G.; Silhanek, Alejandro ULg et al

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 ▲]

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See detailSymmetry-Induced Giant Vortex State in a Superconducting Pb Film with a Fivefold Penrose Array of Magnetic Pinning Centers
Kramer, R. B. G.; Silhanek, Alejandro ULg; Van de Vondel, J. et al

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 ▲]

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See detailCritical field enhancement in hybrid superconductor/ferromagnet mesoscopic disks
Schildermans, N.; Silhanek, Alejandro ULg; Sautner, J. et al

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 ▲]

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See detailTransition from turbulent to nearly laminar vortex flow in superconductors with periodic pinning
Gutierrez, J.; Silhanek, Alejandro ULg; Van de Vondel, J. et al

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 ▲]

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See detailCurrent-induced giant vortex and asymmetric vortex confinement in microstructured superconductors
Chao, X. H.; Zhu, B. Y.; Silhanek, Alejandro ULg et al

in Physical Review. B : Condensed Matter (2009), 80(5),

Based on the time-dependent Ginzburg-Landau equations, we study numerically current-driven vortices in a micrometer size square type-II superconductor. We demonstrate that the applied current ... [more ▼]

Based on the time-dependent Ginzburg-Landau equations, we study numerically current-driven vortices in a micrometer size square type-II superconductor. We demonstrate that the applied current significantly influences the dynamics of the vortices entering the sample. Strikingly, we find that a giant vortex can be created by the current-assisted collision of two singly quantized vortices. [less ▲]

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See detailA Study of Chirality in Gold Nanostructures with Second Harmonic Generation
Valev, V. K.; Silhanek, Alejandro ULg; Gillijns, W. et al

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 ▲]

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See detailPlasmonic Ratchet Wheels: Switching Circular Dichroism by Arranging Chiral Nanostructures
Valev, V. K.; Smisdom, N.; Silhanek, Alejandro ULg et al

in Nano Letters (2009), 9(11), 3945-3948

We demonstrate circular dichroism (CD) in the second harmonic generation (SHG) signal from chiral assemblies of G-shaped nanostructures made of gold. The arrangement of the G shapes is crucial since upon ... [more ▼]

We demonstrate circular dichroism (CD) in the second harmonic generation (SHG) signal from chiral assemblies of G-shaped nanostructures made of gold. The arrangement of the G shapes is crucial since upon reordering them the SHG-CD effect disappears. Microscopy reveals SHG "hotspots" assemblies, which originate in enantiomerically sensitive plasmon modes, having the novel property of exhibiting a chiral geometry themselves in relation with the handedness of the material. These results open new frontiers in studying chirality. [less ▲]

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See detailMagnetically controlled superconducting weak links
Van de Vondel, J.; Silhanek, Alejandro ULg; Raes, B. et al

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 ▲]

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See detailSwitchable magnetic dipole induced guided vortex motion (vol 93, 022507, 2008)
Verellen, N.; Silhanek, Alejandro ULg; Gillijns, W. et al

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

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

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See detailNucleation of superconductivity and vortex matter in superconductor-ferromagnet hybrids
Aladyshkin, A Yu; Silhanek, Alejandro ULg; Gillijns, W. et al

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 ▲]

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See detailPinning centers produced by magnetic microstructures
Silhanek, Alejandro ULg; Van de Vondel, J.; Leo, A. et al

in Superconductor Science and Technology (2009), 22(3),

We investigate the flux pinning and dynamic properties of superconducting vortices in an Al film with an array of magnetic bars deposited on top. The dimensions of each bar are chosen in such a way that ... [more ▼]

We investigate the flux pinning and dynamic properties of superconducting vortices in an Al film with an array of magnetic bars deposited on top. The dimensions of each bar are chosen in such a way that they host a single magnetic domain. These micromagnets are distributed periodically in a rectangular array with 0.5 mu m separation parallel to the longest side of the bars and displaced laterally by a distance w. We show that, for w > Lambda, where Lambda is the effective field penetration depth, the pinning strength is almost independent of w whereas the critical temperature at zero field, T(c)(0), decreases as similar to w(-1). For w < Lambda the opposite behavior is observed, i.e. T(c)(0) seems to saturate to a nearly w-independent value and the transition from large to small w is accompanied by a large suppression of the critical current j(c) together with a clear change in the shape of the current-voltage characteristics. In particular, the substantial weakening of the pinning potential for w < Lambda gives rise to an unexpected flux flow response. The field evolution of this regime allows us to determine whether the magnetic bars induce vortex-antivortex pairs in the system. The present findings suggest that practical application of magnetic pinning centers are restricted to low field values. [less ▲]

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See detailSelf-organized mode-locking effect in superconductor/ferromagnet hybrids
Van de Vondel, J.; Silhanek, Alejandro ULg; Metlushko, V. et al

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 ▲]

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See detailDifferent regimes of nucleation of superconductivity in mesoscopic superconductor/ferromagnet hybrids
Schildermans, N.; Aladyshkin, A Yu; Silhanek, Alejandro ULg et al

in Physical Review. B : Condensed Matter (2008), 77(21),

The competition between two regimes of the nucleation of superconductivity is investigated experimentally and theoretically in a mesoscopic disk-shaped superconductor/ferromagnet hybrid. By changing the ... [more ▼]

The competition between two regimes of the nucleation of superconductivity is investigated experimentally and theoretically in a mesoscopic disk-shaped superconductor/ferromagnet hybrid. By changing the magnetic state of a multilayered Co/Pt disk one can reversibly affect the magnetic-field dependence of the critical temperature T(c)(H) of an Al layer. We demonstrate that an enhancement of the magnetic field near the edge of the out-of-plane magnetized disk either stimulates the nucleation of superconductivity at the disk perimeter due to the field compensation effect or prevents it due to edge magnetic barrier (for relatively low parallel to H parallel to values). As a consequence, the presence of such magnetic-field pattern makes it possible to eliminate boundary effects for mesoscopic superconducting samples. Switching from one nucleation regime to another while sweeping H leads to an abrupt change of the slope of the T(c)(H) envelope. [less ▲]

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See detailMagnetic tunable confinement of the superconducting condensate in superconductor/ferromagnet hybrids
Aladyshkin, A Yu; Gillijns, W.; Silhanek, Alejandro ULg et al

in Physica C: Superconductivity (2008), 468(7-10), 737-740

The effect of a nonuniform magnetic field induced by a ferromagnet on the magnetoresistance of thin-film superconductor/ferromagnet hybrid structures was investigated experimentally. Two different ... [more ▼]

The effect of a nonuniform magnetic field induced by a ferromagnet on the magnetoresistance of thin-film superconductor/ferromagnet hybrid structures was investigated experimentally. Two different magnetic textures with out-of-plane magnetization were considered: a plain ferromagnetic film with bubble domains and a regular array of ferromagnetic dots. The stray fields of the structures are able to affect the spatial profile of the superconducting condensate, leading to a modification of the dependence of the critical temperature T-c on an external magnetic field H. We showed how the standard linear T-c(H) dependence with a single maximum at H = 0 can be continuously transformed into so-called reentrant phase boundary with two T-c peaks. We demonstrated that both domain-wall superconductivity and field-induced superconductivity are different manifestations of the magnetic confinement effect in various magnetic patterns. (C) 2008 Elsevier B.V. All rights reserved. [less ▲]

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See detailPhase slips in mesoscopic superconducting triangles
Schildermans, N.; Salenbien, R.; Silhanek, Alejandro ULg et al

in Physica C: Superconductivity (2008), 468(7-10), 757-760

Voltage-current (V(I)) characteristics are investigated in the superconducting state of an equilateral triangle of 2.2 mu m(2) area. Several steps in the V(I) curves indicate the presence of phase slip ... [more ▼]

Voltage-current (V(I)) characteristics are investigated in the superconducting state of an equilateral triangle of 2.2 mu m(2) area. Several steps in the V(I) curves indicate the presence of phase slip centers in the triangle. The field dependence of these steps follows the Little-Parks oscillations of the phase boundary, associated with the change of vorticity. The influence of the contacts manifests itself as an excess voltage and a negative differential resistance. In addition we show that the position of the current contacts modify the superconducting phase boundary measurements. (C) 2008 Elsevier B.V. All rights reserved. [less ▲]

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See detailSwitchable magnetic dipole induced guided vortex motion
Verellen, N.; Silhanek, Alejandro ULg; Gillijns, W. et al

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 ▲]

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