Vitaly V. Metlushko

Noninvasive magnetic imaging and magnetization measurement of isolated mesoscopic Co rings

A high-resolution scanning Hall probe microscope was used as a noninvasive technique to visualize the magnetization reversal in an array of micron-size Co rings. Two stable “onion” states at remanence and “vortex” states at switching fields were found. To rule out a possible influence of dipole–dipole interaction between ring elements on remagnetization processes, an isolated Co ring was deposited on top of a Hall magnetometer and extremely sharp transitions from onion to vortex and from vortex to onion state of opposite polarity were resolved. Our results were supported by MOKE magnetization measurements and micromagnetic simulations.

Flux phases and quantized pinning force in superconductor with a periodic lattice of pinning centres.

Flux phases have been studied in superconducting Pb/Ge multilayers with a square lattice of submicrometre holes. Sharp peaks in the magnetic-field dependence of the critical current have been observed at integer Hm and rational Hk/l matching fields. At integer fields an artificial flux line crystal is formed, consisting of a stable square lattice of single or multiple flux quanta trapped by the holes. The existence of such a crystal leads to the quantization of the pinning force. New rational flux lines patterns at H = Hk/l, qualitatively different from those previously seen in superconducting networks, have been identified.

Matching effects in Pb/Ge multilayers with the lattice of submicron holes

Abstract The matching effects and the enhancement of the critical current density j c have been studied in superconducting Pb/Ge multilayers with a square lattice of submicrometer holes. The sharp peaks in the j c vs. field curve at the integer and rational matching fields have been observed. The existence of multi-quanta vortices confined by the holes has been confirmed.

Identification of Different Pinning Regimes in Bi2Sr2CaCu2Ox Single Crystals

By analysing the flux line dynamics and the critical current density jc as a function of temperature T and magnetic field H in as-grown Bi2Sr2CaCu2Ox single crystals, we have proposed the identification of different pinning regimes predicted by the collective-pinning theory. The crossover critical current corresponding to the onset of the large-flux-bundle regime corresponds to a sharp kink on the jc(T) curve. A quantitative fit of the jc(T, H) dependences has been achieved for a broad range of temperatures and magnetic fields.

Quantum interference and confinement phenomena in mesoscopic superconducting systems

The superconducting field (H)-temperature (T) phase boundary has been measured in mesoscopic AI samples of different topology: lines, open and filled squares, which were made under the same conditions from the same material. These samples clearly show different superconducting H-T phase boundaries which are nicely reproducing the predictions of the theoretical calculations made for their particular confinement geometries. The confinement of the flux lines by the lattice of the submicrometer holes has been studied in the Pb/Ge multilayers. A substantial enhancement of the critical current j, has been achieved. Sharp integer and rational matching peaks in the j,(H) curve are observed. The possibility of the “quantum design” of the superconducting critical parameters (H,(T) and jc(T, H)) of the mesoscopic and nanostructured superconductors by optimizing the confinement geometry for the superconducting condensate and for the flux lines has been demonstrated.

Enhanced pinning and matching effects in superconducting films with a lattice of submicrometer holes

Abstract Magnetization studies have been carried out on single Pb films with a square lattice of submicrometer holes. These submicrometer hole patterns create the possibility to trap an integer number, as well as a certain rational number of flux lines per hole corresponding to a regular organization of flux lines inside the structure. The stable flux line configurations lead to the appearance of sharp peaks in the magnetization M ( H ⊥ ).

Relation between structural and physical properties in magnetic and superconducting superlattices

The physical properties of superlattices are strongly affected by the chemical and physical properties of the individual layers and by the superlattice structure. In this paper the relationship between structure and properties will be illustrated by the giant magnetoresistance (GMR) effect, as well as the dimensional transitions and pinning mechanisms in respectively magnetic and superconducting superlattices. In the first example we will analyse the effect of the sample structure and interface quality on the GMR of polycrystalline and epitaxial Fe/Cr superlattices grown by molecular beam epitaxy. Secondly, the effect of thermal annealing and ion irradiation on the electrical and magnetic properties of Ag/Fe multilayers is discussed. In a third example, the influence of a very thin Fe interlayer on the coupling phenomena in Fe/Nb multilayers is analysed. Finally, it will be shown that an additional lateral modulation (lattice of submicron holes) in Pb/Ge multilayers substantially changes the critical superconducting parameters.

Novel flux phases in superconductors with a periodic pinning array

Abstract Flux-line confinement by triangular and square regular arrays of microholes (“antidots”) has been studied in superconducting films (Pb, WGe) and multilayers (Pb/Ge). For relatively large antidots sharp cusp-like magnetization anomalies appear at the matching fields Hm. These anomalies are cuased by the formation of the multi-quanta vortex lattices at each subsequent Hm. The multi-quanta vortex lattices make possible a peacefull coexistence of the flux penetration at the antidots and the presence of a substantial superfluid density in the space between them. This leads to a very strong enhancement of the critical current density in films with an antidot lattice. For smaller antidots the vortices are forced to occupy the interstitial positions after the saturation of the pinning sites at antidots. This leads to the formation of the novel composite flux-line lattices consisting from the interpenetrating sublattices of weakly pinned interstitial single-quantum vortices and multi-quanta vortices strongly pinned by the antidots. When the interstitial flux-line lattice melts, it forms the interstitial flux liquid coexisting with the flux solid at antidots.

Quantization and Confinement Effects in Superconducting Films with an Antidot Lattice.

We have studied flux line confinement by regular arrays of submicron holes (triangular and square antidot lattices) in superconducting films. Critical current density j c and pinning force f P are very strongly enhanced due to pinning of single or multi-quanta vortices by the antidots. Direct measurements of j c and f p for several well-defined itntidot diameters D, proves that pinning centers with size considerably larger than the temperature-dependent coherence length ξ(T) are much more efficient than those with D ≃ ξ(T).

Anomalous field-dependence of the critical-current and flux-creep rate in Pb/Ge multilayers

Abstract Critical current densities J c and normalized flux creep rate S (≡ dln M /dln t ) have been measured as a function of the perpendicular field H ⊥ in low- T c Pb/Ge multilayers, which can be used to model the superconducting properties of the layered high- T c cuprates. By taking a proper Ge separator thickness a field-induced crossover between two different pinning regimes can be observed. This transition leads to an anomalous behavior of J c ( H ⊥ ) and S ( H ⊥ ) in Pb/Ge multilayers, while a sing le Pb film demonstrates a monotonic suppression of J c by H ⊥ . These data have been analyzed in the framework of the collective pinning (CP) theory of magnetically coupled superconducting planes: the J c ( H ⊥ ) anomaly is interpreted as a crossover from three-dimensional (3D) flux lines in low fields to two-dimensional (2D) pancake vortices in higher fields. This interpretation has also been confirmed by the suppression of the J c ( H ⊥ ) anomaly in the Pb/Ge multilayers with a square lattice of holes, forming additional defects for the confinement of the flux lines and for the expansion of the 3D regime.