I. F. Lyuksyutov

Ferromagnet–superconductor hybrids

The new class of phenomena described in this review is based on the interaction between spatially separated, but closely located ferromagnets and superconductors, the so-called ferromagnet–superconductor hybrids (FSH). Typical FSH are: coupled uniform and textured ferromagnetic and superconducting films, magnetic dots over a superconducting film, magnetic nanowires in a superconducting matrix, etc. The interaction is provided by the magnetic field generated by magnetic textures and supercurrents. The magnetic flux from magnetic structures or topological defects can pin vortices or create them, changing the transport properties and transition temperature of the superconductor. On the other hand, the magnetic field from supercurrents (vortices) strongly interacts with the magnetic subsystem, leading to formation of coupled magnetic–superconducting topological defects.  Each time the Nambu and spin matrices are written together we mean the direct product. The proximity of ferromagnetic layer dramatically changes the properties of the superconducting film. The exchange field in ferromagnets not only suppresses the Cooper-pair wavefunction, but also leads to its oscillations, which in turn leads to oscillations of observable values: the transition temperature and Josephson current. In particular, in the ground state of the Josephson junction the relative phase of two superconductors separated by a layer of ferromagnetic metal is equal to π instead of the usual zero (the so-called π-junction). Such a junction carries a spontaneous supercurrent and possesses other unusual properties. Theory predicts that rotation of magnetization transforms s-pairing into p-pairing. The latter is not suppressed by the exchange field and serves as a carrier of long-range interaction between superconductors.

On-chip manipulation of levitated femtodroplets

We report diamagnetic levitation of droplets and∕or particles of pico–femtoliter volume and demonstrate their on-chip storage and high precision manipulation (translation, merging, assembling and rotation). We also demonstrate a levitation based microfluidic processor to process droplets∕particles with up to a billion times smaller volume than in typical microfluidic devices. The levitated particles can be positioned with up to 300nm accuracy and precisely rotated and assembled, providing a different physical approach for micro-electro-mechanical systems. Force can be applied to the droplets∕particles via magnetic, electric, and gravitational fields with up to femto-Newton accuracy, and potential energy can be controlled with up to 0.2zeptoJ(0.05kBT) precision, thus providing experimental tools for fundamental studies.

FROZEN FLUX SUPERCONDUCTORS

We propose a new class of composite superconductors with embedded magnetic nanoparticles/nanorods. These nanomagnets can create a thermodynamically stable, frozen, dense flux line network. We discuss the methods of fabrication of these novel materials. We show that due to the frozen flux created by magnetic nanorods, the critical current in this composite superconductor can be higher than in the superconductor with a comparable density of columnar defects.

MAGNETIZATION CONTROLLED SUPERCONDUCTIVITY IN A FILM WITH MAGNETIC DOTS

We consider a superconducting film with a magnetic dots array (MDA) placed upon it. Magnetic moments of the dots are {ital normal} to the film and strong enough to create vortices in the superconducting film. Magnetic interaction between dots is negligible. Zero-field cooling leads to random magnetization of the MDA well above the superconducting temperature. With this cooling, the film is in a resistive state below the (expected) superconducting transition. Paradoxically, when field cooled, the film with MDA can be superconducting. {copyright} {ital 1998} {ital The American Physical Society}

Interaction of Mesoscopic Magnetic Textures with Superconductors.

The fabrication and experimental study [1] of mesoscopic heterogeneous magnetic/superconducting systems together with recent theoretical predictions [2–4] open a new class of physical effects. Earlier two of us (I.F.L. and V.L.P.) proposed to separate superconductivity and magnetism in space employing the modern technique of nanofabrication [2]. The proximity effect which suppresses both order parameters can be easily avoided by growing insulator oxide layers between ferromagnetic (FM) and superconducting (SC) components. Inhomogeneous magnetization of the magnetic texture generates magnetic field penetrating into the superconductor. The magnetic field from the SC currents interacts with magnetic subsystem [3,4]. We have proposed different realizations of mesoscopic magneto-superconducting systems: arrays of magnetic dots on the top of a superconducting film [2], magnetic/superconducting bi-layers [3], magnetic nanorods embedded into a superconductor [4]. In the majority of proposed systems a magnetic texture interacts with the superconducting current. An inhomogeneous magnetization generates magnetic field outside the magnet. The magnetic field generates screening currents in superconductors which, in turn, change the magnetic field. The problem must be solved self-consistently . In this article we develop a general formalism for interacting inhomogeneous magnetization (texture) and superconductors in the London’s approximation. Employ

Theory of the hysteresis loop in ferromagnets

We consider three mechanisms of hysteresis phenomena in alternating magnetic field: the domain-wall motion in a random medium, the nucleation, and the retardation of magnetization due to slow (critical) fluctuations. We construct a quantitative theory for all these processes. The hysteresis is characterized by two dynamic threshold fields, a coercive field and the so-called reversal field. Their ratios to the static threshold field is shown to be a function of two dimensionless variables constituted from the frequency and amplitude of the ac field as well as from some characteristics of the magnet. The area and the shape of the hysteresis loop are found. We consider different limiting cases in which power dependencies are valid. Numerical simulations show the domain-wall formation and propagation and confirm the main theoretical predictions. Theory is compared with available experimental data. {copyright} {ital 1999} {ital The American Physical Society}

Thickness dependence of the microstructures and magnetic properties of electroplated Co nanowires.

The correlation between the crystal structure and the magnetic properties of Co nanowires of diameter 65 and 200 nm fabricated by electroplating Co into the pores of anodic aluminum oxide membranes has been investigated. Strikingly different microstructures have been observed in these Co nanowires by means of x-ray diffraction and selected area electron diffraction measurements. The 65 nm thick Co nanowires are composed of long and ordered hexagonal close-packed Co grains (>5 microm), while the 200 nm thick Co nanowires are composed of submicron-long hexagonal close-packed and face-centered cubic Co grains. Correspondingly, different magnetic properties have been observed for these Co nanowires. Magnetization measurements have found that the 65 nm thick Co nanowires have a magnetic hysteresis that is significantly larger than that of the 200 nm thick Co nanowires. Spontaneous magnetic moments of the nanowires are parallel to the nanowires in the 65 nm thick Co nanowires, but they are transverse to the nanowires in the 200 nm thick Co nanowires, as observed by the magnetic force microscopy. The correlation between their different magnetic properties and microstructures is discussed.

Magnetic Nanorods/Superconductor Hybrids

We discuss properties of hybrid systems which consist of an array of magnetic nanorods (nanowires) embedded in/or covered by a superconductor film. Such hybrid nanostructures are expected to exhibit fascinating characteristics, which are important from both fundamental and applications point of view.

Merged-beams for slow molecular collision experiments

Molecular collisions can be studied at very low relative kinetic energies, in the milliKelvin range, by merging codirectional beams with much higher translational energies, extending even to the kiloKelvin range, provided that the beam speeds can be closely matched. This technique provides far more intensity and wider chemical scope than methods that require slowing both collision partners. Previously, at far higher energies, merged beams have been widely used with ions and/or neutrals formed by charge transfer. Here, we assess for neutral, thermal molecular beams the range and resolution of collision energy that now appears attainable, determined chiefly by velocity spreads within the merged beams. Our treatment deals both with velocity distributions familiar for molecular beams formed by effusion or supersonic expansion, and an unorthodox variant produced by a rotating supersonic source capable of scanning the lab beam velocity over a wide range.

Flux pinning with a magnetic nanorod array

We report measurements of the phase diagram and critical current of a PbBi superconducting film with an embedded square array of Ni nanorods. Strong hysteresis of the superconducting properties and an increase in the critical current which varies from several times to several orders of magnitude depending on the magnetic field and temperature is found.