Revealing Josephson vortex dynamics in proximity junctions below critical current

Made of a thin non-superconducting metal (N) sandwiched by two superconductors (S), SNS Josephson junctions enable novel quantum functionalities by mixing up the intrinsic electronic properties of N with the superconducting correlations induced from S by proximity. Electronic properties of these devices are governed by Andreev quasiparticles [1] which are absent in conventional SIS junctions whose insulating barrier (I) between the two S electrodes owns no electronic states. Here we focus on the Josephson vortex (JV) motion inside Nb-Cu-Nb proximity junctions subject to electric currents and magnetic fields. The results of local (Magnetic Force Microscopy) and global (transport) experiments provided simultaneously are compared with our numerical model, revealing the existence of several distinct dynamic regimes of the JV motion. One of them, identified as a fast hysteretic entry/escape below the critical value of Josephson current, is analyzed and suggested for low-dissipative logic and memory elements.Comment: 11 pages, 3 figures, 1 table, 43 reference

Ultrastrong photon-to-magnon coupling in multilayered heterostructures involving superconducting coherence via ferromagnetic layers

The critical step for future quantum industry demands realization of efficient information exchange between different-platform hybrid systems that can harvest advantages of distinct platforms. The major restraining factor for the progress in certain hybrids is weak coupling strength between the elemental particles. In particular, this restriction impedes a promising field of hybrid magnonics. In this work, we propose an approach for realization of on-chip hybrid magnonic systems with unprecedentedly strong coupling parameters. The approach is based on multilayered microstructures containing superconducting, insulating, and ferromagnetic layers with modified photon phase velocities and magnon eigenfrequencies. The enhanced coupling strength is provided by the radically reduced photon mode volume. Study of the microscopic mechanism of the photon-to-magnon coupling evidences formation of the long-range superconducting coherence via thick strong ferromagnetic layers in superconductor/ferromagnet/superconductor trilayer in the presence of magnetization precession. This discovery offers new opportunities in microwave superconducting spintronics for quantum technologies

Thin Films: Growth Mechanisms, Superconducting and Functional properties, Modelling of Vortex Behaviour

A hundred years passed since the discovery of superconductivity, but it is only during last decades superconductors transformed from exotic object for physical experiments to practically promising materials. During this time new theoretical concepts, measurement techniques and fabrication methods were invented in order to study these materials. New types of superconductors were discovered. It is hard to overestimate current impact of superconductivity in modern life: the most sensitive devices (SQUIDs) based on Josephson effect, loss-free cables for electric networks in Tokyo and New York, high-speed levitating trains in China and Japan, high-field magnets for magneto-resonance imaging and power generators, and other applications have become available. One of the most popular and well studied superconducting material is a YBa2Cu3O7 (YBCO) high-temperature superconductor (HTSC). High working temperatures, high transport currents and technological effectiveness enable a potential of YBCO application in all possible areas from industrial power applications to microelectronics. Hence, a technological control or enhancement of YBCO performance is of current importance. Moreover, well studied YBCO is a good model material to study some physical aspects of HTSC behaviour which are not answered or not quantified. Enhancement of superconducting properties or even novel physical effects can be obtained combining superconducting and ferromagnetic materials (so-called hybrids). FePt ferromagnetic material is of a particular interest for application in highfield hybrids because of high coercive force and magnetic anisotropy. In addition FePt is promising for “traditional” applications of ferromagnets, such as magnetic recording media, micro-electro mechanical systems or high energy product magnets. However, high degree of technological control is required in order to reach necessary performance of FePt. Functional materials, such as YBCO or FePt, demonstrate their best performance or reveal certain physical effects in form of thin films. In this work large degree of control on structure of quasi-single crystal superconducting YBCO thin films and hard ferromagnetic L10-FePt thin films is demonstrated simply by varying laser frequency of pulsed laser deposition. Significant structural modifications have been obtained for the films of constant thicknesses. These modifications are shown to dramatically affect corresponding physical properties of thin films. In particular, critical temperature of YBCO thin films is shown to decrease with laser frequency. Moreover, a strong dependence on the laser frequency is discovered for the critical current density behaviour as a function of the applied magnetic field (Jc(Ba)) of YBCO films with the unexpected reversal of Jc(Ba) curves with temperature. The mechanisms of structure modifications of YBCO films and corresponding properties variations are discussed. In case of FePt thin films the experimental dependence of structure on laser frequency demonstrates the opposite growth development to the trend reported for the modulated flux in the literature. A theoretical model based on the mean field approach is developed, which quantitatively describes the structural changes obtained experimentally. Structure modulation of FePt thin films obtained by varying laser frequency result in modification of demagnetization mechanism and subsequent 20-fold increase of coercive field. A sensitivity of physical properties to structure variation provides an instrumental ability for tuning the practical characteristics of these films by changing the laser frequency of their deposition. One of the most practically important characteristics of any superconductor is a critical current dependence on applied magnetic field (Jc(Ba)). Relatively high working temperature of YBCO modifies the concepts of critical current and vortex pinning, making description of Jc(Ba) more sophisticated. A new model of critical current density for high-quality YBCO thin films has been proposed, combining thermally activated flux creep with a vortex pinning on columnar defects. The pinning for thermally activated vortices has been described as a strong pinning on chains of individual edge dislocations that form low-angle domain boundaries in pulsed laser deposited high-quality YBCO thin films. An electric field criteria (Ecr) introduced into the model allows to apply it for description of critical current obtained by different measurement techniques. In addition, the dependence of the irreversibility field on the Ecr criterion can be obtained within the model. The dependences (Jc(Ba) for high-quality YBCO thin films over the entire applied magnetic field range were obtained using MPMS, VSM PPMS and four-probe method. Thus, the measurements of Jc(Ba) were done employing Ecr varied in 9 order of magnitude range. Transport current and quasi-equilibrium magnetization measurement data were successfully fitted by the developed model with appropriate electric field criteria. Hence, for the first time the model consolidates substantially different Jc(Ba) dependences of YBCO thin films obtained by different measurement techniques. At the same time the dynamic magnetization measurements of the Jc(Ba) obtained using VSM strongly depend on instrumentally defined parameters, introducing inconsistencies in the experimental results. The model calculations are able to explain the Jc(Ba) curves only if the instrumental vibrations affecting vortex behaviour are minimised. Further, a systematic study on the behaviour of superconductors with varied VSM settings (frequency f and amplitude A of vibration) is presented. It was shown that the vibration affects critical currents of all possible types of superconductors during their measurement employing VSM magnetometer regardless geometry of the sample, vortex pinning properties or measurement temperature. Enhancement of vibration frequency or amplitude leads to a progressive reduction of critical current density with applied magnetic field. The vibration effect is more intense for thin films, it is responsible for development of unexpected kinks on Jc(Ba) curves in thin films. The study of magnetic moment dependence of YBCO film on applied magnetic field measured at different VSM settings revealed the asymmetry of the vibration effect on ascending and descending magnetic field branches of magnetization, indicating uneven states of vortex lattice in these branches. In addition, certain thermal response of magnetization to vibration with high f and A was detected. Investigation of magnetization relaxation process of YBCO film showed that the vibration effect in general can be interpreted as a suppressed pinning potential where the thermal response is one of the sources for pinning reduction. Critical current measurements in YBCO film using VSM with magnetic field sweep rate scaled with vibration frequency (f/(dBa/dt)) revealed a strong nonthermal effect of vibration on dynamics of vortices. The vibration effect, reducing the Jc(Ba), decreases irreversibility field of YBCO thin films. It was demonstrated how a technically defined irreversibility field B*irr can be a useful characteristic that reflects the vibration effect on entire Jc(Ba) dependence. It was discussed that the effect of vibration on magnetization, relaxation of magnetization, critical current and irreversibility field is governed by thermal and magnetic fluctuations of vortex structure in oscillating superconducting sample. The role of thermal fluctuations is relatively small: estimated enhancement of temperature induced by vibration does not exceed 1 K. The major impact is expected from magnetic fluctuations, which promote relaxation of vortex lattice in sweeping magnetic field, reduce effectively the pinning potential and degrade the critical current. The mechanism of forced relaxation is expected to be similar to “vortex shaking” mechanism where magnetic periodic perturbations create local ac currents and “shake” vortices out of pinning centres. In order to understand a microscopic nature of vibration effect on YBCO film, a behaviour of vortices in 300 nm YBCO film in 1 T and 2 T of applied magnetic field and 77 K were simulated employing Langeving molecular dynamic simulation. In general, results of simulations confirmed a dominating role of out-of-plane dislocations of relatively low density in establishing the critical current of YBCO films; weak pinning centres of much higher density have only marginal effect. Depinning currents obtained in simulated magnetic fields without vibration are in a good agreement with critical currents measured by VSM. Vibration at 1 T makes interaction of vortices with weaker pinning centres completely negligible, while pinning on out-of-plane dislocations still is effective. Shaking of vortices in 2 T decreases depinning current more substantially then at 1 T, assisting to depinning from all kinds of pinning centres including dislocations. In addition, shaking induces a peak effect behaviour of I-V characteristic. The origin of peak effect (vortex lattice orderdisorder transition vs. matching effect) is determined by applied magnetic field. This difference in vortex behaviour is responsible for switching theJc(Ba) trend at magnetic field of the kink developed by vibration of YBCO film. Finally it was noticed, that vibration of Nb film subjected to flux jumps reduce magnetization (Mfj) and enhance the field second magnetization peak (BSMP ). The increase of BSMP implies that vibration induces transition from critical state to undercritical flux jump mediated state. Behaviour of Mfj and BSMP together with other evidences presented justify the self-organised criticality nature of flux jumps in Nb films during magnetometery (represented by a sandpile) instead of commonly assumed thermo-magnetic instability origin. A simple expression for fitting Mfj(Ba) affected by vibration was designed considering avalanche process in a flow-like regime. Fitting of Mfj(Ba) allowed to estimate a nonuniformity of magnetic field in a sample space (approx. 10−5)

Quantitative model for tunable microstructure in magnetic FePt thin films by pulsed laser deposition

Pulsed laser deposition (PLD) is employed to fabricate FePt L10 thin films from elemental targets. Dramatic structure variations are obtained by varying the laser frequency while keeping the thickness of the films constant. A new theoretical model based on the mean field approach is proposed, which quantitatively describes the structural changes obtained experimentally. The experiment and the model exhibit the opposite growth development to the trend reported for the modulated flux in the literature. The new model considers different growth rates in the lateral and transverse directions due to different responses of the normal and tangential film surfaces being deposited to the incident flux and the migration kinetics of adatoms and clusters. The quantitative results obtained confirm that the migration kinetics and self-assembly can easily be controlled by the PLD frequency which is consistent with the experiments. Magnetic properties of the films are shown to be extremely sensitive to the structure variations allowing practical tunability. © 2013 IOP Publishing Ltd

Significant tunability of thin film functionalities enabled by manipulating magnetic and structural nano-domains

The influence of laser frequency on the structure and physical properties of thin films grown by pulsed laser deposition has been studied. Different types of thin films, hard ferromagnetic FePt L10 and quasi-single crystal superconducting YBa2Cu3O7 (YBCO), have been used for demonstration of the effect. Significant structural modifications have been obtained for the films with similar thicknesses. These modifications are shown to dramatically control their corresponding properties, providing an instrumental ability for tuning the practical characteristics of the films by changing the laser frequency of their deposition. In particular, 20-fold increase of coercive field and modification of demagnetization mechanism are obtained for FePt films by varying the frequency from 1 Hz to 6 Hz. Over a similar frequency range, a strong dependence on the laser frequency is discovered for the YBCO films for the critical current density behavior as a function of the applied magnetic field [Jc(Ba)] with the unexpected reversal of Jc(Ba) curves with temperature. The mechanisms of structure modifications and corresponding property variations are proposed

Observation of transient overcritical currents in YBCO thin films using high-speed magneto-optical imaging and dynamic current mapping

The dynamics of transient current distributions in superconducting YBa2Cu3O7−δ thin films were investigated during and immediately following an external field ramp, using high-speed (real-time) Magneto-Optical Imaging and calculation of dynamic current profiles. A number of qualitatively unique and previously unobserved features are seen in this novel analysis of the evolution of supercurrent during penetration. As magnetic field ramps up from zero, the dynamic current profile is characterized by strong peaks, the magnitude of which exceed the conventional critical current density (as determined from static current profiles). These peaks develop close to the sample edges, initially resembling screening currents but quickly growing in intensity as the external field increases. A discontinuity in field and current behaviour is newly observed, indicating a novel transition from increasing peak current toward relaxation behaviour. After this transition, the current peaks move toward the centre of the sample while reducing in intensity as magnetic vortices penetrate inward. This motion slows exponentially with time, with the current distribution in the long-time limit reducing to the expected Kim-model profile

Vibration effect on magnetization and critical current density of superconductors

In this work the effect of vibrations on critical current density ( Jc) of superconductors has been studied. The vibrations are shown to affect Jc of all types of superconductors during their measurements, employing a vibrating sample magnetometer (VSM). Increasing vibration frequency ( f ) and/or amplitude (A) leads to progressive reduction of Jc as a function of magnetic field (Ba). The effect of vibrations is substantially stronger in thin films. It leads to development of unexpected kinks on Jc (Ba) curves. Analysis of magnetization loops and relaxation of magnetization in YBCO films revealed that the vibration effect can be treated as the effective reduction of pinning potential. The asymmetry of the vibration effect in ascending and descending Ba is observed, indicating differences in free energy of the corresponding vortex structures. Thermal effects induced by vibrations with large f and A are shown to have rather insignificant influence, while the vibrational vortex dynamics exhibit a strong impact. The irreversibility field (Birr) is shown to be instrumentally defined, and its value depends on VSM settings. In addition, the practical importance of Birr for Jc modeling is demonstrated

Large, controllable spikes of magnetoresistance in La2/3Ca 1/3MnO3/SrTiO3 superlattices

We have investigated superlattices consisting of up to 30 epitaxial nanomultilayers (3-7 nm thick) of ferromagnetic La2/3Ca 1/3MnO3 (LCMO) and insulating SrTiO3 (STO) hybrids. The superlattices demonstrate dramatic shifts of Curie temperature, indicating the possibility of its tunability. The metal-insulator transition (MIT) has been observed around 140 K. Below the MIT temperature, the superlattices have shown sharp drops of resistivity, facilitating the largest and sharpest magnetoresistance peaks (\u3e2000%) ever observed in LCMO films and superlattices at low temperatures. The observed experimental results can be explained in the frame of the phase separation model in manganites with well-organized structures. The results of magnetic and transport measurements of such hybrid structures are discussed, indicating a magnetodielectric effect in STO interlayers. The magnetic and transport properties of the superlattices are shown to be technology-dependent, experiencing dimensional transitions, which enables the creation of structures with prescribed magnetoresistance characteristics for a broad range of applications

Origin of magnetic flux-jumps in Nb films subject to mechanical vibrations and corresponding magnetic perturbations

In this paper the origin of flux-jumps in Nb thin films is established during magnetization measurements using a vibrating sample magnetometer (VSM). Magnetization measurements of the flux avalanche activity show its strong dependence on frequency and amplitude of VSM vibration. In particular, under certain conditions the vibrations induce a transition from a stable superconducting critical state to an undercritical state, accompanied by the 20-fold drop in the magnetic moment. These features allow the elucidation of the origin of the flux-jumps. In contrast to the commonly assumed thermomagnetic instabilities to be responsible for the flux-jumps in Nb films, our results provide solid support for an alternative explanation being due to criticality-built instability well represented by a sandpile. Considering properties of the flux-flow during a flux avalanche regime allows us to estimate nonuniformity of a magnetic field in a VSM sample space developed as a result of vibrations