Current-driven Vortex Research Articles

The dynamics of current-driven vortex in two-band superconductor with s+d wave pairing

Two-band mesoscopic superconductors are known to exhibit specific vortex states. In this paper, finite element method (FEM) is used to numerically solve the time-dependent Ginzburg-Landau (TDGL) equation with s+d wave coupling for a 3D mesoscopic superconducting strip. We discuss the effect of external magnetic field H and applied current I on s+d wave coupling at the temperatures Ts<T<Td, where Ts and Td are the critical temperatures of s- and d-bands, respectively. The obtained results show that this coupling is enhanced by the applied current and reflected by the vortex dynamics in two bands. A small amount of Cooper pairs in the s-band form a series of different vortex and antivortex states. Mutual attraction of vortex and antivortex induces simultaneous motion of s- and d-vortices at applied current I. This work provides theoretical background for the design of mesoscopic superconducting devices.

Velocity fluctuations of vortices in driven two-dimensional vortex matter

A molecular dynamic simulation is conducted for a dense two-dimensional vortex matter. At the critical moment when depinning events were concentrated and dynamic phase transition occurred, the simulation shows unexpectedly slow fluctuation in collective velocity of vortices along the direction of driving force (longitudinal direction). A detailed analysis exhibited heavy braiding of vortex flow-channels, and it was revealed that the velocity fluctuation is well synchronized with fluctuation of local density of flow-channels upon the depinning transition. In addition, vortex transport properties of a superconducting MgCNi3 single crystal seemed to show consistency between the experiment and the theory. At last, the density of flow-channels is proposed as a dynamic order parameter of current-driven vortex phase.

Characterization of the critical current and physical properties of superconducting epitaxial NbTiN sub-micron structures

Measurements of critical current in NbTiN as a function of applied magnetic field and temperature are reported for two samples: 700-nm-wide bridge and 100-nm-wide meander. In 700-nm-wide NbTiN bridge we pinpointed the limiting factors for the critical current density to be current-driven vortex de-pinning at low temperatures and thermally activated flux flow closer to the transition temperature. In 100-nm-wide NbTiN meander we found phase slips activation, accompanied by hotspots formation at all measured temperatures. These two types of structures demonstrate different dependence of the critical current on the applied magnetic field. Although our NbTiN meander structures has high de-pairing critical current densities ∼107A/cm2 at low temperatures, the real critical currents are smaller due to the presence of the local constrictions.

Nonstationary current-driven dynamics of vortex domain walls in films with in-plane anisotropy

Micromagnetic simulation of a current-driven vortex domain wall motion in a film with in-plane anisotropy was carried out. The current density values j>jc were considered corresponding to the nonstationary motion, with the domain wall structure dynamic transformation occurred. A nonlinear dependence of the jc value on the film thickness was obtained. The nonstationary motion regime existence restricted the possibility to increase the domain wall velocity by increasing j and decreasing the damping parameter.

Current-driven domain wall mobility in polycrystalline Permalloy nanowires: A numerical study

A complete understanding of domain wall motion in magnetic nanowires is required to enable future nanowire based spintronics devices to work reliably. The production process dictates that the samples are polycrystalline. In this contribution, we present a method to investigate the effects of material grains on domain wall motion using the GPU-based micromagnetic software package MuMax3. We use this method to study current-driven vortex domain wall motion in polycrystalline Permalloy nanowires and find that the influence of material grains is fourfold: an extrinsic pinning at low current densities, an increasing effective damping with disorder strength, shifts in the Walker breakdown current density, and the possibility of the vortex core to switch polarity at grain boundaries.

Influence of material defects on current-driven vortex domain wall mobility

Many future concepts for spintronic devices are based on the current-driven motion of magnetic domain walls through nanowires. Consequently a thorough understanding of the domain wall mobility is required. However, the magnitude of the nonadiabatic component of the spin-transfer torque driving the domain wall is still debated today as various experimental methods give rise to a large range of values for the degree of nonadiabaticity. Strikingly, experiments based on vortex domain wall motion in magnetic nanowires consistently result in lower values compared to other methods. Based on the micromagnetic simulations presented in this contribution we can attribute this discrepancy to the influence of distributed disorder which vastly affects the vortex domain wall mobility, but is most often not taken into account in the models adopted to extract the degree of nonadiabaticity.

The Quantuam Melting Transitions with the Magnetic Field in Weakly Disordered Josephson Junction Arrays

Using the resistively shunted junction model, we study the magnetic field induced dynamic melting transitions of a current-driven vortex system in two-dimensional weakly disordered Josephson junction arrays at zero temperature. From the unified model simulations, we find that the intrinsic quantum vortex liquid (QVL) phenomenon, which consistent with the recent experimental reports in disordered and superconducting MoGe films. The enhancement of critical current in the QVL phase arises from intrinsic quantum fluctuations in the moving vortex flow.

Current-Driven Vortex–Antivortex Pair Rotation in a Magnetic Thin Film with Multiple Contacts

We theoretically analyzed the current-driven vortex–antivortex pair rotation in a magnetic thin film where spatially localized spin-polarized current is injected through two nonmagnetic nanocontacts. We performed the micromagnetic simulations and found that the dynamics of magnetization strongly depends on the distance between the contacts. If the distance between the contacts is large enough to separate the trajectories of vortex–antivortex (VA) pair rotation, the two VA pairs rotate around each contact with an angular frequency identical to that of a single contact system. In contrast, if the distance between the contacts is so short that the trajectory of each VA pair rotation can overlap with each other, two VA pair rotations are merged into one VA pair rotation whose trajectory encloses the two contacts as if a large elliptical contact exists at their midpoint. We showed that the rotation frequency decreases with increasing length of the trajectory. The rotation frequency for the trajectory enclosing...

Experimental Study on Plasticity of Current-driven Vortex Lattices Interacting with Disorder in Bi2Sr2Ca1Cu2O8 Single Crystal

From the I-V characteristics for as-grown and irradiated Bi2Sr2CaCu2O8+δ single crystals at T=5K (i.e. T → 0) in a magnetic field applied parallel to c axis, we have seen two types of vortex dynamics above the depinning threshold, depending on the vortex-vortex interaction strength (λab/a0). For the as-grown sample, at a low field, the I-V curves show steps that clearly indicate a “fingerprint phenomenon” since they reflex the current dependence of the differential resistance rd(I). This can be ascribed to vortices flowing through uncorrelated channels for the highly defective lattice. As field sufficiently increase, these peaks merge to become broader ones indicating a crossover from filament type to braid river type. In contrast, in the irradiated sample, the pinning is found to be individual at low fields and collective when the vortex-vortex interactions are involved.

Magnetic interaction effect on the critical switching current in vortex arrays

In this paper, we investigate the influence of the dipolar magnetic interaction on the current-driven vortex dynamics in a two-dimensional array of nanodisks. A general formula is established for the prediction of the correlation between the restoring force of the target nanodisk and the polarities and circulations of the neighboring vortices. Large numbers of possible magnetization configuration combinations among the target and the surrounding neighbors are reduced into four groups under an extreme condition. The maximum and minimum restoring forces are analytically derived, which can be used to predict the critical switching current dependence on the magnetic configuration combinations of vortices.

Influence of geometry on current-driven vortex oscillations in nanocontact devices

We present a computational study of current-driven vortex dynamics in a particular geometry, a hybrid Co/Au/Py nanocontact, in which the Co layer is not flat. The experimental measurements validate the numerical results. We identify the Py layer as dynamically active. The nonuniform magnetization configuration in the Co layer, which acts as spin polarizer, and the interlayer magnetostatic stray field, both of which are mostly determined by geometry, are shown to have crucial influence on the dynamic properties of the system. The frequency as a function of current at zero field and also as a function of an out-of-plane field for a fixed current are computed. An excellent quantitative agreement with experimental data is obtained, demonstrating a novel approach for tailoring vortex nano-oscillators.

Peak effect and dynamic melting transitions of driven vortex system in weakly disordered Josephson junction arrays

Using the resistively shunted junction model, we study the magnetic frustration induced dynamic melting transitions of a current-driven vortex system in two-dimensional weakly disordered Josephson juction arrays at zero temperature and very low temperatures. From the unified model simulations, we simultaneously obtain two kinds of dynamic melting transitions of the vortex flow, just below the vortex glass phase and above a dynamic melting threshold, respectively. The reenter pinned dilute vortex liquid behavior and the intrinsic quantum vortex liquid (QVL) phenomenon are also obtained, consistent with the recent experimental reports in disordered and superconducting MoGe films. The peak effect of critical current in the vortex glass is induced by intrinsic collective pinning effects in the plastic flow and the enhancement of critical current and voltage noise in the QVL phase arises from intrinsic quantum fluctuations in the moving vortex flow.

Comparison on plasticity of current-driven vortex lattices in as-grown and heavy ion irradiated microbridge Bi2Sr2Ca1Cu2O8+δ single crystals

From the I–V characteristics for as-grown and irradiated Bi2Sr2CaCu2O8+δ single crystals at T=5K in a magnetic field applied parallel to the c axis, we have seen two types of vortex dynamics near the depinning threshold. For the as-grown sample, at low field, the I–V curves show steps that clearly indicate a “fingerprint phenomenon” since they reflect the current dependence of differential resistance Rd=dV/dI. This can be ascribed to vortices flow through uncorrelated channels for the highly defective lattice. As fields sufficiently increase, these peaks merge, giving broader ones, indicating a crossover from filamentary strings to braid river. In contrast, in the irradiated sample, the pinning is found to be individual at low magnetic fields and collective when the vortex–vortex interactions are involved. Our result suggests a dynamic nature of the peak effect, in agreement with recent numerical simulations and experimental works.

Injection locking of the gyrotropic vortex motion in a nanopillar

Spin-torque oscillators (STOs) are a promising application for the spin-transfer torque effect. The major challenge lies in pushing the STO’s microwave output power to useful levels, e.g., by operating an array of STOs in a synchronized, phase-locked mode. Our experiment on metallic, giant magnetoresistance-type nanopillars focuses on the influence of external high-frequency signals on the current-driven vortex dynamics and demonstrates the injection locking of the gyrotropic mode. We find a gap of about three orders of magnitude between the high-frequency power emitted by one oscillator and the power needed for phase-locking.

Vortex nucleation in spin-torque nanocontact oscillators

We present an experimental study of the nucleation rate associated with current-driven vortex oscillations in magnetic nanocontacts. We find that the nucleation and subsequent steady-state oscillation of a vortex can be initiated using current pulses as short as a few nanoseconds, yielding instant on oscillator capability. The nucleation rate is governed by an Arrhenius law, with an exponential dependence on the applied current magnitude. The mechanism for the vortex nucleation is discussed and compared to analytical estimates assuming the transient presence of a vortex-antivortex pair.

Large microwave generation from current-driven magnetic vortex oscillators in magnetic tunnel junctions

Spin-polarized current can excite the magnetization of a ferromagnet through the transfer of spin angular momentum to the local spin system. This pure spin-related transport phenomenon leads to alluring possibilities for the achievement of a nanometer scale, complementary metal oxide semiconductor-compatible, tunable microwave generator that operates at low bias for future wireless communication applications. Microwave emission generated by the persistent motion of magnetic vortices induced by a spin-transfer effect seems to be a unique manner to reach appropriate spectral linewidth. However, in metallic systems, in which such vortex oscillations have been observed, the resulting microwave power is much too small. In this study, we present experimental evidence of spin-transfer-induced vortex precession in MgO-based magnetic tunnel junctions, with an emitted power that is at least one order of magnitude stronger and with similar spectral quality. More importantly and in contrast to other spin-transfer excitations, the thorough comparison between experimental results and analytical predictions provides a clear textbook illustration of the mechanism of spin-transfer-induced vortex precession.

Current-driven vortex oscillations in metallic nanocontacts: zero-field oscillations and training effects

We present an experimental and theoretical study of the low-field dynamics of current-driven vortex oscillations in nanocontacts based on spin-valve multilayers. These oscillations appear as low-frequency (250–500 MHz) excitations in the electrical power spectrum which arise from variations in the giant magnetoresistance. We show that the vortex oscillations, once nucleated at large fields applied perpendicular to the film plane, persist at zero applied magnetic fields. Some training effects on the oscillation frequency and linewidth are also observed for small in-plane magnetic fields.

Exchange-compelled vortices on magnetic core-shell cylinders and their spin-transfer torque driven dynamics

Magnetic core-shell cylinders comprising a hard dot and a soft ring are proposed. Due to energy redistribution, a compelled vortex is formed on the central dot. Current-driven vortex dynamics is clarified. Because of the confinement effect, the vortex-core gyrotropic motion, and further the vortex-antivortex pair mediated reversal are suppressed. Instead, the vortex-core reversal is achieved through a traveling Bloch-point mediated process. Depending on the current density, the Bloch-point-mediated reversal process adopts different modes (A-, B-, and C-modes). The switching time is a nonmonotonic function of the current density. For the 80-nm-wide cylinder, the A-mode gives a “clean” vortex-core reversal.

Simulation study on Hall effect in logarithmically interacting vortex system

We simulate the current-driven vortex motion in a two-dimensional square sample of type-II superconductors, by using a logarithmically interacting point vortex model involving effect of temperature, transport current, random pinning centers, magnetic field, and Magnus force. We also take into account both the vortex pair nucleation and the vortex pair annihilation processes in the model. Simulation results show that the negative Hall effect is observed for a certain range of the model parameters. Such an anomalous behavior is thought to originate from freely moving antivortices with the long life-time due to the collective dynamics of this vortex system on the background of weakly pinned vortex lattice.

Oppositely rotating eigenmodes of spin-polarized current-driven vortex gyrotropic motions in elliptical nanodots

The authors found that there exist two different rotational eigenmodes of oppositely rotating sense in spin-polarized current-driven vortex gyrotropic motions in soft magnetic elliptical nanodots. Simple mathematical expressions were analytically calculated by adopting vortex-core (VC)-rotation-sense-dependent dynamic susceptibility tensors based on the linearized Thiele equation [Phys. Rev. Lett. 30, 230 (1973)]. The numerical calculations of those analytical expressions were confirmed by micromagnetic simulations, revealing that linear-regime steady-state VC motions driven by any polarized oscillating currents can be interpreted simply by the superposition of the clockwise and counterclockwise rotational eigenmodes. The shape of the orbital trajectories of the two eigenmodes is determined only by the lateral dimension of elliptical dots. Additionally, the orbital radii and phases of the two eigenmodes’ VC motions were found to markedly vary with the frequency of applied currents, particularly across the vortex eigenfrequency and according to the vortex polarization, which results in overall VC motions driven by any polarized oscillating currents.