Vortex Matter in Superconductor / Ferromagnet Hybrids

No Heading Vortex pinning in type-II superconducting films can be effectively controlled by combining these films with different ferromagnetic nanostructures. In this article an overview is presented of divergent types of ferromagnetic pinning centers. The investigated hybrid structures consist of Pb films that are deposited on top of arrays of ferromagnetic dots with in-plane magnetization (IPM) or out-of-plane magnetization (OPM), ferromagnetic films with IPM or OPM that contain arrays of submicron holes (antidots), or continuous films with OPM and a magnetic domain structure. Interesting effects such as field-polarity dependent vortex pinning and the dependence of the pinning strength on the domain structure of the ferromagnet are observed. Our experiments demonstrate that vortex pinning in superconductors is strongly influenced by the magnetic properties of the different ferromagnetic pinning centers.PACS numbers: 74.25.Qt, 74.25.Ha, 74.78.Na     

Spin Transport in Ferromagnet/Ferromagnet/s-Wave Superconductor Hybrid System

We study the spin transport in ferromagnet/ferromagnet/s-wave superconductor hybrid system with the extended Blander-Tinkham-Klapwijk formalism. We show the subgap spin conductance dominated by Andreev reflection is susceptible to the relative orientation of the two magnetizations. A giant spin tunneling magnetoresistance which is defined with the spin conductance for the collinear and the non-collinear magnetization configurations is obtained. The oscillatory behavior in the conductance spectra and Andreev bound states in the junctions caused by interference effect are also investigated. All these results may possess potential applications in future device design in spintronics.     

Superconducting Proximity Effects in Ferromagnet/Superconductor Heterostructures

We consider in this paper superconducting proximity effects in clean Ferromagnet/Ferromagnet/Superconductor (F1/F2/S) heterostructures. We make use of a numerical self-consistent solution of the microscopic Bogoliubov?Cde Gennes equations to investigate the superconducting transition temperatures as a function of the angle ?? between the magnetizations in the ferromagnetic layers. Unlike in F1/S/F2 trilayers, we find that the superconducting transition temperature is a nonmonotonic function of ??.     

Long-Range Spin-Triplet Superconductivity Induced by Magnetic Field in d Wave Superconductor/Ferromagnet Hybrid System

We theoretically study the long-range spin- triplet superconductivity in d wave superconductor/ ferromagnet/ferromagnet (S/F₁/F₂) trilayer junction, in which the magnetization of F₁ layer could be rotated in the y–z plane by an external magnetic field. The four-component Eilenberger equations were constructed to calculate the superconducting order parameters and density of states (DOS). Near the clean limit, the p wave equal-spin triplet component could be induced when the magnetization directions of F₁/F₂ layers are non-collinear, and the DOS exhibits a split zero-bias conductance peak. The various parameters such as ferromagnetic exchange energy, thickness of ferromagnetic layers, and angles between F₁/F₂ magnetization directions are studied for the effect on inducing triplet superconductivity. By magnetic field controlling the emergence of equal-spin triplet pairings or not, such a tunable S/F₁/F₂ trilayer junction based on long-range spin-triplet superconductivity could be used as a superconducting switch device, which would open up a new view of spintronics.     

On the Sarma-Type Phase Transitions in Ferromagnet/Superconductor/Ferromagnet Tunneling Junctions with Highly Spin-Polarized Ferromagnets

In the ferromagnet/superconductor/ferromagnet double tunneling junctions, the spin-polarized tunneling current in the antiparallel alignment of the magnetization induces spin imbalance in the superconductor, which has a pair breaking effect depressing superconductivity in the same way that the Zeeman effect does in the paramagnetic limit. In particular, it is shown that when the ferromagnets are highly spin polarized, the strong spin imbalance may lead to a first-order phase transition from the superconducting phase to the normal phase at low temperature and low bias voltage. This phase transition accompanies a large discontinuous drop in superconducting gap parameter bring in distinctive features in low energy transport.     

Magnetization Properties of Microstructured NiFe Rings Investigated Using Semiconductor/Ferromagnet Hybrid Structures

Magnetization reversal processes in microstructured NiFe rings are studied by fringe-field-induced local Hall magnetometry. This semiconductor-based technique yields a high sensitivity of magnetic stray fields and allows us to detect magnetization hysteresis loops of single NiFe rings. The transition fields can be controlled by the ratio between inner- and outer-ring diameter. Comparison between Hall measurements and numerical simulation suggests that there are four different magnetization states in two integrated rings.

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Transport Properties of Topological Insulator-Based Ferromagnet/f-Wave Superconductor Junction

We investigate effect of magnetically-induced relativistic mass and also anisotropic f-wave pair coupling on the tunneling conductance on the surface of a 3D topological insulator ferromagnet/superconductor junction, which two types of pairing for superconductivity are possible. A topological insulator as a new state of condensed-matter caused by spin–orbit interaction and time-reversal symmetry has a bulk band gap and gapless surface states. We use the BTK formalism to find the charge carriers behavior. Due to two different nature of order parameters of the f-wave superconductivity, the tunneling conductance is found to be linearly dependent on the magnetic gap in terms of f ₁ and the exponential for f ₂. It is shown that the conversion of the conductance peak from ZBCP to ZBCD occurs in the f ₁ case with increasing m, while this is not observed in f ₂. Also, we find that the conductance behaves as a unit step function for the superconductor electrostatic potential in f ₁, and this should be usable in nanoelectronic switch devices. In addition, we illustrate how the magnetic gap affects the transmission coefficient in quite different behaviors for order parameters.     

Anomalously Weak Cooper Pair-breaking by Exchange Energy in Ferromagnet/Superconductor Bilayers

We report the superconducting transition temperature T _c versus thickness d _F of Ferromagnet/Superconductor (F/S) bilayers, where F is a strong 3d ferromagnet (Ni, Ni _0.81 Fe _0.19 (Permalloy), Co _0.5Fe _0.5) and S = Nb, taken from superfluid density measurements rather than resistivity. By regrouping the many physical parameters that appear in theory, we show that the effective exchange energy is determined from the F film thickness d _F where T _c versus d _F begins to flatten out. Using this rearranged theory, we conclude 1) the effective exchange energy, E _{e x} , is about 15 times smaller than measured by ARPES and five times smaller than deduced in previous studies similar to ours; 2) the dirty-limit coherence length, ξ _F , for Cooper pairs in F is larger than the electron mean free path, ? _F ; and 3) the 3d-F/Nb interface is enough of a barrier that Cooper pairs typically must hit it several times before getting through. The Py/Nb and CoFe/Nb interfaces are more transparent than the Ni/Nb interface.     

Vortex in Holographic Two-band Superconductor

A single static vortex in a holographic two-band superconductor is constructed. We investigate the effect of the interband coupling to the condensate profile. We estimate the first critical magnetic field, and compute characteristic lengths like penetration and coherence lengths and conclude both bands in our superconductor behave type II.     

Anisotropic Charge Conductance and Huge Tunneling Magnetoresistance in Ferromagnet/Noncentrosymmetric Superconductor Junctions

We study theoretically the tunneling charge conductance in quasi-two-dimensional ferromagnet/noncentrosymmetric superconductor (FM/NCS) junctions on the basis of the Blonder-Tinkham-Klapwijk (BTK) formalism. It is found that, spin-orbit coupling in the noncentrosymmetric superconductor may induce significant dependence of subgap tunneling charge conductance on the orientation of magnetization in the ferromagnet electrode, giving rise to a novel magnetoresistance effect; this effect becomes very pronounced when the ferromagnet is in the half metallic limit. We also find that, magnetization-orientation dependent tunneling spectroscopy may provide a clue to determine the structure of spin-orbit interaction and the pairing symmetry in noncentrosymmetric superconductor.     

Channel Width Dependence of Spin Polarized Transports in NiFe/InGaAs Hybrid Two-Terminal Structures

We investigated spin polarized transports in NiFe/InGaAs hybrid two-terminal structures at 1.5K as well as their channel width dependence. The two-terminal structures were fabricated in order to neglect the local Hall effect (LHE) by fringe fields of NiFe contacts. First, we measured magneto-resistance (MR) characteristics of the samples under vertical magnetic fields, and obtained clear oscillations indicating the ohmic formation at NiFe/InGaAs interfaces. Next, we measured spin valve (SV) properties under parallel magnetic fields, and successfully observed clear SV peaks without LHE hysterisis loops. Furthermore, we also confirmed unique behavior of SV peaks depending on the channel width. Such dependence also indicates spin injection/detection through NiFe/InGaAs interfaces.     

Vortex Structure in an Antiferromagnetic Superconductor

The structure of a superconducting vortex in a dirty antiferromagnetic superconductor (AFSC) has been studied theoretically. We show that a superconducting vortex in AFSC has an antiferromagnetic core. The existence of superconducting vortex with antiferromagnetic cores may have nontrivial influence upon transport properties of type-II AFSC in magnetic field.     

Giant Tunneling Magnetoresistance Effect in Ferromagnet/Spin-triplet Superconductor Junctions Caused by Andreev Reflection

We study theoretically the tunneling magnetoresistance effect in ferromagnet/chiral triplet superconductor junctions using the generalized Blonder-Tinkham-Klapwijk theory. This effect rely entirely on the dependence of Andreev reflection on the relative orientation between the magnetic moment in ferromagnet and the d-vector in spin-triplet superconductor, The tunneling differential conductance and magnetoresistance are calculated for three types of chiral p-wave gap functions, and huge tunneling magnetoresistance effect is obtained in the strong spin-polarization and low-temperature regime. This effect might provide a valuable new source of device application.     

Vortex Avalanches in a Type II Superconductor

We report on a study of the spatiotemporal variation of magnetic induction in a superconducting niobium sample during a slow sweep of external magnetic field. A sizable fraction of the increase in the local vortex population occurs in abrupt jumps. We compare the size distribution of these avalanches with the predictions of self-organized-criticality models for vortex dynamics.     

Magnetic Proximity Effects in V/Fe Superconductor/Ferromagnet Single Bilayer Revealed by Waveguide-Enhanced Polarized Neutron Reflectometry

Polarized neutron reflectometry is used to study the magnetic proximity effect in a superconductor/ferromagnet (SC/FM) system of composition Cu(32 nm)/ V(40 nm)/Fe(1 nm)/MgO. In contrast to previous studies, here a single SC/FM bilayer, is studied and multilayer artefacts are excluded. The necessary signal enhancement is achieved by waveguide resonance, i.e., preparing the V(40 nm)/Fe(1 nm) SC/FM bilayer sandwiched by the highly reflective MgO substrate and Cu top layer, respectively. A new magnetic state of the system was observed at temperatures below 0.7T _C manifested in a systematic change in the height and width of the waveguide resonance peak. Upon increasing the temperature from 0.7T _C to T _C, a gradual decay of this state is observed, accompanied by a 5% growth of the diffuse scattering. This behavior can be explained in a natural way by the polarization of the superconducting electrons upon the SC transition, i.e., an appearance of additional induced magnetization within the SC, due to the proximity of the FM layer.     

Vortex Escape from Columnar Defect in a Current-Loaded Superconductor

The problem of Abrikosov vortices depinning from extended linear (columnar) defect in 3D-anisotropic superconductor film under non-uniformly distributed Lorentz force is studied for the case of low temperatures, disregarding thermal activation processes. We treat it as a problem of mechanical behavior of an elastic vortex string settled in a potential well of a linear defect and exerted to Lorentz force action within the screening layer about the London penetration depth near the specimen surface. The stability problem for the vortex pinning state is investigated by means of numerical modeling, and conditions for the instability threshold are obtained as well as the critical current density \(j_\mathrm{c}\) and its dependence on the film thickness and magnetic field orientation. The instability leading to vortex depinning from extended linear defect first emerges near the surface and then propagates inside the superconductor. This scenario of vortex depinning mechanism at low temperatures is strongly supported by some recent experiments on high-Tc superconductors and other novel superconducting materials, containing columnar defects of various nature.     

Temperature Dependence of Vortex Nucleation in Gaseous Bose-Einstein Condensates

The formation of quantized vortices in trapped, gaseous Bose-Einstein condensates is considered. The thermodynamic stability of vortex states and the essential role of the surface excitations as a route for vortex penetration into the condensate are discussed. Special focus is on finite-temperature effects of the vortex nucleation process. It is concluded that the critical angular frequencies for exciting surface modes with the relevant multipolarities yield, also at finite temperatures, the appropriate thresholds for the nucleation of vortices in dilute Bose-Einstein condensates, in fair agreement with the recent experiments.     

Imaging Domain Reversal in an Ultrathin Van der Waals Ferromagnet

The recent isolation of 2D van der Waals magnetic materials has uncovered rich physics that often differs from the magnetic behavior of their bulk counterparts. However, the microscopic details of fundamental processes such as the initial magnetization or domain reversal, which govern the magnetic hysteresis, remain largely unknown in the ultrathin limit. Here a widefield nitrogen-vacancy (NV) microscope is employed to directly image these processes in few-layer flakes of the magnetic semiconductor vanadium triiodide (VI₃). Complete and abrupt switching of most flakes is observed at fields H_c ≈ 0.5–1 T (at 5 K) independent of thickness. The coercive field decreases as the temperature approaches the Curie temperature (T_c ≈ 50 K); however, the switching remains abrupt. The initial magnetization process is then imaged, which reveals thickness-dependent domain wall depinning fields well below H_c. These results point to ultrathin VI₃ being a nucleation-type hard ferromagnet, where the coercive field is set by the anisotropy-limited domain wall nucleation field. This work illustrates the power of widefield NV microscopy to investigate magnetization processes in van der Waals ferromagnets, which can be used to elucidate the origin of the hard ferromagnetic properties of other materials and explore field- and current-driven domain wall dynamics.