Ferromagnet-superconductor Interface Research Articles

Anomalous Josephson Effect in a Planar Hybrid Structure with the Spin–Orbit Coupling

The way of formation of controlled phase inhomogeneity in a hybrid structure consisting of a short Josephson junction between two superconducting thin films, with one electrode partially coated with a ferromagnetic insulator, has been theoretically studied. The joint action of spin splitting and the Rashba spin–orbit coupling at the superconductor–ferromagnet interface leads to the generation of a spontaneous supercurrent, which changes the transport properties of the junction. The critical current and the current–phase relation of this hybrid structure have been calculated; it has been shown that this structure can be used to form an anomalous φ0 Josephson junction with the phase shift φ0 smoothly varying over a wide range.

Aspects of long range spin–triplet correlations in superconductor/ferromagnet hetero-structures

The notion of competing ferromagnetic (F) and superconducting (S) orders in F/S hybrid structures was transformed by the first realization of ferromagnetic Josephson π-junctions and the almost simultaneous prediction of a possibility of spin–triplet correlations in such structures, almost two decades back. Such hybrid structures in various configurations are now studied as rich sources of emergent states and new effects. Unlike the spin singlet Cooper pairs, the spin triplet Cooper pairs are much less affected by the exchange field of a ferromagnet and, therefore, immediately finds interest in the field of spintronics. Theoretically, it has been shown that the basic protocol for spin–singlet to spin–triplet supercurrent conversion is the presence of magnetic non-collinearity at the superconductor–ferromagnet interface. Therefore, almost all experiments in this direction have utilized transport measurements on F/S systems with artificial magnetic non-collinearity formed by combination of several ferromagnetic layers next to the superconducting layer. Here we highlight two aspects of studying these heterostructures. Firstly we show that natural magnetic inhomogeneities, found in domain walls of ferromagnets, can also be used to achieve singlet–triplet conversion, instead of artificial magnetic inhomoheneities. This possibility was explored via transport measurements in nano-scale planar Nb–Ni–Nb junctions and nano-SQUIDs, where a domain wall was pinned at the Josephson junction barrier. By this method we were able to show Josephson coupling across about 70 nm of strong ferromagnetic planar barrier. Secondly we show that spin–triplet correlations at the F/S interface are robust enough to be probed by the diamagnetic screening currents at the interface. This was probed by studying the change in sperconducting transition temperature of Nb/Co/Py/Nb multilayers in presence of small in-plane magnetic field. The Co/Py combination, which is a soft-hard type magnetic exchange spring, worked as magnetic inhomogeneity for triplet generation at the interface of the superconducting Nb. These observations may promote new experiments in the field of superconducting-spintronics.

Electric Field and Light Field Modulated Josephson Effect in Silicene-Based SNS Josephson Junction: Andreev Reflection and Free Energy

We investigate the Josephson effect in superconductor-normal-superconductor junction (SNS) base on an unbiased silicene under the perpendicular electric field and off-resonance circularly polarized light both analytically and numerically. The Andreev reflection (during the subgap transport), free energy, Josephson current, and the reversal of the Josephson effect are explored. The Andreev reflection is complete (for tunnel limit) in the NS interface even for the clean sample and without the Fermi wave vector mismatch, which is opposite to the case of ferromagnet-superconductor interface. The dynamical polarization of the related degrees of freedom is also related to the 0 − π transition and the generation of ϕ0-junction. In the short junction limit, the approximated results about the Andreev level and free energy are also discussed. Beside the low-energy limit of the tight-binding model, the finite-size effect is needed to be taken into account. The effect of the time-dependent drive and the Green’s function method also represented in the end of this paper.

Andreev bound states in superconductor/ferromagnet point contact Andreev reflection spectra

As charge carriers traverse a single superconductor ferromagnet interface they experience an additional spin-dependent phase angle which results in spin mixing and the formation of a bound state called the Andreev Bound State. This state is an essential component in the generation of long range spin triplet proximity induced superconductivity and yet the factors controlling the degree of spin mixing and the formation of the bound state remain elusive. Here we demonstrate that point contact Andreev reflection can be used to detect the bound state and extract the resulting spin mixing angle. By examining spectra taken from La1.15Sr1.85Mn2O7 single crystal - Pb junctions, together with a compilation of literature data on highly spin polarised systems, we show that the existence of the Andreev Bound State both resolves a number of long standing controversies in the Andreev literature as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly we find that for these high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have chosen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances.

Inverse proximity effect at superconductor-ferromagnet interfaces: Evidence for induced triplet pairing in the superconductor

Considerable evidence for proximity-induced triplet superconductivity on the ferromagnetic side of a superconductor-ferromagnet (S-F) interface now exists; however, the corresponding effect on the superconductor side has hardly been addressed. We have performed scanning tunneling spectroscopy measurements on NbN superconducting thin films proximity coupled to the half-metallic ferromagnet La2/3Ca1/3MnO3 (LCMO) as a function of magnetic field. We have found that at zero and low applied magnetic fields the tunneling spectra on NbN typically show an anomalous gap structure with suppressed coherence peaks and, in some cases, a zero-bias conductance peak. As the field increases to the magnetic saturation of LCMO where the magnetization is homogeneous, the spectra become more BCS-like and the critical temperature of the NbN increases, implying a reduced proximity effect. Our results therefore suggest that triplet-pairing correlations are also induced in the S side of an S-F bilayer.

Spontaneous formation of superconducting NiBi3 phase in Ni-Bi bilayer films

We report the spontaneous formation of superconducting NiBi3 phase in thermally evaporated Ni-Bi bilayer films. High reaction-diffusion coefficient of Bi is believed to drive the formation of NiBi3 during the deposition of Bi on the Ni film. Cross sectional transmission electron microscopy and glancing incidence X-ray depth profiling confirmed the presence of NiBi3 throughout the top Bi layer. Superconducting transition at ∼3.9 K, close to the bulk value, was confirmed by transport and magnetization measurements. The bilayers were irradiated with varying fluence of 100 MeV Au ions to study the robustness of superconducting order in presence of large concentration of defects. Superconducting parameters of NiBi3, such as transition temperature and upper critical field, remained unchanged upto an ion dose of 1 × 1014 ions/cm2. The diffusive formation of NiBi3 in Ni opens the possibility of studying superconducting proximity effect at a truly clean superconductor-ferromagnet interface.

Effect of Transport-Induced Charge Inhomogeneity on Point-Contact Andreev Reflection Spectra at Ferromagnet–Superconductor Interfaces

We investigate the transport properties of a ferromagnet-superconductor interface within the framework of a modified three-dimensional Blonder-Tinkham-Klapwijk formalism. In particular, we propose that charge inhomogeneity forms via two unique transport mechanisms, namely, evanescent Andreev reflection and evanescent quasiparticle transmission. Furthermore, we take into account the influence of charge inhomogeneity on the interfacial barrier potential and calculate the conductance as a function of bias voltage. Point-contact Andreev reflection (PCAR) spectra often show dip structures, large zero-bias conductance enhancement, and additional zero-bias conductance peak. Our results indicate that transport-induced charge inhomogeneity could be a source of all these anomalous characteristics of the PCAR spectra.

Spin-Resolved Tunneling Studies of the Exchange Field inEuS/AlBilayers

We use spin-resolved electron tunneling to study the exchange field in the Al component of EuS/Al bilayers, in both the superconducting and normal-state phases of the Al. Contrary to expectation, we show that the exchange field H(ex) is a nonlinear function of applied field, even in applied fields that are well beyond the EuS coercive field. Furthermore, the magnitude H(ex) is unaffected by the superconducting phase. In addition, H(ex) decreases significantly with increasing temperature in the temperature range of 0.1-1 K. We discuss these results in the context of recent theories of generalized spin-dependent boundary conditions at a superconductor-ferromagnet interface.

Spin Josephson effect with a single superconductor

A thin ferromagnetic layer on a bulk equal-spin-pairing triplet superconductor is shown to mediate a Josephson coupling between the spin $\uparrow$ and $\downarrow$ condensates of the superconductor. By deriving analytic expressions for the bound states at the triplet superconductor-ferromagnet interface, we show that this spin Josephson effect establishes an effective anisotropy axis in the ferromagnetic layer. The associated Josephson spin current is predicted to cause a measurable precession of the magnetization about the vector order parameter of the triplet superconductor.

Effect of Frozen Magnetic Flux on the Electrical Transport Characteristics of Superconductor–Ferromagnet Junction

We studied electrical transport characteristics of lateral superconductor (SC)–ferromagnet (FM)–superconductor devices which were found to exhibit two strikingly different differential resistance states at zero field. The key difference between these states is that the critical current (Ic) is dramatically lower for one state (state 2) than the other (state 1). Moreover, state 2 exhibits a higher zero bias resistance than state 1. We explain this as an effect of magnetic field from the frozen flux in SC on the Andreev reflection process at the SC–FM interface. The magnetoresistance (MR) curves at different dc current biases are nonhysteretic and symmetric with respect to zero field axis when the device is at state 1. In contrary to this, MR curves at state 2 are asymmetric and show a pronounced hysteresis at high bias current. These effects indicate a coupling between applied field and frozen flux, which affects the transport characteristics of the SC–FM interface.

Influence of Barrier Width on Spin-Polarisation Measured by Point Contact Andreev Reflection

A modification of the Blonder–Tinkham–Klapwijk (BTK) model is proposed for the description of the ferromagnet-superconductor interface. Modelling the contact potential with a rectangular barrier, we investigate the influence of the barrier width at the interface on the value of spin polarisation measured by point contact Andreev reflection. Results suggesting that neglecting the width of the barrier at the interface can lead to an overestimation of the spin polarisation using the original BTK model are presented. This effect is particularly strong for low values of polarization and vanishes for high polarisation. The impact on analysis of the experimental data is also discussed.

Spin-polarized conductance in graphene-based FSF junctions

In this paper, conductance of spin and electron in graphene-based ferromagnet—superconductor (FS) and parallel and antiparallel ferromagnet–superconductor–ferromagnet (FSF) junctions are studied. Using the Dirac–Bogoliubov–de Gennes equations, Andreev and normal reflections are obtained and then using these coefficients, conductance of spin and electrons are calculated at the FS interface(s) analytically. As a result, both the energy dependence of spin and charge differential conductances are investigated and a comparison between electron and spin transport is done in this paper. Effect of exchange energy of ferromagnet h on conductances is studied too.

Controlled Injection of Spin-Triplet Supercurrents into a Strong Ferromagnet

The superconductor-ferromagnet proximity effect describes the fast decay of a spin-singlet supercurrent originating from the superconductor upon entering the neighboring ferromagnet. After placing a conical magnet (holmium) at the interface between the two, we detected a long-ranged supercurrent in the ferromagnetic layer. The long-range effect required particular thicknesses of the spiral magnetically ordered holmium, consistent with spin-triplet proximity theory. This enabled control of the electron pairing symmetry by tuning the degree of magnetic inhomogeneity through the thicknesses of the holmium injectors.

Spin transfer torque in the presence of Andreev reflections

A first-principles method is developed to study the spin transfer torques in magnetic noncollinear textured ferromagnet-superconductor heterostructure. We apply the method to study the long-range spin-triplet pairing $({S}_{z}=\ifmmode\pm\else\textpm\fi{}1)$ induced by the spin-flip scattering near the ferromagnet-superconductor interface. The long-range spin-triplet Andreev reflection will saturate to the same value as usual Andreev reflection with strong enough spin-flip scattering. We find that the giant magnetoresistance effect of a spin valve with a superconductor contact can be restored by a small amount of interfacial spin-flip scattering.

Odd triplet superconductivity in a superconductor/ferromagnet structure with a narrow domain wall

We study proximity effect in superconductor/ferromagnet (SF) structure with a narrow domain wall (DW) at the SF interface. The width of the domain wall is assumed to be larger than the Fermi wavelength but smaller than other characteristic lengths (for example, the ``magnetic'' length). The transmission coefficient is supposed to be small so that we deal with a weak proximity effect. Solving the linearized Eilenberger equation, we find analytical expressions for quasiclassical Green's functions. These functions describe the short-range (SR) condensate components, singlet and triplet, with zero projection of the total spin on the quantization $z$ axis, induced in ferromagnet (F) due to the proximity effect as well as long-range odd triplet component (LRTC) with a nonzero projection of the total spin of Cooper pairs on the $z$ axis. The amplitude of the LRTC essentially depends on the product $h\ensuremath{\tau}$ and increases with increasing the exchange energy $h$ ($\ensuremath{\tau}$ is the elastic-scattering time). We calculate the Josephson current in superconductor/ferromagnet/superconductor junction with a thickness of the F layer much greater than the penetration length of the SR components. The Josephson critical current caused by the LRTC may be both positive and negative depending on chirality of the magnetic structure in F. The density of states (DOS) in a diffusive SF bilayer is also analyzed. It is shown that the contributions of the SR and LR components to the DOS in F have a different dependence on the thickness $d$ of the F layer (nonmonotonous and monotonous).

Nanoscale spin polarization in the dilute magnetic semiconductor (In,Mn)Sb

Results of point-contact Andreev-reflection experiments on (In,Mn)Sb are presented and analyzed in terms of current models of charge conversion at a superconductor-ferromagnet interface. We investigate the influence of surface transparency, and study the crossover from ballistic to diffusive transport regime as contact size is varied. Application of a Nb tip to a (In,Mn)Sb sample with Curie temperature ${T}_{C}$ of 5.4 K allowed the determination of spin polarization when the ferromagnetic phase-transition temperature is crossed. We find a striking difference between the temperature dependence of the local spin polarization and of the macroscopic magnetization, and demonstrate that nanoscale clusters with magnetization close to the saturated value are present even well above the magnetic phase-transition temperature.

Appearance of an inhomogeneous superconducting state in La0.67Sr0.33MnO3-YBa2Cu3O7-La0.67Sr0.33MnO3 trilayers

An experimental study of proximity effect in La0.67Sr0.33MnO3-YBa2CU3O7-La0.67Sr0.33MnO3 trilayers is reported. Transport measurements on these samples show clear oscillations in critical current (Ic) as the thickness of La0.67Sr0.33MnO3 layers (dF) is scanned from ∼50 A to ∼ 1100 A. In the light of existing theories of ferromagnet-superconductor (FM-SC) heterostructures, this observation suggests a long range proximity effect in the manganite, modulated by its weak exchange energy (∼2 meV). The observed modulation of the magnetic coupling between the ferromagnetic LSMO layers as a function of dF, also suggests an oscillatory behavior of the SC order parameter near the FM-SC interface.

Incomplete Andreev reflection in a clean superconductor-ferromagnet-superconductor junction

We study the Josephson effect in a clean superconductor-ferromagnet-superconductor junction for arbitrarily large spin polarizations. The Andreev reflection at a clean ferromagnet-superconductor interface is incomplete, and Andreev channels with a large incidence angle are progressively suppressed with increasing exchange energy. As a result, the critical current exhibits oscillations as a function of the exchange energy and of the length of the ferromagnet and has a temperature dependence which deviates from the one predicted by the quasiclassical theory.

Josephson junction through a thin ferromagnetic layer: negative coupling.

We investigate Josephson coupling through a ferromagnetic thin film using superconductor-insulator-ferromagnet-superconductor planar junctions. Damped oscillations of the critical current are observed as a function of the ferromagnetic layer thickness. We show that they result from the exchange energy gained or lost by a quasiparticle Andreev-reflected at the ferromagnet-superconductor interface. The critical current cancels out at the transition from positive ("0") to negative ("pi") coupling, in agreement with theoretical calculations.

Spontaneous magnetization changes and nonlocal effects in mesoscopic ferromagnet-superconductor structures

We report magnetization studies of individual ferromagnet-superconductor (FS) structures of submicron size. Upon cooling through the superconducting transition in zero field, such structures are found to change their magnetization spontaneously. We attribute this effect to reshuffling of domains in the ferromagnet, which is caused by temperature-dependent screening of domain's stray fields by the superconductor. The spontaneous magnetization is not localized to the actual contact area between the two materials but can propagate along the ferromagnet as far as several microns from the FS interface.