Non-equilibrium superconductivity and spintronics

Proximity coupling across superconductor-ferromagnet bilayers can give rise to the triplet component of the superconducting condensate. Superconductivity and ferromagnetism have been reported to coexist in a Ni/Bi bilayer system. Thus spin polarised triplet supercurrent in ferromagnetic-superconducting Josephson junctions can be expected. The Moodera group has investigated the complex and rich behaviour of this phenomenon in bilayers of Ni/Ga and Ni/Bi systems, including Josephson and quasiparticle tunnelling. Ni/Ga as well as Ni/Bi bilayer systems show unusual superconductivity, with high T_c that can co-exist with ferromagnetism. Tunnelling spectroscopy studies at low temperatures show the presence of three superconducting energy gaps in the bilayers, attributable to surface, interface and bulk states within the bilayers. Ni layers, ranging from 0.8 to 6nm thick, confirmed to be ferromagnetic by the magnetisation studies while spin polarised tunnelling studies revealed that the tunnelling electrons coming from the Ni surface were spin polarised and simultaneously displayed superconducting gap, supporting the co-existence of SC and FM. The interplay of SC and FM with the presence of spin polarised carriers in such bilayer system could be a strong case for triplet pairing. In addition, the observed Josephson current could be spin polarised. Interestingly, the superconductivity in the Ni/Bi bilayer is expected to be topological. The occurrence of zero bias conductance may reflect odd-frequency symmetry in the superconducting condensate, supporting the presence of a non-zero component associated with triplet pair superconductivity insensitive to disorder. Jagadeesh Moodera will present this ongoing work leaving it open for discussion. This work has been done in collaboration with Madison Sutula, Sebastian Bergeret, Jia Song, Valeria Lauter and Niladri Banerjee.

Dirac materials with strong spin-orbit interaction have been shown to generate large surface spin accumulations in response to applied currents. Such materials have, in addition, been demonstrated to exert spin-orbit torques on adjacent ferromagnetic structures. This magnetoelectric effect in these materials is strong due to the efficient spin-momentum locking. In heterostructures consisting of superconductors and three-dimensional superconductors, this spin-momentum locking leads to an induced unconventional superconductivity that may be useful for superconducting spintronics. Here, Cecilia Holmqvist investigates theoretically the quantum transport properties of a ballistic junction consisting of two topological superconductors coupled over a quantum dot that is coupled to a ferromagnet. The spin-orbit torques acting on the ferromagnet are examined and are shown to depend strongly on the magnetisation direction relative to the current direction.

Recently several mechanisms realising the direct coupling between magnetic moment and Josephson current in S/F/S junctions have been proposed. In such junctions, the ac Josephson effect may generate a magnetic precession providing then a feedback to the current. Magnetic dynamics results in several anomalies of current-phase relations (second harmonic, dissipative current) which are strongly enhanced near the ferromagnetic resonance frequency. The simulations of magnetic moment dynamics show that by applying an electric current pulse, it may be possible to realise the full magnetisation reversal which is quite important for the elaboration of superconducting spintronic devices with low dissipation.

In their original formulation of superconductivity, the London brothers predicted the exponential suppression of an electrostatic field inside a superconductor over the London penetration depth. Despite a few experiments indicating hints of perturbation induced by electrostatic fields, no clue has been provided so far on the possibility to manipulate conventional superconductors via field-effect. In this talk, Francesco Giazotto will show the evidence of full field-effect control of the supercurrent in all-metallic transistors made of different BCS superconducting films. At a low temperature, the field-effect transistors (FETs) show a monotonic decay of the critical current under increasing electrostatic field up to total quenching for gate voltage values as large as ±40V in titanium-based devices. A similar behaviour, though less pronounced, was observed in aluminum FETs. In addition, Francesco will report on the realisation of Ti-based Dayem bridge Josephson field-effect transistors. The latter show full suppression of I_C for gate voltages as low as ±8V. Finally, Francesco will show the behaviour of mesoscopic superconductor-normal metal-superconductor Josephson field-effect transistors that will reveal the impact of electrostatic fields even on proximity metals thereby suggesting that the field effect is universal. Possible electronic and circuital schemes based on this all-metallic technology will be discussed.

Jacob Linder’s group predicts two interesting phenomena related to superconductivity in hybrid structures driven out of equilibrium by application of an electric voltage. Firstly, they theoretically demonstrate that superconductivity in thin films can be stabilised in high magnetic fields if the superconductor is driven out of equilibrium by a voltage bias. For realistic material parameters and temperatures, they show that superconductivity is restored in fields many times larger than the Chandrasekhar–Clogston limit. After motivating the effect analytically, they present rigorous numerical calculations to corroborate the findings, and discuss concrete experimental signatures. Secondly, Linder discusses how the magnetic exchange interaction in a spin-valve is influenced by non-equilibrium superconductivity. Linder shows that the sign of the exchange interaction in a spin-valve, determining whether a parallel or antiparallel magnetic configuration is favoured, can be controlled via an electric voltage. This occurs due to an interplay between a non-equilibrium quasiparticle distribution and the presence of spin-polarized Cooper pairs. These findings may be of relevance for spin-based superconducting devices which in practice most likely have to be operated precisely by non-equilibrium effects.

The spin transfer torque is one of the most active field of spintronics due to its potential for use in memory and logic applications. This control can be achieved via a spin polarised current with the mechanism of spin-filtering torque (SFT) or through a pure spin current via the mechanism of spin-orbit torque (SOT). Over the past several years, SOT has gained increased attention due to the new possibilities that it offers for data storage applications. However, the quantification and comparison of both mechanisms’ efficiencies remains uncertain, due to the uncertainty in material parameters needed to quantify the torque. In this work, researchers at NIST compared for the first time the SFT and SOT efficiencies acting on the same nanomagnetic element. To do so, they created 3-terminal spin-torque oscillators (STO) composed of spin-valves (SV) patterned on top of Pt nanowires. The devices are excited either by SFT or by SOT depending on whether the current is applied through the SV or through the Pt wire. By comparing the magnetization dynamics obtained with the different STT mechanisms, they quantify the relative efficiencies of the SOT and SFT in the system as a function of the dimensions of the SV and Pt nanowires.

Ferromagnetic resonance is a powerful method to extract information on the magnetic torques, spin damping and magnetic anisotropies. In Chiara Ciccarelli’s group ferromagnetic resonance methods are applied to quantitatively evaluate the spin torques in asymmetric ferromagnets, magnetic insulators and synthetic antiferromagnets. The focus of this talk will be on some recent work where spin pumping experiments are conducted on HM/SC/FM and SC/FM structures (SC=niobium, HM=platinum and FM=permalloy). Ferromagnetic resonance in the permalloy is excited via a waveguide and the damping measured through the superconducting transition temperature of niobium. These results show that while in the SC/FM structures spin pumping is suppressed when the niobium turns superconducting, in the HM/SC/FM structures the presence of platinum leads to an enhanced spin transfer through superconducting niobium with respect to its normal state and even with respect to bare platinum.

In mesoscopic superconducting systems the Meissner response of a superconducting film can be very different from its bulk behaviour. It was theoretically shown that adding a normal metal (N) to a mesoscopic superconductor (S) can lead to a greatly enhanced Meissner screening, opposite to the case of adding a homogeneous ferromagnet (F) which is expected to reduce the screening due to its strong pair-breaking effect. However, a recently developed theory shows that for the S/F case, spontaneous screening currents can appear, generated by the vector-potential of magnetisation near the S/F interface. Using low energy muon spin spectroscopy, which is an exquisite tool to probe the local flux inside a thin film, Michiel Flokstra presents experimental data on various N/S/F type systems and observe enhanced screening due to the addition of a normal metal, as well as clear signatures of the newly predicted electromagnetic proximity effect generated at the S/F interfaces.