Voltage-control over a high-field superconducting transition and the superconducting exchange interaction
Professor Jacob Linder, Norwegian University of Science and Technology, Norway
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.
Comparison of the spin-transfer torque mechanisms in three terminal spin-torque-oscillators
Dr Emilie Jué, National Institute of Standards and Technology, USA
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 studies of ferromagnets and superconductors
Dr Chiara Ciccarelli, University of Cambridge, UK
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.
Anomalous Meissner effect in superconductor-ferromagnet proximity systems
Dr Machiel Flokstra, University of St Andrews, UK