Evidence for spin selectivity of triplet pairs in superconducting spin valves.

Spin selectivity in a ferromagnet results from a difference in the density of up- and down-spin electrons at the Fermi energy as a consequence of which the scattering rates depend on the spin orientation of the electrons. This property is utilized in spintronics to control the flow of electrons by ferromagnets in a ferromagnet (F1)/normal metal (N)/ferromagnet (F2) spin valve, where F1 acts as the polarizer and F2 the analyser. The feasibility of superconducting spintronics depends on the spin sensitivity of ferromagnets to the spin of the equal spin-triplet Cooper pairs, which arise in superconductor (S)-ferromagnet (F) heterostructures with magnetic inhomogeneity at the S-F interface. Here we report a critical temperature dependence on magnetic configuration in current-in-plane F-S-F spin valves with a holmium spin mixer at the S-F interface providing evidence of a spin selectivity of the ferromagnets to the spin of the triplet Cooper pairs.This work was funded by the Royal Society through a University Research Fellowship “Superconducting Spintronics” held by J.W.A.R. M.G.B acknowledges funding from the UK EPSRC and the European Commission through an ERC Advanced Investigator Grant "Superspin". C.B.S. and R.G.J.S were supported by the Erasmus exchange programme and the Leiden Outbound Grant. C.B.S. acknowledges Prof. Jan Aarts’ for scientific input. The work of F.S.B and A. O. have been supported by the Spanish Ministry of Economy and Competitiveness under Project FIS2011-28851-C02-02. The work of A. O. have also been supported by the CSIC and the European Social Fund under JAE-Predoc program and the EU-FP 7 MICROKELVIN project (Grant No. 228464).This is the accepted version of an article originally published in Nature Communications. The final version is available at http://www.nature.com/ncomms/2014/140109/ncomms4048/full/ncomms4048.html. © Nature Publishing Group. Reuse rights are available at http://www.nature.com/authors/policies/license.html

A spin triplet supercurrent through the half-metallic ferromagnet CrO2

In general, conventional superconductivity should not occur in a ferromagnet, though it has been seen in iron under pressure. Moreover, theory predicts that the current is always carried by pairs of electrons in a spin singlet state, so conventional superconductivity decays very rapidly when in contact with a ferromagnet, which normally prohibits the existence of singlet pairs. It has been predicted that this rapid spatial decay would not occur when spin triplet superconductivity could be induced in the ferromagnet. Here we report a Josephson supercurrent through the strong ferromagnet CrO2, from which we infer that it is a spin triplet supercurrent. Our experimental setup is different from those envisaged in the earlier predictions, but we conclude that the underlying physical explanation for our result is a conversion from spin singlet to spin triplets at the interface. The supercurrent can be switched with the direction of the magnetization, analogous to spin valve transistors, and therefore could enable magnetization-controlled Josephson junctions.Comment: 14 pages, including 3 figure

Reconfigurable superconducting vortex pinning potential for magnetic disks in hybrid structures

High resolution scanning Hall probe microscopy has been used to directly visualise the superconducting vortex behavior in hybrid structures consisting of a square array of micrometer-sized Py ferromagnetic disks covered by a superconducting Nb thin film. At remanence the disks exist in almost fully flux-closed magnetic vortex states, but the observed cloverleaf-like stray fields indicate the presence of weak in-plane anisotropy. Micromagnetic simulations suggest that the most likely origin is an unintentional shape anisotropy. We have studied the pinning of added free superconducting vortices as a function of the magnetisation state of the disks, and identified a range of different phenomena arising from competing energy contributions. We have also observed clear differences in the pinning landscape when the superconductor and the ferromagnet are electron ically coupled or insulated by a thin dielectric layer, with an indication of non-trivial vortex-vortex interactions. We demonstrate a complete reconfiguration of the vortex pinning potential when the magnetisation of the disks evolves from the vortex-like state to an onion-like one under an in-plane magnetic field. Our results are in good qualitative agreement with theoretical predictions and could form the basis of novel superconducting devices based on reconfigurable vortex pinning sites

Spectroscopic evidence of odd frequency superconducting order

Spin filter superconducting S/I/N tunnel junctions (NbN/GdN/TiN) show a robust and pronounced zero bias conductance peak at low temperatures, the magnitude of which is several times the normal state conductance of the junction. Such a conductance anomaly is representative of unconventional superconductivity and is interpreted as a direct signature of an odd frequency superconducting order.Comment: 5 pages, 3 figures + supplementary informatio

Superconducting spintronics

The interaction between superconducting and spin-polarized orders has recently emerged as a major research field following a series of fundamental breakthroughs in charge transport in superconductor-ferromagnet heterodevices which promise new device functionality. Traditional studies which combine spintronics and superconductivity have mainly focused on the injection of spin-polarized quasiparticles into superconducting materials. However, a complete synergy between superconducting and magnetic orders turns out to be possible through the creation of spin-triplet Cooper pairs which are generated at carefully engineered superconductor interfaces with ferromagnetic materials. Currently, there is intense activity focused on identifying materials combinations which merge superconductivity and spintronics in order to enhance device functionality and performance. The results look promising: it has been shown, for example, that superconducting order can greatly enhance central effects in spintronics such as spin injection and magnetoresistance. Here, we review the experimental and theoretical advances in this field and provide an outlook for upcoming challenges related to the new concept of superconducting spintronics.J.L. was supported by the Research Council of Norway, Grants No. 205591 and 216700. J.W.A.R. was supported by the UK Royal Society and the Leverhulme Trust through an International Network Grant (IN-2013-033).This is the accepted manuscript. The final version is available at http://www.nature.com/nphys/journal/v11/n4/full/nphys3242.html

Pt and CoB trilayer Josephson π junctions with perpendicular magnetic anisotropy

We report on the electrical transport properties of Nb based Josephson junctions with Pt/Co68B32/Pt ferromagnetic barriers. The barriers exhibit perpendicular magnetic anisotropy, which has the main advantage for potential applications over magnetisation in-plane systems of not affecting the Fraunhofer response of the junction. In addition, we report that there is no magnetic dead layer at the Pt/Co68B32 interfaces, allowing us to study barriers with ultra-thin Co68B32. In the junctions, we observe that the magnitude of the critical current oscillates with increasing thickness of the Co68B32 strong ferromagnetic alloy layer. The oscillations are attributed to the ground state phase difference across the junctions being modified from zero to π. The multiple oscillations in the thickness range 0.2 ⩽ dCoB ⩽ 1.4 nm suggests that we have access to the first zero-π and π-zero phase transitions. Our results fuel the development of low-temperature memory devices based on ferromagnetic Josephson junctions

Remotely induced magnetism in a normal metal using a superconducting spin-valve

Superconducting spintronics has emerged in the past decade as a promising new field that seeks to open a new dimension for nanoelectronics by utilizing the internal spin structure of the superconducting Cooper pair as a new degree of freedom1, 2. Its basic building blocks are spin-triplet Cooper pairs with equally aligned spins, which are promoted by proximity of a conventional superconductor to a ferromagnetic material with inhomogeneous macroscopic magnetization3. Using low-energy muon spin-rotation experiments we find an unanticipated effect, in contradiction with the existing theoretical models of superconductivity and ferromagnetism: the appearance of a magnetization in a thin layer of a non-magnetic metal (gold), separated from a ferromagnetic double layer by a 50-nm-thick superconducting layer of Nb. The effect can be controlled either by temperature or by using a magnetic field to control the state of the remote ferromagnetic elements, and may act as a basic building block for a new generation of quantum interference devices based on the spin of a Cooper pair

Proposal for a phase-coherent thermoelectric transistor

Identifying materials and devices which offer efficient thermoelectric effects at low temperature is a major obstacle for the development of thermal management strategies for low-temperature electronic systems. Superconductors cannot offer a solution since their near perfect electron-hole symmetry leads to a negligible thermoelectric response; however, here we demonstrate theoretically a superconducting thermoelectric transistor which offers unparalleled figures of merit of up to ~ 45 and Seebeck coefficients as large as a few mV/K at sub-Kelvin temperatures. The device is also phase-tunable meaning its thermoelectric response for power generation can be precisely controlled with a small magnetic field. Our concept is based on a superconductor-normal metal-superconductor interferometer in which the normal metal weak-link is tunnel coupled to a ferromagnetic insulator and a Zeeman split superconductor. Upon application of an external magnetic flux, the interferometer enables phase-coherent manipulation of thermoelectric properties whilst offering efficiencies which approach the Carnot limit.This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.489344

Giant triplet proximity effect in superconducting pseudo spin valves with engineered anisotropy

The proximity coupling of a thin superconducting layer and an inhomogeneous ferromagnet can lead to a significant reduction of the critical temperature due to the generation of spin-polarized triplet Cooper pairs. We report critical temperature measurements of Co/Cu/NiFe(Py)/Cu/Nb superconducting pseudo spin valves (PSVs) in which the magnetization of the soft layer (Py) can be independently rotated in-plane with a magnetic field to create an angle (θ) between it and the magnetization of Co. Here we observe results consistent with spin-triplet theory and demonstrate large changes in ΔTC up to −120 mK as the Py layer is rotated from 0° (Co and Py are parallel) to 90° (Co and Py are orthogonal), which offers the potential for active control of the superconducting state. The key to this achievement is engineered magnetic anisotropy in Py, which enables well-defined control over the magnetization configuration of the PSV.The work was funded by the Royal Society, the Leverhulme Trust through an International Network Grant and the European Research Council (AIG "Superspin").This is the accepted manuscript version. The final version is available from APS at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.140508