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Spin triplet pairings in ferromagnet Josephson junctions

Periodic Reporting for period 1 - SPIN3 (Spin triplet pairings in ferromagnet Josephson junctions)

Reporting period: 2015-04-01 to 2017-03-31

For a long time, the coexistence of conventional superconductivity and ferromagnetism was believed to be impossible. In 2001 it was theoretically predicted that under certain conditions both phases could coexist in hybrid structures , giving rise to a race for the discovery of an entirely new kind of superconducting electron pairing state in which the electrons are in the triplet state. The novel hypothesis of this Action relies on the fact that triplet pairs can be formed combining ferromagnets, normal metals and superconductors which will represent an enormous improvement in comparison to the presently established spin-singlet-based devices.

This Action consists of two supplementary stages starting from the maximization of spin-triplet current densities in hybrid ferromagnet junctions (materials science) to the understanding of the basic mechanisms of the spin triplet pairs and the nanofabrication of hybrid Josephson junctions in which the spin triplet supercurrent will be controlled (condensed matter physics). Once the objectives of this Action will be achieved, besides its inherent immediate impact on spintronics and condensed matter, the generation of a radically new technology will emerge. This new technological paradigm, the superconducting spintronics , will take advantage of the unique properties of the two macroscopic phases that were believed to be incompatible and has the potential to overcome significant limitations of logic circuits based separately on superconductivity and spintronics.

The ultimate goal of this action focuses on the experimental study on unconventional electronic properties of charge transport in hybrid devices combining superconductors (S), ferromagnets (F) and normal metals (N) towards the complete understanding of spin-triplet-currents and spin-triplet-dynamics in such junctions. The action is inherently multidisciplinary and it will develop by combining a technological/materials science approach, focused on the maximization of the triplet spin currents in SFS and SF-N-FS junctions with a more fundamental/condensed matter physics one, where the spin triplet dynamics in S/F/N junctions will be studied by drawing on insights from the field of spintronics and superconductivity.
"From the beginning of the project I have been growing and characterising ferrimagnetic insulators like YIG, TmIG, odd-frequency superconductors and half-metallic manganites like by pulsed laser deposition (PLD). The properties have been extensively characterised by means of x-ray diffraction and x-ray reflectivity, to specify its crystal orientation and thickness, by atomic force microscope, for the surface roughness, and by vibrating sample magnetometry (VSM), low temperature magnetometry using a commercial SQUID, and ferromagnetic resonance (FMR) to extract their magnetic properties when present.

Unconventional superconductivity has been found in PCCO/graphenes performing local scanning tunnelling microscopy (STM) and spectroscopy, we have observed an intrinsic p-wave superconducting density of states (DoS) in single layer graphene. As a result from the odd-frequency pairing of the graphene/PCCO sample, a spin-triplet (spin polarized) DoS arises in the graphene.

For YIG I have obtained the best growing conditions (temperature, laser energy and O2 pressure) for atomically flat layer-by-layer deposition of thin (<100 nm) films of YIG on GGG. The resulting material shows outstanding properties in terms of roughness and magnetisation per unit volume since for such low thicknesses. The generation of Superconductor/ferromagnetic (Al or Nb) multilayers aiming to produce tunnel junctions is being explored. SF layers will be the building block for the creation of spin polarized (spin triplets). A detailed characterization of the effect that the YIG (F-layer) has on the transition temperature on the superconducting layer as a function of the thickness of the S has been performed. We have observed striking properties of bare YIG such as deformation blockage of the unit cell when growing the film below 50nm which enhances the magnetization per unit volume noticeably.
Outreach/offline roadshows

During the whole duration of the Action I have shown the results/concepts of the MC action in different outreach events. During the Cambridge Science Festival: 19th March 2015 and 21st March 2016 I have lectured ""Approaching the absolute zero"" where a broad audience of +200 were present in a demonstration of how superconductivity works and its application with ferromagnetic materials. Also, during the Science at work (September 2015) UK public school's students came to visit the different department at the University. I showed them the main research lines of the group during a series of lectures with different schools. On June the 16th 2015 I presented the masterlines of the MC action in the Armourers and Brassiers meeting where external actors (publishers, researchers from other institutions and benefactors were present). Finally, I was selected to show the results obtained during the MC action to the NSF/WTEC panel who visit the Cambridge Graphene Centre in 2015. The WTEC is the World Technology Evaluation Center and it consists in a panel of experts in condensed matter physics based on the United States of America.

Dissemination events

During the first year of the MC action I have attended several international conferences where the results obtained during the action have been presented both in poster and oral presentations:

I have also given invited presentations in several international universities worldwide.

I have attracted and supervised part III students (Ben T. Walker and James Devine-Stoneman) a NanoDTC Ph.D student (Lauren McKenzie-Shell) and I am currently the co-supervisor of a Ph.D. student (Yang Li) all of them from the University of Cambridge

I have been involved in the creation of the Superspin EPSRC grant which has been awarded with a budget of 2.5Mpounds for the next 5 years. This project will study the generation of spin triplets in SF junctions and will be the natural evolution and continuation of the MC action SPIN3. My salary will be covered for extra 5 years once the MC action will be finished."
The quality of our as grown YIG thin films have turned out to be extraordinary. For a 10 nm thick film we have obtained a Gilbert Damping parameter of 10^-3 which turns out to be within the state of the art obtained so far in YIG thin films grown by PLD with no ex situ postannealing and/or treatment of the surface. The high quality of the material has served us to start new collaborations with different group within Europe: Dr. A.J.Ferguson and Prof. H.Sirringhaus in Hitachi Laboratory in the University of Cambridge, Prof. M. April in the LPS-Orsay, France, and Prof. E. Garcia-Michel and Dr. A. Mascaraque in Madrid, Spain. With this collaborations, a new bunch of effects like spin pumping, spin triplet generation in tunnel junctions and the kinetics in the growth of high spin-orbit materials on YIG will be studied. These collaborations are creating new links across different groups within Europe which have never worked before together, hence, they will definitively create a network of laboratories working in the same direction in the field of spintronics and S/F junctions.
Hall bar on graphene/YIG deposited on GGG 110-oriented substrate
Hall bar device in a WS2/graphene/YIG device. WS2 will be used to lift the SO coupling in graphene
AFM image of a typical YIG on GGG110. Terraces with atomic steps of 4.5A appear.