Observation of spin-triplet superconductivity in Co-based Josephson Junctions (original) (raw)
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Spin-triplet supercurrent in Co-based Josephson junctions
2011
Abstract In the past year several groups have reported experimental evidence for spin-triplet supercurrents in Josephson junctions containing strong ferromagnetic materials. In this paper we present several new experimental results that follow up on our previous work. We study Josephson junctions of the form S/X/N/SAF/N/X/S, where S is a superconductor (Nb), N is a normal metal, SAF is a synthetic antiferromagnet of the form Co/Ru/Co and X is an ferromagnetic layer necessary to induce spin-triplet correlations in the structure.
Spin-triplet supercurrents in Josephson junctions containing strong ferromagnetic materials
Philosophical transactions. Series A, Mathematical, physical, and engineering sciences, 2018
The proximity effect between a superconducting material and a non-superconducting normal metal can extend over distances of the order of micrometres at sufficiently low temperatures. If the normal metal is replaced by a ferromagnetic material, the spatial extent of the proximity effect drops precipitously due to the exchange splitting between the majority and minority spin bands in the ferromagnet. In 2001, several theorists predicted that spin-triplet pair correlations could be induced in proximity systems involving multiple ferromagnetic materials (or multiple domains in one material) with non-collinear magnetizations. Such spin-triplet pair correlations should extend deep into the ferromagnet, producing a long-range proximity effect. In this paper, we review our experimental work in this area, which has focused primarily on Josephson junctions containing strong ferromagnetic materials. We show that Josephson junctions containing particular combinations of strong ferromagnetic mat...
Supercurrent in ferromagnetic Josephson junctions with heavy metal interlayers
2018
The lengthscale over which supercurrent from conventional BCS, s-wave, superconductors (S) can penetrate an adjacent ferromagnetic (F) layer depends on the ability to convert singlet Cooper pairs into triplet Cooper pairs. Spin aligned triplet Cooper pairs are not dephased by the ferromagnetic exchange interaction, and can thus penetrate an F layer over much longer distances than singlet Cooper pairs. These triplet Cooper pairs carry a dissipationless spin current and are the fundamental building block for the fledgling field of superspintronics. Singlet-triplet conversion by inhomogeneous magnetism is well established. Here, we describe an attempt to use spin orbit coupling as a new mechanism to mediate singlet-triplet conversion in S-F-S Josephson junctions. We report that the addition of thin Pt spin-orbit coupling layers in our Josephson junctions significantly increases supercurrent transmission, however the decay length of the supercurrent is not found to increase. We attribut...
11 Area-dependence of spin-triplet supercurrent in ferromagnetic Josephson junctions
2016
In 2010, several experimental groups obtained compelling evidence for spin-triplet supercurrent in Josephson junctions containing strong ferromagnetic materials. Our own best results were obtained from large-area junctions containing a thick central Co/Ru/Co "synthetic antiferromagnet" and two thin outer layers made of Ni or PdNi alloy. Because the ferromagnetic layers in our samples are multi-domain, one would expect the sign of the local current-phase relation inside the junctions to vary randomly as a function of lateral position. Here we report measurements of the area dependence of the critical current in several samples, where we find some evidence for those random sign variations. When the samples are magnetized, however, the critical current becomes clearly proportional to the area, indicating that the current-phase relation has the same sign across the entire area of the junctions.
Area-dependence of spin-triplet supercurrent in ferromagnetic Josephson junctions
arXiv (Cornell University), 2011
In 2010, several experimental groups obtained compelling evidence for spin-triplet supercurrent in Josephson junctions containing strong ferromagnetic materials. Our own best results were obtained from large-area junctions containing a thick central Co/Ru/Co "synthetic antiferromagnet" and two thin outer layers made of Ni or PdNi alloy. Because the ferromagnetic layers in our samples are multi-domain, one would expect the sign of the local current-phase relation inside the junctions to vary randomly as a function of lateral position. Here we report measurements of the area dependence of the critical current in several samples, where we find some evidence for those random sign variations. When the samples are magnetized, however, the critical current becomes clearly proportional to the area, indicating that the current-phase relation has the same sign across the entire area of the junctions.
Spin-polarized triplet supercurrent in Josephson junctions with perpendicular ferromagnetic layers
2020
Josephson junctions containing three ferromagnetic layers with non-collinear magnetizations between adjacent layers carry spin-triplet supercurrent under certain conditions. The signature of the spin-triplet supercurrent is a relatively slow decay of the maximum supercurrent as a function of the thickness of the middle ferromagnetic layer. In this work we focus on junctions where the middle magnetic layer is a [Co/Pd]$_N$ multilayer with perpendicular magnetic anisotropy (PMA), while the outer two layers have in-plane anisotropy. We compare junctions where the middle PMA layer is or is not configured as a synthetic antiferromagnet (PMA-SAF). We find that the supercurrent decays much more rapidly with increasing the number NNN of [Co/Pd] bilayers in the PMA-SAF junctions compared to the PMA junctions. Similar behavior is observed in junctions containing [Co/Ni]$_N$ PMA multilayers. We model that behavior by assuming that each Co/Pd or Co/Ni interface acts as a partial spin filter, so...
2017
We present measurements of Josephson junctions containing three magnetic layers with noncolinear magnetizations. The junctions are of the form S/F^'/N/F/N/F^"/S, where S is superconducting Nb, F^' is either a thin Ni or Permalloy layer with in-plane magnetization, N is the normal metal Cu, F is a synthetic antiferromagnet (SAF) with magnetization perpendicular to the plane, composed of Pd/Co multilayers on either side of a thin Ru spacer, and F^" is a thin Ni layer with in-plane magnetization. The supercurrent in these junctions decays more slowly as a function of the F-layer thickness than for similar spin-singlet junctions not containing the F^' and F^" layers. The slower decay is the prime signature that the supercurrent in the central part of these junctions is carried by spin-triplet pairs. The junctions containing F^'= Permalloy are suitable for future experiments where either the amplitude of the critical current or the ground-state phase differ...
Optimization of Spin-Triplet Supercurrent in Ferromagnetic Josephson Junctions
Physical Review Letters, 2012
In the past year, several groups have observed evidence for long-range spin-triplet supercurrent in Josephson junctions containing ferromagnetic (F) materials. In our work, the spin-triplet pair correlations are created by non-collinear magnetizations between a central Co/Ru/Co "synthetic antiferromagnet" (SAF) and two outer thin F layers. Here we present data showing that the spin-triplet supercurrent is enhanced up to 20 times after our samples are subject to a large in-plane magnetizing field. This surprising result can be explained if the Co/Ru/Co SAF undergoes a "spin-flop" transition, whereby the two Co layer magnetizations end up perpendicular to the magnetizations of the two thin F layers. Direct experimental evidence for the spin-flop transition comes from scanning electron microscopy with polarization analysis and from spin-polarized neutron reflectometry.