Inhomogeneous magnetism and superconductivity
Lead Research Organisation:
University of Cambridge
Department Name: Materials Science & Metallurgy
Abstract
The past fifteen years has seen considerable research into the coupling of superconductivity and magnetism. These two effects are both mediated by coupling between electrons, but ferromagnetism leads to the parallel alignment of spins while conventional (so called spin-singlet) superconductivity requires anti-parallel spin alignment. As a result the coupling of superconductivity into ferromagnets is generally much weaker than the coupling into non-magnetic metals (the so-called proximity effect). However, at very short-range (a few nanometres) the coupling between superconductivity and ferromagnetism at the interface between the two materials results in complex behaviour which is distinct from that of either material. Most notably, the pairs of electrons which are responsible for superconductivity have a rapidly oscillating phase in the ferromagnet which can lead to negative rather than positive supercurrents appearing in devices in which a thin ferromagnetic barrier separates two superconductors. Devices based on this effect are currently being developed for quantum computation. More controversially, a few very recent experiments have detected a much longer-ranged proximity effect in which superconductivity can penetrate a ferromagnet over distances of hundreds on nanometres. This effect seems to be confirmation of theoretical predications that if the magnetism is inhomogeneous (i.e. all the spins do not point in a single direction) or the electrons are 100% spin polarised then a so-called spin-triplet state of superconductivity should appear. The aim of our proposed project is to investigate carefully the conditions required for the formation of this spin-triplet state and to understand how to control it so that potential applications can be developed. In particular we will look at classes of ferromagnet which have a spiral rather than linear magnetic order, we will also grow artificial magnetic structures in which such spirals can be changed by applying a magnetic field and we will explore how the presence of magnetic domain walls (which are regions in which the magnetism changes direction in a material) affects the superconducting properties.
Organisations
People |
ORCID iD |
Mark Blamire (Principal Investigator) | |
Zoe Barber (Co-Investigator) |
Publications
Usman I
(2011)
Evidence for spin mixing in holmium thin film and crystal samples
in Physical Review B
Robinson JW
(2009)
Crossover induced by spin-density-wave interference in the coherence of singlet electron pairs in Cr.
in Physical review letters
Robinson JW
(2010)
Enhanced supercurrents in Josephson junctions containing nonparallel ferromagnetic domains.
in Physical review letters
Robinson JW
(2010)
Controlled injection of spin-triplet supercurrents into a strong ferromagnet.
in Science (New York, N.Y.)
Robinson J
(2009)
Strong ferromagnetic Josephson devices with optimized magnetism
in Applied Physics Letters
Halász G
(2009)
Critical current of a Josephson junction containing a conical magnet
in Physical Review B
Blamire M
(2013)
Field modulation of the critical current in magnetic Josephson junctions
in Superconductor Science and Technology
Description | The superconductor-ferromagnet proximity effect describes the fast decay of a spin-singlet supercurrent originating from the superconductor upon entering the neighboring ferromagnet. After placing a conical magnet (holmium) at the interface between the two, we detected a long-ranged supercurrent in the ferromagnetic layer. The long-range effect required particular thicknesses of the spiral magnetically ordered holmium, consistent with spin-triplet proximity theory. This enabled control of the electron pairing symmetry by tuning the degree of magnetic inhomogeneity through the thicknesses of the holmium injectors. |
First Year Of Impact | 2012 |
Sector | Electronics |
Impact Types | Societal Policy & public services |
Description | EPSRC |
Amount | £330,000 (GBP) |
Funding ID | EP/I038047/1 |
Organisation | Engineering and Physical Sciences Research Council (EPSRC) |
Sector | Public |
Country | United Kingdom |
Start | 12/2011 |
End | 12/2014 |