344 research outputs found
Controlling the superconducting transition by spin-orbit coupling
Whereas there exists considerable evidence for the conversion of singlet
Cooper pairs into triplet Cooper pairs in the presence of inhomogeneous
magnetic fields, recent theoretical proposals have suggested an alternative way
to exert control over triplet generation: intrinsic spin-orbit coupling in a
homogeneous ferromagnet coupled to a superconductor. Here, we proximity-couple
Nb to an asymmetric Pt/Co/Pt trilayer, which acts as an effective spin-orbit
coupled ferromagnet owing to structural inversion asymmetry. Unconventional
modulation of the superconducting critical temperature as a function of
in-plane and out-of- plane applied magnetic fields suggests the presence of
triplets that can be controlled by the magnetic orientation of a single
homogeneous ferromagnet. Our studies demonstrate for the first time an active
role of spin-orbit coupling in controlling the triplets -- an important step
towards the realization of novel superconducting spintronic devices.Comment: 11 pages + 4 figures + supplemental informatio
Critical current of a Josephson junction containing a conical magnet
We calculate the critical current of a
superconductor/ferromagnetic/superconductor (S/FM/S) Josephson junction in
which the FM layer has a conical magnetic structure composed of an in-plane
rotating antiferromagnetic phase and an out-of-plane ferromagnetic component.
In view of the realistic electronic properties and magnetic structures that can
be formed when conical magnets such as Ho are grown with a polycrystalline
structure in thin-film form by methods such as direct current sputtering and
evaporation, we have modeled this situation in the dirty limit with a large
magnetic coherence length (). This means that the electron mean free
path is much smaller than the normalized spiral length which in
turn is much smaller than (with as the length a complete
spiral makes along the growth direction of the FM). In this physically
reasonable limit we have employed the linearized Usadel equations: we find that
the triplet correlations are short ranged and manifested in the critical
current as a rapid oscillation on the scale of . These rapid
oscillations in the critical current are superimposed on a slower oscillation
which is related to the singlet correlations. Both oscillations decay on the
scale of . We derive an analytical solution and also describe a
computational method for obtaining the critical current as a function of the
conical magnetic layer thickness.Comment: Extended version of the published paper. Additional information about
the computational method is included in the appendi
Whole-brain patterns of 1H-magnetic resonance spectroscopy imaging in Alzheimer's disease and dementia with Lewy bodies
Acknowledgements We thank Craig Lambert for his help in processing the MRS data. The study was funded by the Sir Jules Thorn Charitable Trust (grant ref: 05/JTA) and was supported by the National Institute for Health Research (NIHR) Newcastle Biomedical Research Centre and the Biomedical Research Unit in Lewy Body Dementia based at Newcastle upon Tyne Hospitals National Health Service (NHS) Foundation Trust and Newcastle University and the NIHR Biomedical Research Centre and Biomedical Research Unit in Dementia based at Cambridge University Hospitals NHS Foundation Trust and the University of Cambridge.Peer reviewedPublisher PD
Depairing critical current achieved in superconducting thin films with through-thickness arrays of artificial pinning centers
Large area arrays of through-thickness nanoscale pores have been milled into
superconducting Nb thin films via a process utilizing anodized aluminum oxide
thin film templates. These pores act as artificial flux pinning centers,
increasing the superconducting critical current, Jc, of the Nb films. By
optimizing the process conditions including anodization time, pore size and
milling time, Jc values approaching and in some cases matching the
Ginzburg-Landau depairing current of 30 MA/cm^2 at 5 K have been achieved - a
Jc enhancement over as-deposited films of more than 50 times. In the field
dependence of Jc, a matching field corresponding to the areal pore density has
also been clearly observed. The effect of back-filling the pores with magnetic
material has then been investigated. While back-filling with Co has been
successfully achieved, the effect of the magnetic material on Jc has been found
to be largely detrimental compared to voids, although a distinct influence of
the magnetic material in producing a hysteretic Jc versus applied field
behavior has been observed. This behavior has been tested for compatibility
with currently proposed models of magnetic pinning and found to be most closely
explained by a model describing the magnetic attraction between the flux
vortices and the magnetic inclusions.Comment: 9 pages, 10 figure
Spin-transfer switching and low-field precession in exchange-biased spin valve nano-pillars
Using a three-dimensional focused-ion beam lithography process we have
fabricated nanopillar devices which show spin transfer torque switching at zero
external magnetic fields. Under a small in-plane external bias field, a
field-dependent peak in the differential resistance versus current is observed
similar to that reported in asymmetrical nanopillar devices. This is
interpreted as evidence for the low-field excitation of spin waves which in our
case is attributed to a spin-scattering asymmetry enhanced by the IrMn exchange
bias layer coupled to a relatively thin CoFe fixed layer.Comment: 11 pages, 4 figures. To appear in APL, April 200
Critical Current Oscillations in Strong Ferromagnetic Pi-Junctions
We report magnetic and electrical measurements of Nb Josephson junctions with
strongly ferromagnetic barriers of Co, Ni and Ni80Fe20 (Py). All these
materials show multiple oscillations of critical current with barrier thickness
implying repeated 0-pi phase-transitions in the superconducting order
parameter. We show in particular that the Co barrier devices can be accurately
modelled using existing clean limit theories and so that, despite the high
exchange energy (309 meV), the large IcRN value in the pi-state means Co
barriers are ideally suited to the practical development of superconducting
pi-shift devices.Comment: 4 pages 3 figures 1 table. Revised version as accepted for
publication. To appear in Physical Review Letter
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