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

Journal name:
Nature Physics
Year published:
DOI:
doi:10.1038/nphys3486
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Accepted
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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.

At a glance

Figures

  1. Sample architecture and experimental arrangement.
    Figure 1: Sample architecture and experimental arrangement.

    Schematic of the sample architecture (NSFnF), centred between the positron detectors within a homogeneous applied field (Hext) along the z-direction. The momentum (p) of the incoming muon (μ) is normal to the sample plane (along the y-direction) and its initial spin (s) points towards the left positron detector. The direction of the exchange field of the (free) F layer closest to the S layer is saturated along the applied field direction, whereas the second (pinned) F layer is always directed along the pinning direction from the anti-ferromagnet (Hpin). The sample orientations used were either with Hpin aligned with Hext (collinear arrangement) or perpendicular to it (orthogonal arrangement). Muon stopping profiles are overlaid on the front face of the sample to indicate the probability distribution for muons with increasing energies between 4 and 24keV with 4keV steps. The higher the energy the further the muons penetrate on average into the sample, but this also broadens the profile. Up to 12keV all muons stop within the N layer and only for higher energies does an increasing fraction stop within the S layer.

  2. Fit results to LE-[mu]SR data on the NSFnF architecture.
    Figure 2: Fit results to LE-μSR data on the NSFnF architecture.

    a, Magnetic flux profile B(y) obtained from fitting all data simultaneously (at fixed temperature), for both the collinear () and orthogonal () arrangement. Red for T = 10K and blue for T = 3K. For the latter an exponentially decaying model function was used whereas the former is taken to be constant. b, Average magnetic flux left fenceBright fence(left fenceyright fence) obtained from fitting each data set individually (that is, the conventional treatment) compared to the calculated values from the profiles of a. Top axis shows the corresponding muon energies of the data points. c, Temperature dependence of the average flux left fenceBright fence in the orthogonal arrangement, taken at a muon energy of 12keV (muon stopping profile shown in inset) to ensure all muons stopped in the Au layer. In b and c, the error bars indicate the asymptotic standard error in left fenceBright fence.

  3. Thin Au cap sample.
    Figure 3: Thin Au cap sample.

    Difference between the induced magnetic flux at T = 3K and that at T = 10K, with error bars indicating the asymptotic standard error in left fenceBright fence3Kleft fenceBright fence10K, for the NSFnF architecture with a very thin 5nm N (Au) cap in the orthogonal arrangement (shown with muon stopping profiles overlaid on the front face). The highest energy (12keV) includes contributions from the n-spacer. It is only in the region of the FnF interface that any difference is detected between above and below Tc.

  4. Spin-transfer mechanisms.
    Figure 4: Spin-transfer mechanisms.

    Schematic of the proposed mechanisms to transfer spin across the superconductor (S) with gap energy Δ when there is a spin accumulation in the ferromagnet (F) resulting in a shift between the chemical potentials μ of the spin-up and spin-down band. a, During a crossed Andreev reflection (CAR) a singlet Cooper pair (CP) is created from an electron at energy +ε with spin down (+ε) originating from the F layer and an electron at energy −ε with spin up (−ε) originating from the normal metal (N) layer (blue arrows). CAR can also annihilate a CP by donating electron +ε into the N and −ε into the F layer (red arrows). b, During an elastic co-tunnelling (EC) process a singlet CP attracts electron +ε from the F layer while simultaneously donating its own +ε electron into the N layer (blue arrows). EC can also attract electron −ε from the N layer and donate its own electron −ε into the F layer (red arrows). c, A flow of polarized (triplet) Cooper pairs can transfer spin across the S layer, without generating a moment inside the S layer. Triplet pairs of +ε electrons move from the F to the N layer while an equal flow of triplet pairs of −ε electrons move from the N to the F layer.

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Author information

Affiliations

  1. School of Physics and Astronomy, SUPA, University of St Andrews, St Andrews KY16 9SS, UK

    • M. G. Flokstra &
    • S. L. Lee
  2. School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK

    • N. Satchell,
    • J. Kim &
    • G. Burnell
  3. Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK

    • P. J. Curran &
    • S. J. Bending
  4. ISIS, Rutherford Appleton Laboratory, Oxfordshire OX11 0QX, UK

    • J. F. K. Cooper,
    • C. J. Kinane &
    • S. Langridge
  5. SEPnet and Hubbard Theory Consortium, Department of Physics, Royal Holloway, University of London Egham, Surrey TW20 0EX, UK

    • A. Isidori,
    • N. Pugach &
    • M. Eschrig
  6. Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University (SYNP MSU), Leninskie Gory, Moscow 119991, Russia

    • N. Pugach
  7. Labor für Myonspinspektroskopie, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

    • H. Luetkens,
    • A. Suter &
    • T. Prokscha

Contributions

J.K. and G.B. developed the samples; M.G.F., S.L.L., N.S., J.F.K.C., H.L. and T.P. performed the muon measurements, in which H.L., A.S. and T.P. provided the beamline support; M.G.F., S.L.L., N.S., J.F.K.C., P.J.C., S.J.B., C.J.K. and S.L. performed various support and characterization measurements; A.I., N.P. and M.E. provided theoretical interpretation of the data and helped writing the paper; G.B. and M.E. helped designing the study; M.G.F. and S.L.L. designed the study, analysed data and wrote the paper. All authors discussed the results and commented on the manuscript.

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The authors declare no competing financial interests.

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