Nodal superconducting exchange coupling

Abstract

A superconducting spin valve consists of a thin-film superconductor between two ferromagnetic layers. A change of magnetization alignment shifts the superconducting transition temperature (ΔΤc) due to an interplay between the magnetic exchange energy and the superconducting condensate. The magnitude of ΔΤc scales inversely with the superconductor thickness (dS) and is zero when dS exceeds the superconducting coherence length (ξ). Here, we report a superconducting spin-valve effect involving a different underlying mechanism in which magnetization alignment and ΔΤc are determined by nodal quasiparticle excitation states on the Fermi surface of the d-wave superconductor YBa2Cu3O7–δ sandwiched between insulating layers of ferromagnetic Pr0.8Ca0.2MnO3. We observe ΔΤc values that approach 2 K with the sign of ΔΤc oscillating with dS over a length scale exceeding 100ξ and, for particular values of dS, the superconducting state reinforces an antiparallel magnetization alignment. These results pave the way to all-oxide superconducting memory in which superconductivity modulates the magnetic state.

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Fig. 1: Structure of PCMO (50 nm)/YBCO (5 u.c.)/PCMO (100 nm).
Fig. 2: Electronic and magnetic properties of PCMO/YBCO/PCMO trilayers.
Fig. 3: Exchange coupling in PCMO/YBCO/PCMO.
Fig. 4: Different types of superconducting spin valve.

Data availability

Supporting research data has been deposited in the University of Cambridge research repository and is publicly available at https://doi.org/10.17863/CAM.37835.

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Acknowledgements

A.D.B., S.K. and J.W.A.R. acknowledge funding from the EPSRC through the International Network grant no. EP/N017242/1. A.D.B. acknowledges funding from St John’s College, Cambridge. J.W.A.R. acknowledges funding from the Royal Society.

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Contributions

J.W.A.R. supervised the project and with A.D.B. conceived and designed it. A.D.B. prepared the samples and performed electrical and magnetic measurements with help from S.K. TEM imaging and compositional analysis were performed by G.D. The theoretical model was developed by M.C and P.G. with support from G.L. and input from A.D.B. and J.W.A.R. The paper was written by A.D.B., J.W.A.R. and M.C. with input from all authors.

Corresponding authors

Correspondence to A. Di Bernardo or J. W. A. Robinson.

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Supplementary Information

Supplementary Figs. 1–19, Notes 1–4 and refs. 1–31.

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Di Bernardo, A., Komori, S., Livanas, G. et al. Nodal superconducting exchange coupling. Nat. Mater. 18, 1194–1200 (2019). https://doi.org/10.1038/s41563-019-0476-3

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