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Coexistence of superconductivity and magnetism at the nanoscale

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The main mechanisms of the interplay between magnetism and superconductivity and the coexistence between these two different long ranged orders in the bulk magnetic superconductors will be reviewed. The antagonism of ferromagnetism with a singlet superconductivity leads to spectacular effects such as a re-entrant superconductivity and the nanoscopic domain magnetic structure formation. In the case of the weak exchange interaction (or triplet superconductivity), the competition between a self-induced vortex phase and a short-period domain structure may occur.

The very special character of the proximity effect in superconductor-ferromagnet heterostructures is revealed in the damped oscillatory behaviour of the Cooper pair wave function. And the formation of the special π-Josephson junctions is possible. Such “π -junction” incorporated in a superconducting circuit may generate a spontaneous current. The quantum oscillations and the “π”-states should also be present in multiply connected ferromagnet–superconductor hybrids, for example in a thin-walled superconducting shell surrounding a ferromagnetic cylinder.

The proximity effect in S/F structures is usually short-ranged though it can become long ranged when the magnetic structure is non-collinear. Recently it has been demonstrated that the Josephson junctions with a composite ferromagnetic interlayer indeed reveal the triplet long-ranged superconducting current. The triplet superconducting correlations provide the possibility to generate the magnetization in the Josephson junction. Such induced magnetization occurs at a relatively large distance, and is sensitive to the superconducting phase difference. By tuning the Josephson current, one may manipulate the long-range induced magnetic moment. The induced magnetic moment controlled by the Josephson current may therefore be used in spintronics devices instead of the spin-torque effect.

Coupling between the superconducting current and magnetization opens the very interesting perspectives for emerging superconducting spintronics.

Acknowledgement: A.B. Visiting Professorship at University of Cambridge is supported by the Leverhulme Trust Award.

This talk is part of the Theory of Condensed Matter series.

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