Ultra-strong coupling of two ferromagnets via Meissner currents

TL;DR

This paper demonstrates ultra-strong magnon-magnon coupling in a superconductor/ferromagnet heterostructure mediated by Meissner currents, with tunable interaction strength and anisotropy, promising for magnonic applications.

Contribution

It reveals a new mechanism for ultra-strong magnon-magnon coupling via Meissner currents in S/F/I/F/S structures, distinct from traditional dipole interactions.

Findings

Magnon-magnon coupling strength exceeds dipole-dipole interactions.

Coupling strength is tunable by temperature below superconducting critical temperature.

Coupling anisotropy is opposite to magnon-photon coupling anisotropy.

Abstract

In this work, we study the magnetization dynamics in a ferromagnet/insulator/ferromagnet trilayer sandwiched between two superconductors (S/F/I/F/S heterostructure). It is well-known that a conceptually similar S/F/S system is a platform for implementing ultra-strong magnon-photon coupling. Here, we demonstrate that in such S/F/I/F/S heterostructure, ultra-strong magnon-magnon coupling between the two F layers also appears. The strength of this interaction is many times greater than the strength of the usual dipole-dipole interaction. It is mediated via Meissner currents excited in the superconductor layers by the magnon stray fields. The strength of the magnon-magnon coupling is anisotropic, and its anisotropy is opposite to the anisotropy of the magnon-photon coupling, which allows them to be separated. Both couplings become much stronger when the temperature drops below the critical temperature of the superconductor layers. It enables the implementation of an efficient tuning of the wavenumber in the S/F/I/F/S heterostructures controlled by temperature in a wide range of frequencies. Overall, the rich and tunable spectrum of S/F/I/F/S multilayers opens broad prospects for their application in magnonics.