Strong coupling of Fe-Co alloy with ultralow damping to superconducting co-planar waveguide resonators

TL;DR

This paper demonstrates strong coupling between a ferromagnetic Fe-Co alloy and superconducting coplanar waveguide resonators, achieving ultrastrong coupling rates and revealing how device dimensions influence coupling strength, with implications for scalable hybrid systems.

Contribution

It reports the first demonstration of ultrastrong coupling between Fe-Co ferromagnetic films and superconducting resonators, with insights into how geometry affects coupling efficiency.

Findings

Coupling rate scales with the square root of the number of spins.

Achieved coupling rate over 700 MHz, nearing ultrastrong coupling.

Reducing the conductor width increases coupling and cooperativity.

Abstract

We report on the strong coupling between a metallic ferromagnetic Fe75Co25 thin film patterned element and a range of superconducting Nb half-wavelength co-planar waveguide (CPW) resonators. By varying the volume of the ferromagnet we demonstrate that the coupling rate scales linearly with the square root of the number of spins and achieve a coupling rate over 700 MHz, approaching the ultrastrong coupling regime. Experiments varying the center conductor width while maintaining constant magnetic volume verify that decreasing the center conductor width increases coupling and cooperativity. Our results show that the frequency dependence of the coupling rate is linear with the fundamental and higher order odd harmonics of the CPW, but with differing efficiencies. The results show promise for scaling planar superconducting resonator/magnetic hybrid systems to smaller dimensions.