Josephson Coupling and Fiske Dynamics in Ferromagnetic Tunnel Junctions

TL;DR

This study reports on high-quality Nb/AlO_x/PdNi/Nb Josephson junctions exhibiting 0- to -coupling transition, analyzing their Fiske modes and damping mechanisms, with implications for quantum circuits.

Contribution

The paper presents the fabrication and detailed analysis of high-quality ferromagnetic Josephson junctions with tunable coupling and high-frequency damping characterization.

Findings

Transition from 0- to -coupling at d_F  6 nm

High quality factors up to 30 at 200 GHz

Damping dominated by surface resistance at high frequencies

Abstract

We report on the fabrication of Nb/AlO_x/Pd_{0.82}Ni_{0.18}/Nb superconductor/insulator/ferromagnetic metal/superconductor (SIFS) Josephson junctions with high critical current densities, large normal resistance times area products, high quality factors, and very good spatial uniformity. For these junctions a transition from 0- to \pi-coupling is observed for a thickness d_F ~ 6 nm of the ferromagnetic Pd_{0.82}Ni_{0.18} interlayer. The magnetic field dependence of the \pi-coupled junctions demonstrates good spatial homogeneity of the tunneling barrier and ferromagnetic interlayer. Magnetic characterization shows that the Pd_{0.82}Ni_{0.18} has an out-of-plane anisotropy and large saturation magnetization, indicating negligible dead layers at the interfaces. A careful analysis of Fiske modes provides information on the junction quality factor and the relevant damping mechanisms up to about 400 GHz. Whereas losses due to quasiparticle tunneling dominate at low frequencies, the damping is dominated by the finite surface resistance of the junction electrodes at high frequencies. High quality factors of up to 30 around 200 GHz have been achieved. Our analysis shows that the fabricated junctions are promising for applications in superconducting quantum circuits or quantum tunneling experiments.