Escape Rate Measurements and Microwave Spectroscopy of 0, pi, and 0-pi ferromagnetic Josephson Tunnel Junctions

TL;DR

This study investigates the microwave spectral properties and escape rates of various ferromagnetic Josephson junctions, revealing their eigenfrequencies, nonlinear responses, and temperature-dependent switching behaviors through experiments and simulations.

Contribution

It provides the first detailed microwave spectroscopy analysis of 0, pi, and 0-pi ferromagnetic Josephson junctions, including their eigenfrequencies and nonlinear dynamics.

Findings

Observation of harmonic, subharmonic, and superharmonic pumping.

Temperature-dependent decrease and saturation of switching current histogram width.

Comparison of experimental data with numerical simulations highlighting model discrepancies.

Abstract

We present experimental studies of high quality underdamped 0, pi, and 0-pi ferromagnetic Josephson tunnel junctions of intermediate length L (lambda_J < L < 5 lambda_J, where lambda_J is the Josephson penetration depth). The junctions are fabricated as Nb/Al_2O_3/Cu_40Ni_60/Nb Superconductor-Insulator-Ferromagnet-Superconductor heterostructures. Using microwave spectroscopy, we have investigated the eigenfrequencies of 0, pi, and 0-pi Josephson junctions in the temperature range 1.9K...320mK. Harmonic, subharmonic and superharmonic pumping is observed in experiment, and the experimental data are compared with numerical simulations. Escape rate measurements without applied microwaves at temperatures T down to 20mK show that the width of the switching current histogram decreases with temperature and saturates below T=150mK. We analyze our data in the framework of the short junction model. The differences between experimental data and theoretical predictions are discussed.