Critical Current Oscillations of Elliptical Josephson Junctions with Single-Domain Ferromagnetic Layers

TL;DR

This study investigates how the critical current in elliptical Josephson junctions with ferromagnetic layers oscillates with barrier thickness, revealing phase transitions between 0 and π states, which are crucial for cryogenic memory and quantum computing.

Contribution

The paper provides experimental evidence of critical current oscillations and phase shifts in Nb-based elliptical Josephson junctions with specific ferromagnetic barriers, enhancing understanding of 0-$f{ ext{π}}$ transitions.

Findings

Critical current oscillates with ferromagnetic layer thickness.

Fraunhofer patterns indicate single-domain magnetic behavior.

Transitions between 0 and π phase states are observed.

Abstract

Josephson junctions containing ferromagnetic layers are of considerable interest for the development of practical cryogenic memory and superconducting qubits. Such junctions exhibit a phase shift of $\pi$ for certain ranges of ferromagnetic layer thickness. We present studies of Nb based micron-scale elliptically-shaped Josephson junctions containing ferromagnetic barriers of Ni$_{81}$Fe$_{19}$ or Ni$_{65}$Co$_{20}$Fe$_{15}$. By applying an external magnetic field, the critical current of the junctions are found to follow characteristic Fraunhofer patterns, and display sharp switching behavior suggestive of single-domain magnets. The high quality of the Fraunhofer patterns enables us to extract the maximum value of the critical current even when the peak is shifted significantly outside the range of the data due to the magnetic moment of the ferromagnetic layer. The maximum value of the critical current oscillates as a function of the ferromagnetic barrier thickness, indicating transitions in the phase difference across the junction between values of zero and $\pi$. We compare the data to previous work and to models of the 0-$\pi$ transitions based on existing theories.