High quality ferromagnetic 0 and pi Josephson tunnel junctions

TL;DR

This paper reports the fabrication and characterization of high-quality Nb/Al2O3/Ni0.6Cu0.4/Nb Josephson tunnel junctions with tunable 0 and π ground states, demonstrating low damping and promising parameters for quantum applications.

Contribution

The study introduces high-quality superconductor-insulator-ferromagnet-superconductor Josephson junctions with controlled ground states and detailed theoretical understanding of their parameters.

Findings

Junctions exhibit homogeneous interfaces with near-perfect Fraunhofer patterns.

Critical current density in π state reaches up to 5 A/cm^2 at 2.1 K.

McCumber parameter β_c reaches approximately 700 at 2.1 K.

Abstract

We fabricated high quality $\Nb/\Al_2\O_3/\Ni_{0.6}\Cu_{0.4}/\Nb$ superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic $\Ni_{0.6}\Cu_{0.4}$ layer and on the ambient temperature, the junctions were in the 0 or $\pi$ ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The $\Al_2\O_3$ tunnel barrier allows to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter $\beta_c$ increases exponentially with decreasing temperature and reaches $\beta_c\approx700$ at $T=2.1 {\rm K}$. The critical current density in the $\pi$ state was up to $5\:\rm{A/cm^2}$ at $T=2.1 {\rm K}$, resulting in a Josephson penetration depth $\lambda_J$ as low as $160\:\rm{\mu m}$. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the $\Ni_{0.6}\Cu_{0.4}$ layer and different transparencies of the interfaces.