Ferromagnetic Josephson Junctions with Critical Current Density Artificially Modulated on a “Short” Scale

Abstract

We study the Josephson effect in junctions with a ferromagnetic (F) barrier having its properties (interface transparency or the F-layer thickness) artificially modulated on a scale less than the Josephson penetration length. Within the framework of the quasiclassical Usadel equations, we describe SIFS and SIFNS (S is a superconductor, I is an insulator, N is a normal metal) structures with a step-like transparency of the FS or NS interface. The step-like change in parameters may lead to oscillations (including sign change) of the critical current density J C (y) along the junction in the vicinity of the step, resulting in the formation of a 0–π nano-junction near the step. Such structures exhibit an unusual behaviour in an external magnetic field H. The properties of arrays of nano-junctions with several transparency steps are also investigated. We propose a method to realize a φ Josephson junction by combining alternating 0 and π parts made of “clean” SFS sub junctions with different F-layer thickness and an intrinsically non-sinusoidal current–phase relation (CPR). The latter can significantly enlarge the parameter range of the φ ground state and make the practical realization of φ Josephson junctions feasible. Such junctions may also have two different stable solutions, such as 0 and π, 0 and φ, or φ and π.