/Nb Josephson junctions*

Josephson junctions with a ferromagnetic barrier (F-JJ) so-called π-junctions have attracted a great deal of interests because their rich physics concept orientating from the co-existence of magnetism and superconductivity allows to develop novel electronics which are potentially used in cryogenic memory and superconducting circuits. More recently, some theoretical studies on ferromagnetic insulator-based Josephson junctions (FI-JJ) have been proposed to develop new type π-junctions which are promising candidates to avoid strong dissipation associated with low-energy quasiparticle excitations in ferromagnetic metals and to increase the voltage jump across the junction as well. To prove the viability of the theoretical approaches, we have established an experimental program aiming at developing FI-JJ thin-film platforms consisting of two Niobium superconducting electrodes separated by a magnetite (Fe₃O₄) tunneling barrier. For this purpose, we have sequentially grown a series of Nb/Fe₃O₄/Nb epitaxial thin films with gradually varying thickness of the Fe₃O₄ barrier from 2 nm to 15 nm and fabricated small junction arrays on the thin films. Then a series of out-of-plane transport measurements across the junctions have been carried out at various sample temperatures, and magnetic fields to investigate the characteristic features of π-junctions such as superconducting phase shift and super-spin current. We will discuss the prerequisite signatures of the π-junctions in their transport characteristics.