Investigation Of The Sputtering Conditions Of Niobium And Magnetite Thin Films For Realizing Ferromagnetic Josephson Junctions

Ferromagnetic Josephson junctions have attracted a great deal of interest for superconducting electronics and cryogenic memory devices since their sandwiched heterostructures exhibit the coexistence of superconductivity and magnetism, which comprises rich physics concepts such as Fulde-Ferrell-Larkin-Ovchinnikov pairing, Andreev bound states, anomalous current-phase behavior due to phase shifting, spin triplet supercurrent valving due to ferromagnetic tunneling domains and mutual proximity effects. These versatile junctions offer new approaches in developing new cryogenic quantum devices such as phase qubits, spin valve-based memory circuits and phase shifters for programmable logic circuits. In this work, an optimized sputtering system have been systematically studied to achieve high superconducting characteristics in niobium layers and ferromagnetic-insulator characteristics in iron oxide layers for ultimately realizing new-type π-junctions. For this purpose, a series of niobium and iron oxide thin films on periclase and sapphire substrates have been grown under various magnetron sputtering conditions from niobium and magnetite targets respectively. Their structural electrical and magnetic characteristics have been analyzed by X-ray diffraction, Raman spectroscopy, vibrating sample magnetometry, magnetic force microscopy, and electrical transport measurements. The best growth condition was employed to deposit each layer of coplanar Nb/Fe3O4/Nb junctions. In addition to the film deposition by sputtering, thermal and e-beam evaporation, electron beam lithography and non-reactive ion etching was employed to form the coplanar junction arrays. The resulting effects of different in-situ growing temperatures on surface morphology, crystal structure, electrical and magnetic responses were investigated for iron oxide thin films. The effects of crystalline quality and size on the superconductive transition of niobium thin films are discussed. For electrical characterization of the coplanar Josephson junctions, 4-point transport measurements are carried out under various magnetic fields in a cryogenic system from room temperature down to 2 K. The prerequisite signatures of π-junctions were investigated in their Fraunhofer patterns, and temperature dependence of Ic and IcRN products. The effects of the growth and fabrication conditions on these properties are discussed in order to realize ideal ferromagnetic-insulator based Josephson junction layers for applications of cryogenic memory and phase qubits