Nanoscale Domain Wall Engineered Spin-Triplet Josephson Junctions and SQUID.

Spin-singlet Cooper pairs convert to spin-triplet Cooper pairs on passing through a magnetically noncollinear structure at a superconductor(S)/ferromagnet(F) interface. In this context, the generation of triplet supercurrents through intrinsic ferromagnetic domain walls, which are naturally occurring noncollinear magnetic features, was proposed theoretically in the past decade. However, an experimental demonstration has been lacking in the literature, particularly because of the difficulty in accessing a single domain wall, which is typically buried between two domains in a ferromagnetic material. By patterning a ferromagnetic nanoconstriction, we have been able to realize a nanoscale S/F/S planar junction, where a single domain wall (pinned at the nanoconstriction) acts as a Josephson barrier. In this geometry, we are able to show the predicted long-range triplet supercurrent across a ferromagnetic barrier exceeding 70 nm. Using this technique, we have demonstrated a ferromagnetic planar nano-SQUID device consisting of two Nb/Ni/Nb spin-triplet Josephson junctions.

Superconducting exchange coupling between ferromagnets.

Recent discoveries from superconductor (S)/ferromagnet (FM) heterostructures include π-junctions, triplet pairing, critical temperature (Tc) control in FM/S/FM superconducting spin valves (SSVs) and critical current control in S/FM/N/FM/S spin valve Josephson junctions (N: normal metal). In all cases, the magnetic state of the device, generally set by the applied field, controls the superconducting response. We report here the observation of the converse effect, that is, direct superconducting control of the magnetic state in GdN/Nb/GdN SSVs. A model for an antiferromagnetic effective exchange interaction based on the coupling of the superconducting condensation energy to the magnetic state can explain the Nb thickness and temperature dependence of this effect. This superconducting exchange interaction is fundamentally different in origin from the various exchange coupling phenomena that underlie conventional spin electronics (spintronics), and provides a mechanism for the active control of the magnetic state in superconducting spintronics.

Nano-optical single-photon response mapping of waveguide integrated molybdenum silicide (MoSi) superconducting nanowires.

We present low temperature nano-optical characterization of a silicon-on-insulator (SOI) waveguide integrated SNSPD. The SNSPD is fabricated from an amorphous Mo83Si17 thin film chosen to give excellent substrate conformity. At 350 mK, the SNSPD exhibits a uniform photoresponse under perpendicular illumination, corresponding to a maximum system detection efficiency of approximately 5% at 1550 nm wavelength. Under these conditions 10 Hz dark count rate and 51 ps full width at half maximum (FWHM) timing jitter is observed.

Spin-filter Josephson junctions.

Josephson junctions with ferromagnetic barriers have been intensively investigated in recent years. Of particular interest has been the realization of so called π-junctions with a built-in phase difference, and induced triplet pairing. Such experiments have so far been limited to systems containing metallic ferromagnets. Although junctions incorporating a ferromagnetic insulator (I(F)) have been predicted to show a range of unique properties including π-shifts with intrinsically low dissipation and an unconventional temperature dependence of the critical current I(c), difficulties with the few known I(F) materials have prevented experimental tests. Here we report supercurrents through magnetic GdN barriers and show that the field and temperature dependence of I(c)is strongly modified by the I(F). In particular we show that the strong suppression of Cooper pair tunnelling by the spin filtering of the I(F) barrier can be modified by magnetic inhomogeneity in the barrier.

Vertically oriented TiO(x)N(y) nanopillar arrays with embedded Ag nanoparticles for visible-light photocatalysis.

We present a straightforward method to produce highly crystalline, vertically oriented TiO(x)N(y) nanopillars (up to 1 µm in length) with a band gap in the visible-light region. This process starts with reactive dc sputtering to produce a TiN porous film, followed by a simple oxidation process at elevated temperatures in oxygen or air. By controlling the oxidation conditions, the band gap of the prepared TiO(x)N(y) can be tuned to different wavelength within the range of visible light. Furthermore, in order to inhibit carrier recombination to enhance the photocatalytic activity, Ag nanoparticles have been embedded into the nanogaps between the TiO(x)N(y) pillars by photoinduced reduction of Ag(+) (aq) irradiated with visible light. Transmission electron microscopy reveals that the Ag nanoparticles with a diameter of about 10 nm are uniformly dispersed along the pillars. The prepared TiO(x)N(y) nanopillar matrix and Ag:TiO(x)N(y) network show strong photocatalytic activity under visible-light irradiation, evaluated via degradation of Rhodamine B.

Unconventional domain wall magnetoresistance of patterned Ni/Nb bilayer structures below superconducting transition temperature of Nb.

Scattering of spin-up and spin-down electrons while passing through a ferromagnetic domain wall (DW) leads to an additional resistance for transport current, usually observed prominently in constricted magnetic structures. In this report we use the resistance of the DW as a probe to find an indirect signatures of the theoretically predicted spin-singlet supercurrent to spin-triplet supercurrent conversion effect of ferromagnetic DWs. Here we examine the DW induced resistance in Ni stripe in a bilayer Ni/Nb geometry in the normal state and in the superconducting state of Nb. By making a 3µm wide gap in the top Nb layer we routed the transport current through the Ni layer in the normal state and in the superconducting state of Nb. In the normal state of Nb, in-field transport measurements showed a clear domain wall magneto-resistance (DWMR) peak of amplitude ∼5.9 mΩ near the coercive field, where the DW density is expected to be maximum. Interestingly, however, below the superconducting transition temperature of Nb, the DWMR peak of the Ni layer showed a sharp drop in the field range where the number of DWs become maximum. This observation may be a possible signature of magnetic DW induced spin-triplet correlations in the Ni layer due to the direct injection of spin-singlet Cooper pairs from Nb into the magnetic DWs.

The response of osteoblasts to nanocrystalline silicon-substituted hydroxyapatite thin films.

Magnetron co-sputtering has been employed to fabricate thin nanocrystalline coatings of silicon-substituted hydroxyapatite (SiHA) of different Si compositions: 0.8 wt%, 2.2 wt%, and 4.9 wt%. A human osteoblast-like (HOB) cell model was used to study the long-term interaction between the HOB cells and coatings in vitro. Results showed that the number of cells growing on all coated titanium (Ti) samples were statistically significantly higher than on uncoated Ti. In addition, HOB cells growing on all SiHA surfaces displayed enhanced cell spreading, with extensive extracellular matrix synthesis. DNA staining revealed normal phenotype nuclear morphology for HOB cells, with several dense chromosomes surrounded by a periphery of intact nuclear membrane. Furthermore, immunofluorescent staining indicated that cells showed improved adhesion on the coated surfaces with increasing Si content, developing mature cytoskeletons with numerous distinct and well-defined actin stress fibres in the cell membranes. Results also demonstrated that the bone mineralisation process was greatest in the presence of the highest Si level (4.9 wt%). However, at very early culturing time point, cells did not attach so readily on the surface of this coating due to rapid dissolution. Thus, this work suggests that a Si content of 2.2 wt% may be the optimum loading to improve the bioactive property of HA thin films.

In Situ Observations of Phase Transitions in Metastable Nickel (Carbide)/Carbon Nanocomposites.

Nanocomposite thin films comprised of metastable metal carbides in a carbon matrix have a wide variety of applications ranging from hard coatings to magnetics and energy storage and conversion. While their deposition using nonequilibrium techniques is established, the understanding of the dynamic evolution of such metastable nanocomposites under thermal equilibrium conditions at elevated temperatures during processing and during device operation remains limited. Here, we investigate sputter-deposited nanocomposites of metastable nickel carbide (Ni3C) nanocrystals in an amorphous carbon (a-C) matrix during thermal postdeposition processing via complementary in situ X-ray diffractometry, in situ Raman spectroscopy, and in situ X-ray photoelectron spectroscopy. At low annealing temperatures (300 °C) we observe isothermal Ni3C decomposition into face-centered-cubic Ni and amorphous carbon, however, without changes to the initial finely structured nanocomposite morphology. Only for higher temperatures (400-800 °C) Ni-catalyzed isothermal graphitization of the amorphous carbon matrix sets in, which we link to bulk-diffusion-mediated phase separation of the nanocomposite into coarser Ni and graphite grains. Upon natural cooling, only minimal precipitation of additional carbon from the Ni is observed, showing that even for highly carbon saturated systems precipitation upon cooling can be kinetically quenched. Our findings demonstrate that phase transformations of the filler and morphology modifications of the nanocomposite can be decoupled, which is advantageous from a manufacturing perspective. Our in situ study also identifies the high carbon content of the Ni filler crystallites at all stages of processing as the key hallmark feature of such metal-carbon nanocomposites that governs their entire thermal evolution. In a wider context, we also discuss our findings with regard to the much debated potential role of metastable Ni3C as a catalyst phase in graphene and carbon nanotube growth.