Superconducting fluctuations and magnetic properties of NbN/NiCu and NbTiN/NiCu bilayer nanostructures for photon detection

Wolfgang Lang, Bernd Aichner, Georg Zechner, Florian Jausner, A. Klimov, W. Slysz, M. Guziewicz, R. Kruszka, M. Wegrzecki, R. Puzniak, Roman Sobolewski

We present results of our studies of superconducting order parameter fluctuations and existence of both the ferromagnetic and superconducting phases in NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) ultrathin bilayers and nanostripes. NbN and NbTiN underlayers with thicknesses from 4 to 6 nm were grown using dc-magnetron sputtering on epitaxial quality sapphire substrates. After high-temperature rapid thermal annealing, the S films were coated with ~6-nm thick, Ni0.52Cu0.48 overlayers using co-sputtering. Compositions of our S/F nanostructures were confirmed by EDX spectroscopy analysis, while TEM studies demonstrated excellent epitaxial quality of the S layers and the very sharp (<2-nm-wide) F/S interface. Magnetic properties of S/F bilayers were studied using a SQUID magnetometer technique in low and high magnetic fields. Low-temperature tests confirmed that in all cases NiCu films were ferromagnetic with the Curie temperature of above 30 K. Below the bilayer critical temperature of ~12-13 K, the structures were fully proximitized with the strong diamagnetic signal and the critical current density exceeding 1 MA/cm2 at 4.2 K. Fluctuation-enhanced conductivity (para-conductivity) as well as magneto-conductivity of our S/F nanostructures strongly deviated from those of pure NbN and NbTiN reference films. The temperature dependence of magneto-resistance demonstrated an unusual negative region in the S/F nanostructures that extended almost to room temperature and that was not present in the single S layer. Both para-conductivity and magneto-conductivity data fell below the theoretical expectation, indicating a strong reduction of superconducting fluctuations. Such suppression of the fluctuation amplitude in S/F bilayers should reduce dark counts in photon detectors based on S/F nanostripes.

Electronic Properties of Materials
External organisation(s)
Universität Wien, Instytut Technologii Elektronowej, Polish Academy of Sciences (PAS), University of Rochester
Publication date
Peer reviewed
Austrian Fields of Science 2012
Superconductivity, Nanophotonics
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