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

A. Klimov, R. Puzniak, B. Aichner, W. Lang, E. Joon, R. Stern, W. Slysz, M. Guziewicz, M. Juchniewicz, M. A. Borysiewicz, R. Kruszka, M. Wegrzecki, A. Laszcz, A. Czerwinski, Roman Sobolewski

Performance of superconducting single-photon detectors based on resistive hotspot formation in nanostripes upon optical photon absorption depends strongly on the critical current density JC of the fabricated nanostructure. Utilization of an ultrathin, weak-ferromagnet cap layer on the top of a superconducting film enhances of the structure€™s JC due to an extra flux pinning. We have fabricated a number of both NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) ultrathin bilayers and microbridges. NbN and NbTiN underlayers with thicknesses varying from 4 to 7 nm were grown using dc-magnetron sputtering on chemically cleaned sapphire single-crystal substrates. After rapid thermal annealing at high temperatures, the S films were coated with Ni0.54Cu0.46 overlayers with thicknesses of about 6 nm, using cosputtering. Compositions of the deposited films were confirmed by EDX spectroscopy analysis, while TEM studies demonstrated excellent epitaxial quality of our S layers with ∼2-nm-thick F/S transition layer and atomically-sharp S/substrate interface. Magnetic properties of bilayers were studied using both the SQUID and Vibrating Sample Magnetometer techniques 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 approx. 12-13 K, the structures were fully proximitized with the strong superconducting signal. For superconducting transport properties characterization, we used bilayers patterned into 40-1/4m-long microbridges with the width varying from 0.4 1/4m to 2 1/4m. The same S/F nanostructures were also used to study their superconducting fluctuations. The temperature dependence of magnetoresistance demonstrated highly 2-dimensional character with an unusual negative region that extended almost to room temperature. In the S/F sample, the fluctuations were observed to be substantially below theoretical expectations.

Electronic Properties of Materials
External organisation(s)
Polish Academy of Sciences (PAS), NICPB, National Institute of Chemical Physics & Biophysics (NICPB), Institute of Electron Technology
No. of pages
Publication date
Peer reviewed
Austrian Fields of Science 2012
103026 Quantum optics, 103021 Optics, 103033 Superconductivity, 103009 Solid state physics
ASJC Scopus subject areas
Electronic, Optical and Magnetic Materials, Condensed Matter Physics, Applied Mathematics, Electrical and Electronic Engineering, Computer Science Applications
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