6 May 2015 Superconducting and ferromagnetic properties of NbN/NiCu and NbTiN/NiCu bilayer nanostructures for photon detection
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Abstract
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-μm-long microbridges with the width varying from 0.4 μm to 2 μm. 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.
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A. Klimov, A. Klimov, R. Puźniak, R. Puźniak, B. Aichner, B. Aichner, W. Lang, W. Lang, E. Joon, E. Joon, R. Stern, R. Stern, W. Słysz, W. Słysz, M. Guziewicz, M. Guziewicz, M. Juchniewicz, M. Juchniewicz, M. A. Borysiewicz, M. A. Borysiewicz, R. Kruszka, R. Kruszka, M. Węgrzecki, M. Węgrzecki, A. Łaszcz, A. Łaszcz, A. Czerwinski, A. Czerwinski, Roman Sobolewski, Roman Sobolewski, } "Superconducting and ferromagnetic properties of NbN/NiCu and NbTiN/NiCu bilayer nanostructures for photon detection", Proc. SPIE 9504, Photon Counting Applications 2015, 950405 (6 May 2015); doi: 10.1117/12.2179697; https://doi.org/10.1117/12.2179697
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