Superconducting order parameter fluctuations in bilayer hybrid films of NbN/NiCu and NbTiN/NiCu

Bernd Aichner, Florian Jausner, Wolfgang Lang, Andrii Klimov, Wojciech Slysz, Marek Guziewicz, Renata Kruszka, Maciej Wegrzecki, Roman Puzniak, Roman Sobolewski

Thermodynamic fluctuations of the superconducting order parameter lead to an excess conductivity above the critical temperature Tc, the so-called paraconductivity. While superconducting fluctuations have been extensively investigated in both metallic and cuprate superconductors, little is known about their properties in superconductor/ferromagnet hybrids. We present results of our studies of superconducting order parameter fluctuations in NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) ultrathin bilayers.
The NbN and NbTiN layers 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 Ni0.5Cu0.5 overlayers with thicknesses of few nanometers, using co-sputtering. Low-temperature magnetization tests confirmed that the NiCu films are ferromagnetic with the Curie temperature of above 30 K [1]. The temperature dependence of magnetoresistance shows an unusual negative region in the S/F bilayers that extends almost to room temperature and is not present in the S single layers. We take this behavior as an indication of remarkably different magnetotransport properties of the S/F bilayers.
The paraconductivity of the NbN and NbTiN single layer films is in excellent Agreement with the parameter-free theory for order-parameter fluctuations in two-dimensional superconductors, confirming the validity of our approach. The same holds for the magnetoconductivity, which probes the suppression of superconducting fluctuations in a magnetic field and provides a measure for the Ginzburg-Landau coherence length [2]. However, the addition of a ferromagnetic top layer changes the magnetotransport properties significantly. In the S/F hybrids both paraconductivity and magnetoconductivity are significantly modified and deviate from phenomenological and microscopic theories for fluctuations in superconductors [3]. Our observations are tentatively explained by a ferromagnetic domain structure in the NiCu layer that could reduce the fluctuation amplitude even in zero external magnetic field due to the spontaneous magnetization of individual domains.

[1] A. Klimov et al., Proc. of SPIE Vol. 9504, p. 950405 (2015).
[2] B. Aichner et al., Proc. of SPIE Vol. 10229, p. 102290H (2017).
[3] A. Larkin and A. Varlamov, Theory of fluctuations in superconductors, Clarendon Press, Oxford, 2005.

Electronic Properties of Materials
Publication date
Publication status
Peer reviewed
Austrian Fields of Science 2012
103033 Superconductivity, 103009 Solid state physics, 103018 Materials physics, 103026 Quantum optics
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