Nanoscale physics

Tuning the critical current in nanoscale superconductor-ferromagnet multilayers
Polackova M.1, Zhitlukhina E.2, Gregor M.1
1 Faculty of Mathematics, Physics and Informatics, Comenius University in Bratislava, Mlynská dolina F2, 842 48 Bratislava, Slovakia.
2 O. O. Galkin Donetsk Institute for Physics and Engineering, National Academy of Sciences of Ukraine, 46 Nauki Avenue, 03028 Kyiv, Ukraine. 
E-mail: elena_zhitlukhina@ukr.net

Critical current density Jc is one of the most important properties for practical applications of superconductors. Hybrid superconductor (S) – ferromagnet (F) nanostructures provide a perspective way for controlling Jc via coupling the two metals with very distinct properties. Bulaevskii et al. [1] proposed to enhance the Jc values of S layers by pinning magnetic fluxes of superconducting vortices in the nearby F films, rather than their cores inside S layers. Notice that magnetic pinning, different from the conventional one introduced by non-magnetic defects, has no temperature limitation since it does not weaken with increasing temperature.
In this work, we have followed this idea using NbN as a superconductor with a weak F alloy Ni0.45Cu0.55 and strong ferromagnets Ni and Co as F layers. Thicknesses of both films deposited on the Al2O3 substrate were fixed at 50 nm. Measured dc current-voltage characteristics were interpreted assuming the current flowing only across the S layer. Separation of the F and S films with 20 nm-thick AlN interlayers led to the growth in the critical temperature Tc from 13.5 – 13.8 K to 15.4 K. Unexpectedly, the Jc values increased several times in contacts with an F film in comparison with control samples without it and the most prominent effect was observed in NbN/Co bilayers. The effect of an external magnetic field B was unusual as well. In low fields, Jc in hybrid samples was higher than that in individual S films, while in fields above 0.2 T, the critical current density always exceeded its values for bilayers.
The work is supported by the Slovak Research and Development Agency under contracts no. APVV-19-0303 and APVV-19-0365 and the NAS of Ukraine Targeted Fundamental Research Program “Promising basic research and innovative development of nanomaterials and nanotechnologies for industry, healthcare, and agriculture” (Project No. 17/21-H).

1. Bulaevskii L.N., Chudnovsky E.M., Maley M.P. Magnetic pinning in superconductor-ferromagnet multilayers // Appl. Phys. Lett.-2000. -76, N 18.-P. 2594-2596.