Lattice-mismatch-induced granularity in CoPt-NbN and NbN-CoPt superconductor-ferromagnet heterostructures: effect of strain

Abstract

The effect of strain due to lattice mismatch and of ferromagnetic exchange field on superconductivity (SC) in NbN-CoPt bilayers is investigated. Two different bilayer systems with reversed deposition sequence are grown on MgO (001) single crystals. While robust superconductivity with high critical temperature (T_c≈15.3 k) and narrow transition width (ΔT_c≈0.4 k) is seen in two types of CoPt-NbN/MgO heterostructures where the magnetic anisotropy of CoPt is in plane in one case and out of plane in the other, the NbN-CoPt/MgO system shows markedly suppressed SC response. The reduced SC order parameter of this system, which manifests itself in T_c, temperature dependence of critical current density J_c(T), and angular variation of flux-flow resistivity ρ_f, is shown to be a signature of the structure of NbN film and not a result of the exchange field of CoPt. The ρ_f (H,T) data further suggest that the domain walls in the CoPt film are of the Neel type and hence do not cause any flux in the superconducting layer. A small but distinct increase in the low-field critical current of the CoPt-NbN couple is seen when the magnetic layer has perpendicular anisotropy.