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 (FM) 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 (Tc ~ 15.3 K) and narrow transition width DelTc ~ 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 Tc, temperature dependence of critical current density Jc (T), and angular (Phi) variation of flux-flow resistivity Rhof is shown to be a signature of the structure of NbN film and not a result of the exchange field of CoPt. The Rhof (H,T,Phi) 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.