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