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

TL;DR

This study investigates how lattice mismatch-induced strain affects superconductivity in NbN-CoPt heterostructures, revealing that structural factors, rather than ferromagnetic exchange fields, primarily influence superconducting properties.

Contribution

It demonstrates that strain from lattice mismatch significantly impacts superconductivity in NbN-CoPt heterostructures, with structural factors outweighing magnetic exchange effects.

Findings

Superconductivity remains robust in CoPt-NbN/MgO heterostructures with different magnetic anisotropies.

Superconductivity is suppressed in NbN-CoPt/MgO due to structural factors, not exchange fields.

Domain walls in CoPt are of Neel type and do not induce flux in the superconductor.

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 (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.