Particle size dependence of the magnetic, dielectric and gas sensing properties of Co substituted NiFe2O4 nanoparticles

•Defect-free Ni0.8Co0.2Fe2O4 nanoparticles prepared using egg-white as bio-template.•Annealing from 600 C to 900 C controls particle size from 10 to 22 nm.•Increase in particle size increases saturation magnetization but lowers coercivity.•Core-shell model is used to explain size dependence of magnetic parameters.•Smaller particles perform better as gas-sensors for LPG, H2, NH3, and CO.