NMR and Mossbauer studies of core–shell FeCo@C ferromagnetic nanoparticles near the superparamagnetic transition

•Nanopowder Fe0.66Co0.34@C has a high saturation magnetization (189 emu/g).•Nanoparticles with an average linear size of 9 nm have been obtained.•Annealed nanoparticles Fe0.66Co0.34@C are based on ordered FeCo alloy. A series of nanoparticles of the bimetallic core and carbon shell type FexCo1-x@C (x = 0.4 – 0.8) have been obtained by the gas condensation synthesis method. The microstructural peculiarities and magnetic behaviour of the nanoparticles is analysed depending on the composition and thermal treatment. Collateral phases, which are difficult to detect by traditional methods, are identified. The 59Co,57Fe NMR and 57Fe Mossbauer spectroscopies have shown that homogeneity of the composition and removal of carbon from the metallic core can be reached by annealing. The combination of methods has quantified the fraction of superparamagnetic particles, the proportion of paramagnetic inclusions. A simplified visual interpretation of the 59Co NMR spectra is proposed for binary FeCo alloys.