Structure and magnetic properties of carbon encapsulated FeCo@C and FeNi@C nanoparticles

•Gas-condensation synthesis of FeNi@C and FeCo@C nanoparticles.•Formation of solid solution of carbon in cores of FeNi@C, FeCo@C nanoparticles.•Abnormal variation of susceptibility versus temperature for FeNi@C nanoparticles.•Variations of carbon content in nanoparticles are proposed. A comparative analysis of the structure and magnetic properties of nanoparticles of Fe-Ni and Fe-Co alloys without carbon and encapsulated in carbon obtained by gas-condensation synthesis is carried out. Particular attention is paid to the study of the formation of a solid solution in nanoparticles with carbon. X-ray analysis and magnetic properties convincingly demonstrate that a disordered supersaturated solid solution of carbon in FeNi@C and FeCo@C nanoparticles is formed.