High‐Temperature Ferromagnetism of the Iron‐Based FCC Phase: The Effect of Carbon and Nickel

The phase composition and magnetic properties of FeNi nanoparticles encapsulated in carbon with a nickel content of about 10% are studied in comparison with FeNi nanoparticles of similar composition without carbon coating. The Fe91Ni9 particles have the body centered cubic structure, whereas the Fe88Ni12@C particles have the face centered cubic (FCC) structure. The Fe88Ni12@C particles are characterized with a rather high magnetization of 60 emu g−1 in the field of 30 kOe at 5 K and 40 emu g−1 at 300 K. The monotonous behavior of magnetization with temperature gives grounds to assume that the Fe88Ni12@C particles retain the FCC structure over the entire temperature range down to 5 K. It is concluded that the stabilization of FCC phase of Fe88Ni12@C and the ferromagnetic state of this FCC phase are related to a large concentration of carbon in the core of the nanoparticles. Carbon‐coated iron‐nickel alloy nanoparticles exhibit unique ferromagnetic properties in the face centered cubic (FCC) phase.