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Shell-core structures of Fe(C), Co(C) and Fe-Co(C) nanocapsules, prepared by an arc discharge process in a mixture of methane and helium, have been demonstrated by means of high-resolution transmission electron microscopy (HRTEM). These nanoscale magnetic cores are protected by graphite shells. It has been found that the zero-field-cooled (ZFC) magnetization of Fe-Co(C) nanocapsules that display different characteristics in three temperature ranges can be well interpreted in terms of the unblocking of magnetization of small single-domain particles and the depinning of large multidomain particles. The saturation magnetization of these nanocapsules decreases monotonically, while the coercivity decreases significantly with increasing temperature.

Characterization and magnetic properties of Fe–Co(C) nanocapsules were investigated systematically by means of x-ray diffraction, Mössbauer spectroscopy, x-ray photoelectron spectroscopy, transmission electron microscopy, energy dispersive spectroscopy analysis, chemical analysis, oxygen determination, and magnetization measurement. The effects of elemental carbon, decomposed from a methane atmosphere in carbon arc process, on the phase structures, magnetic states and surface characterization were studied. Carbon atoms favor forming a new core/shell type structure, consisting of a carbon coating and a core containing a carbon solution. The mechanism of formation of the nanocapsules as well as origin of ferromagnetism and paramagnetism are discussed.

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Ultrafine Fe, Fe–Ni, and Ni particles were prepared by using the hydrogen plasma-metal reaction method in a mixture of H2 and Ar of 0.1 MPa. The particles were characterized by x-ray diffraction, transmission electron spectroscopy, energy disperse spectroscopy, chemical analysis, and Mössbauer spectroscopy. In contrast with bulk Fe–Ni alloys, the distinguishing features in corresponding ultrafine particles are that two phases with fcc and bcc structures coexist in a wide composition range. Ultrafine Fe–Ni particles have higher resistance to oxidation than Fe and Ni particles. The mechanism of forming particles was analyzed by means of structural and magnetic measurements. It was found that quenching is a dominant mechanism for forming paramagnetic particles. Hyperfine interactions were studied by Mössbauer spectroscopy in comparison with those in bulk Fe–Ni alloys.

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