Structural and Mössbauer studies of aerosol FeCu nanoparticles in a wide composition range

Structure and magnetic state of aerosol FeCu nanoparticles of 10–30 nm size with Cu content of 0.6–92.1 at.% have been examined by X-ray diffraction and Mössbauer spectroscopy. The FeCu particles have been shown to consist of an iron core surrounded by a copper and Fe oxide shell. With increasing Cu content the iron core having a bcc structure is reduced down to its complete disappearance followed by vanishing ferromagnetism of the particles. Within the copper content from 4.9 to 74.3 at.% the bcc and fcc phases coexist, with the fcc phase having a lattice constant close to that of pure copper and the bcc lattice constant being slightly higher than that for pure Fe due to embedding Cu atoms into the Fe lattice. At Fe-rich FeCu samples a presence of two-spin (ferromagnetic and paramagnetic) components of the fcc Fe is also observed. In the case of a thin copper shell there is only the ferromagnetic fcc Fe, whereas with further thickening of the shell both spin states of the fcc Fe appear existing up to a 20% Cu content. For FeCu samples with a higher Cu content they disappear due to oxidation of the copper grains. The Cu-rich samples with Cu content higher 80 at.% have a fcc structure, with the lattice constant being slightly higher than that of copper and they are paramagnetic. A slight increase of the lattice constant is due to the penetration of small iron aggregations into the Cu grains. In contact with air, the FeCu particles become covered with Fe₃O₄ and Cu₂O. Their long-term exposure to ambient conditions leads to further oxidation process of Cu₂O to CuO.     

Exhibition of High- and Low-spin States of the High-temperature Fcc Phase in Nanoparticles of Fe,Fe-rich and Co-rich Alloys

The results of combined X-ray and Mössbauer studies of structure and local magnetic ordering in massive substances Fe, Fe–Ni, Fe–Mn, Fe–Ni–Mn, Fe–Pt, Fe–Co and aerosol nanoparticles produced by their evaporation in rare Ar atmosphere are discussed. This technique provides a stochiometric composition of alloys in nanoparticles. The smallest (5–8 nm) particles for all alloys containing Fe 60–65% are shown to have a bcc structure whereas with doubling a size the particles acquire a fcc structure. This is explained by the fact that by cooling the particles in the course of preparation they quickly reach a state close to the equilibrium and, according to the constitution diagram, must decompose into two phases. Such decomposition in massive alloys was never observed at temperatures below 300°C because of diffusive difficulties. It is found that as-fresh aerosol particles are covered with an X-ray amorphous oxide shell, which is displayed in the room temperature Mössbauer spectra as a superparamagnetic doublet and is transformed into sextet at lower temperatures. An availability of the oxide shell has no practical influence on the particles structure. The obtained Mössbauer spectra are considered with the model suggested by R.J. Weiss in 1963, on existence of two-spin states in the high-temperature fcc modification of Fe and its alloys. Both states coexist, moreover, in the Mössbauer spectra the ferromagnetic state dominates at high temperature and anti-ferromagnetic one at low temperature. The ferromagnetic state manifests itself as a remnant of the frozen magnetic ordering of the high-temperature fcc modification in the resulting bcc structure, whereas the anti-ferromagnetic state is related to some fcc fraction retained under the particles quenching.     

On the origin of discontinuity of the hyperfine fields at Fe nuclei in bulk iron and aerosol Fe nanoparticles

Advancing the early work in which a discontinuity of hyperfine fields at ⁵⁷Fe nuclei in bulk iron and in aerosol Fe nanoparticles has been revealed by analyzing their Mössbauer spectra the present Letter evidences that the existence of several peaks in the hyperfine distribution (HFD) for bulk Fe is caused with the internal magnetic fields owing to its multidomain structure whereas aerosol Fe nanoparticles are single-domain and show only a unique peak in HFD. This argument has been corroborated by transformation of the HFD pattern for Fe foil after applying the external magnetic field of 0.03 T.     

Aerosol Fe Nanoparticles with the Passivating Oxide Shell

Structure, state, transformations and interactions of iron oxide shell with the iron metallic core in aerosol Fe nano-particles has been studied by X-ray and electron diffraction, TEM, Mössbauer spectroscopy and magnetization measurements. A strong influence of the state of nanoparticles oxide shell has been revealed on their magnetization, coercive force and hysteresis loop shift. A long-term passivation of the particles has been shown to be caused by the primary amorphous oxide. The passivation ability of the oxide shell becomes essentially worse after heat treatment of powder, resulting in its crystallization. Basing on the obtained results, a qualitative mechanism of passivation for nanoparticles covered with amorphous oxide shell has been suggested.     

Magnetic behaviour of Fe–Cr nanoparticle systems

The low-temperature magnetic properties of Fe_1−xCr_x nanoparticles with various chromium content (x=2.4−83.0 at%) have been studied. The multiphase particles (α-FeCr, σ-FeCr and Fe/Cr oxides) have a core (metallic)–shell (oxide) structure. The magnetic properties of the Fe–Cr systems depend on the chromium content as well as on the types and relative contributions of the constituent crystalline phases but, in particular, they are determined by long-range interparticle dipolar interactions. The role of the weakly magnetic σ-FeCr phase is also discussed.