The structure and hyperfine fields of Fe1−xCrx (x=0.0236–0.803) nanoparticles (average size of 27±2 nm) are studied at room temperature by combined x-ray diffraction and Mössbauer spectroscopy techniques. They are produced by fast evaporation of bulk alloys at 3 Torr Ar pressure. The bulk alloys of any composition are shown to exhibit a bcc structure, whereas the nanoparticles demonstrate a mixture of bcc and tetragonal σ phases in the Cr range from 24.4 to 83.03 at. %. At the Cr content of 2.36 at. % the lattice constant for nanoparticles is larger than that of the bulk alloy, though the values of hyperfine fields on Fe nuclei do not differ. The Mössbauer spectrum of nanoparticles contains an oxide doublet in addition to the sextet specific to that of the bulk alloy. In both cases the width of the sextet lines is rather narrow. However, even at ∼8 at. % Cr the lines of the sextet are broadened so much that it can be decomposed by two-three components. This is explained by freezing the high-temperature ferromagnetic fcc phase regions in the bcc lattice. As the Cr content increases, the Mössbauer spectra become more complex, transforming finally into a paramagnetic singlet. A complete ferromagnetic→paramagnetic transition is observed for the bulk alloy at 68 at. % Cr and for nanoparticles at 35 at. % Cr. The results are discussed under the assumption that at high temperatures the alloys are not homogeneous and exhibit fluctuations of the composition. With decrease of temperature these fluctuations result in decomposition of the alloy into two phases for nanoparticles whereas they are frozen at the cluster level in the bulk alloys holding a macroscopic homogeneity.

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The production and structural characterization of gas-evaporated nanoparticles in the Fe 1−x Cr x system, with 0 Ͻ x Ͻ 0.83 are reported. The results show that for x ϳ 0.5 the metastable -FeCr can be stabilized and it constitutes up 60 wt % of the material. The sample with the highest -FeCr content is further analyzed to study the structural and the magnetic properties of this phase and its thermal stability. The -FeCr phase is weakly magnetic with an average magnetic moment of 0.1 B per Fe atom and a Curie temperature below ϳ60 K. It is stable up to 550 K where it starts to transform to bcc-FeCr. Annealing at 700 K yields Cr 2 O 3 due to Cr surface segregation and affects the magnetic behavior of the system, which is dominated by interparticle interactions.

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Ultrafine Fe–Ni (28%–32%) and Fe–Mn (30%) particles with an average size of 10–15 nm are studied by combined x-ray diffraction and Mössbauer spectroscopy techniques with the latter being applied at a temperature range from 298 to 4 K. They are produced by evaporation of bulk alloys at 3 Torr Ar pressure. From the x-ray data the ultrafine Fe–Ni (28%–32%) particles are a mixture of bcc and fcc phases, and the ultrafine Fe–Mn (30%) particles contain bcc, fcc, and hcp phases. It is shown that in the former the paramagnetic fcc phase transforms to the antiferromagnetic state with decreasing temperature from 77 down 4 K. As for the latter, the fcc phase is observed to be antiferromagnetic even at room temperature whereas the hcp phase keeps a paramagnetic state right down to 4 K. The results corroborate the Weiss hypothesis that the high temperature face-centered-cubic lattice of Fe-rich alloys can exist in two (ferro- and antiferromagnetic) spin states. The oxide contribution in the spectra is also separated.