E. A. Shafranovsky

Structure and Mössbauer spectra for the Fe–Cr system: From bulk alloy to nanoparticles

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 Mossbauer 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.u200a%. At the Cr content of 2.36 at.u200a% 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 Mossbauer 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.u200a% 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 fe...

Aerosol nanoparticles in the Fe1−xCrx system: Room-temperature stabilization of the σ phase and σ→α-phase transformation

The production and structural characterization of gas-evaporated nanoparticles in the Fe1−xCrx system, with 0<x<0.83 are reported. The results show that for x∼0.5 the metastable σ-FeCr can be stabilized and it constitutes up 60wt% of the material. The sample with the highest σ-FeCr content is further analyzed to study the structural and the magnetic properties of this phase and its thermal stability. The σ-FeCr phase is weakly magnetic with an average magnetic moment of 0.1μB per Fe atom and a Curie temperature below ∼60K. It is stable up to 550K where it starts to transform to bcc-FeCr. Annealing at 700K yields Cr2O3 due to Cr surface segregation and affects the magnetic behavior of the system, which is dominated by interparticle interactions.

On ferro- and antiferromagnetic ordering in ultrafine particles of Fe-rich Fe–Ni and Fe–Mn alloys

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 Mossbauer 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.

COMPARATIVE STUDY OF THE STRUCTURE AND LOCAL MAGNETIC ORDER IN BULK AND ULTRAFINE PARTICLES OF FE-MN (32%-35%) AND FE3PT

A study of Fe–Mn (32% and 34.65%) and Fe3Pt Invar alloys during the transition from bulk to ultrafine (3–20 nm) particles is done at room temperature by combined x-ray diffraction and Mossbauer spectroscopy techniques. The particles obtained by evaporation of foil or filings of bulk material in an Ar atmosphere at pressures from 0.16 to 50 Torr were rapidly quenched during their production. Mossbauer measurements confirmed the availability of two spin states in both Fe–Mn and Fe3Pt fcc particles like it has been observed in previous studies of fine Fe and Fe–Ni (30%–35%) particles. Various heat treatments of the Fe3Pt foil with the initial bcc structure made it possible to obtain its fcc modification in either the entirely ordered or the entirely disordered state. It was shown that the disordered fcc structure in a bulk sample could exist in two spin states (ferromagnetic and paramagnetic) whereas the ordered structure was only in a ferromagnetic state.

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.

Structure and Magnetic Properties of Aerosol Nanoparticles of Fe and Its Alloys

Structure and magnetic properties of aerosol nanoparticles of Fe and its alloys (FeMn, FeNi, FeNiMn, FePt, FeCr, FeCo, and FeCu) have been reviewed. It has been shown that, compared to a bulk material, the particles have a number of specific features being of much fundamental and applied interest. The effect of both a quenched high-temperature Fe modification and its oxides on the structure and magnetism of nanoparticles has been considered in detail. Particular attention has been paid to the recently observed fine structure in the hyperfine field distribution at iron nuclei in Mossbauer spectra for pure iron and its alloys both as a bulk and aerosol nanoparticles. This phenomenon makes it possible to reveal very weak magnetic interactions in the system under study. The plausible origin of these magnetic interactions has been also discussed.

Structure and local magnetic order in ultrafine particles of Fe65(Ni1-xMnx)35 (0 ≤ x ≤ 1) alloys

Abstract X-ray diffraction and Mossbauer spectroscopy techniques at room temperature are applied to find out the influence of the sequential substitution of Ni by Mn in ternary Fe 65 (Ni 1− x Mn x ) 35 (0≤ x ≤1) alloys on the structure and the local magnetic order in ∼5 and ∼15xa0nm particles prepared by evaporation of foil samples in Ar atmosphere. Small (∼5xa0nm) particles exhibit the bcc structure in contrast to the bulk ternary alloys having the fcc structure. The increase of the size up to ∼15xa0nm as well as the increase of Ni content leads to the appearance of an admixture of the fcc phase in X-ray patterns. Analysis of the Mossbauer spectra with consideration of the X-ray data, indicates the existence of the high-temperature fcc structure of particles in two spin states.