K. Szymański

Magnetic moment distribution in Fe3−xCrxSi alloys

Abstract Polarized and unpolarized neutron diffraction and magnetization studies in the temperature range 10–300 K are report for the Fe 3− x Cr x Si ( x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 1.0) system. The experiments indicate that the single phase having the fcc DO 3 -type structure is maintained for Fe 3 -x Cr x Si for x ≤0.4, while the sample with x = 0.5 shows a small ( 3 Si-type phase, the amount of which increases linearly with Cr concentration. It is inferred from the dependence of the lattice constant on Cr content measured separately by X-ray diffraction that the phase with x = 0.35 ± 0.05 is always formed. Therefore, for the heat treatment reported here the concentration x = 0.4 seems to be the limit of solubility for Cr in DO 3 -type Fe 3 -x Cr x Si system. It was found that Cr is randomly distributed among (A, C) and B sites, while D sites are fully occupied by Si. Room-temperature magnetization measurements show a linear dependence of magnetization on Cr concentration. The Curie temperature varies as (843.0 ± 1.5) K, (740.0 ± 1.5) K and (712.0 ± 1.5) K for x = 0.1, 03. and 0.4, respectively. Also a linear composition dependence of the one-site magnetic moments μ(A, C) and μ(B) was observed. At room temperature the μ(B) moment decreases from 2.61 μ B for x = 0.1 to 2.08 μ B for x = 0.4, while the μ(A, C) moments decrease from 0.95 μ B for x = 0.1 to 0.72 μ B for x = 0.4. Assuming the constancy of magnetic moments one obtains μ Fe A = (1.04±0.04) μ B and μ Fe B = (2.77±0.10) μ B , while μ A Cr = -(1.27±0.14) μ B and μ B Cr = −(2.57±0.38) μ B . A weak temperature dependence of μ(B) moment dμ(B)/d T = (-2.4±0.1)×10 -4 μ B /K in the range 10–300 K was found for samples with x = 0.1 and 0.3.

The crystal and magnetic structures of intermetallic compounds

The powder samples of (x = 4, 5 and 6) alloys have been measured by means of magnetization, Mossbauer effect, x-ray and neutron diffraction techniques in the temperature range 1.5 - 400 K. Both neutron and x-ray diffraction experiments showed that the positions f in all samples are occupied by iron, while the iron atoms in the samples with x = 5 and 6 locate also at j sites. A little f - j disorder exists in . The paper shows a change of the magnetic structure with an increase of iron content. The magnetic moments lie in a basal a - b plane. Iron atoms in alloy order antiferromagnetically. and alloys exhibit in general a ferromagnetic behaviour. However a detailed distribution of magnetic moments among different sites could not be determined unambiguously from the neutron data only. Nevertheless, combining information from neutron and Mossbauer experiments, one can infer that the ordering among iron atoms must be non-collinear. A magnetic ordering among uranium atoms has been found in only. Low-temperature thermal expansion varies nonlinearly with temperature and in all samples is negligible at low temperatures.

Hyperfine fields, local environment effects and site preference in Fe3-xCrxAl alloys

Abstract The properties of Fe 3− x Cr x Al ( x = 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5) alloys with DO 3 -type of ordering have been investigated by Mossbauer spectroscopy at room temperature without and with external magnetic field parallel to the beam direction. The results show that the mean hyperfine magnetic field decreases nonlinearly with chromium concentration and is linearly correlated with the average magnetic moment of the sample under study. The postulated quadratic dependence of h.m.f. on the number of nearest Fe neighbours allows one for unambiguous identification of all the components of the measured spectra. Hyperfine magnetic field, isomer shift and iron magnetic moment are presented as functions of local environments of iron atoms. The analysis of the site occupations showed that chromium atoms occupy preferentially B sites, the effect expected but not found in isostructural Fe 3− x Cr x Si alloys [1].

Structural and magnetic properties of FeCrAl alloys with DO3-type structure

X-ray, neutron, magnetization and Mossbauer studies in the temperature range 10–300 K are reported for Fe3−xCrxAl system with x < 0.6. The experiments indicate that a single phase having the DO3-type structure is maintained in the range of Cr concentration studied. It was found that Cr atoms occupy preferentially B-sites and the lattice constant decreases with increasing concentration of chromium. The magnetic moment of chromium is small and diminishes the value of neighbouring iron atoms by about 0.1 μB per Cr atom. The magnetic moments of iron at (A, C) and B sites were estimated to be 1.54 (22), 2.58 (22), 1.51 (27) and 2.32 (14) μB for T = 10 and 300 K, respectively. The magnetic moment of iron decreases by 0.37 (10) and 0.26 (6) μB at 10 K and T = 300 K, respectively, if one aluminium atom is found as a nearest neighbour of iron.

Mössbauer study of Fe3−xCrxSi alloys with DO3-type ordering

The influence of chromium atoms substituting for iron in the Fe3−xCrxSi alloys with DO3-type ordering is investigated by Mossbauer spectroscopy at room temperature. Using the correlation between local hyperfine magnetic field and magnetic moments observed for Fe3Si it is inferred that the magnetic moment of iron in Fe3−xCrxSi decreases linearly with x for both (A, C) and B sites. A few models intended to describe an influence of Cr on local hyperfine fields are tested. It is found that the local hyperfine magnetic field depends primarily on the chemical composition of the nearest neighbour shell. The isomer shift and magnetic hyperfine field show no substancial dependence on the alloy composition. Chromium atoms occupy, unexpectedly, both B and (A, C) sites, although the B sites are preferentially occupied.

Local environments effects and site preference in Fe3−xCrxAl alloys

Abstract The properties of Fe 3− x Cr x Al alloys for x ≤ 0.5 with DO 3 -type of ordering have been investigated at room temperature by Mossbauer spectroscopy in zero field and in external magnetic field of about 4.3 kOe applied along the beam direction. The results show that the mean hyperfine magnetic field (h.m.f.) decreases nonlinearly with Cr concentration and is linearly correlated with the average atomic magnetic moments of the samples. The postulated quadratic dependence of h.m.f. on the number of nearest neighbour (n.n.) iron atoms allows unambiguous identification of the components of the measured spectra. The analysis of relative intensities of every component leads to the conclusion that Cr preferentially occupies B-sites. The values of iron magnetic moment as a function of n.n. configuration are estimated for both (A,C) and B sites.

On the validity of a hyperfine magnetic field distribution measured by a monochromatic, circularly polarized Mössbauer source

It is demonstrated that when in a standard Mossbauer spectroscopy experiment two different hyperfine magnetic field distributions describe experimental data equally well, measurements by means of a monochromatic circularly polarized source lead to the correct choice of the solution. The problem of the asymmetry of the spectrum in the case of a polarized source and pure magnetic interaction is discussed and constraints for the line intensities are given.

Mössbauer effect on mechanically alloyed FeN

Mossbauer measurements were carried out on mechanically alloyed 10Fe + 2Fe3N compounds. Hyperfine fields of various local environments for compounds ground in Ar and N2 atmospheres are presented. The initial increase of the average hyperfine magnetic field is reported. Gaseous nitrogen accelerates the process of mechanical alloying. There is no indication of Fe16N2 formation.

Magnetic and Mossbauer investigation of FeSi2-xAlx

Abstract Results of X-ray diffraction, magnetic susceptibility and Mossbauer measurements are presented for FeSi 2− x Al x with x = 0 up to 0.06. Almost pure β-FeSi 2 was obtained with crystallite size 1–10 μm and with traces of the FeSi phase. The FeSi phase is created in the initial stage of crystallization in the form of dendrites. The α-FeSi 2 phase was not detected in the annealed alloy with x = 0. Substitution of Si by Al causes appearance of the α-FeSi 2 phase which is visible particularly clear as an additional component in Mossbauer spectra. A large maximum of the low temperature susceptibility was observed between x = 0 and x = 0.040. The maximum is interpreted as the result of the enhancement of the susceptibility in the β-FeSi 2 phase doped with Al.

Structural disorder in Fe−Si alloys near Fe3Si composition

Ordered and slightly disordered Fe1−xSix alloys near X=0.25 are studied by means of Mössbauer spectroscopy. Quantitative analysis of the spectra after correction for the thickness effect is presented. The short range order as well as the long range order parameters are calculated from Mössbauer spectra. Distribution of atoms among different sites is determined. A slight disorder for x<0.252 is mainly due to an interchange of Fe atoms from A sites with Si atoms residing at D sites. However, at larger x the B disorder prevails.