Ludwik Dobrzynski

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.

A SINGLE LINE CIRCULARLY POLARISED SOURCE FOR MOSSBAUER SPECTROSCOPY

Abstract A single line source of linearly polarized radiation in a conventional Mossbauer setup working at room temperature with 57 Co in Cr matrix was constructed. The ordered Fe 3 Si was used as a filter. The estimated degree of polarization of the source depends on the preparation and thickness of the polarizer. The best polarization degree (92xa0±xa08)% was achieved. The measurements with polarized radiation lead to significant simplification of the complex spectra and allow to study components arising from various polarizations separately. In particular, it is possible to extract Δ m xa0=xa0(1,xa0−1) and Δ mxa0 =xa00 transitions by use of a linearly polarized beam. Moreover polarized beam can be used for determination of texture parameters related to spatial arrangements of spins in the plane perpendicular to the radiation beam. Such parameters are not easily measurable by other methods. The Mossbauer spectra of invar Fe 65 Ni 35 measured by monochromatic linearly polarized radiation are shown as an example.

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.

A spectrometer for Compton scattering studies of heavy elements and the problem of bremsstrahlung background

Abstract A high energy Compton spectrometer which operates at either 662 keV or 412 keV with the γ-ray sources 198 Au and 137 Cs, respectively, has been designed and built. Measurements conducted on Al, Cu and Ag samples indicate the usefulness of this spectrometer for studies of electron momentum distributions. Systematic discrepancies observed between the experimental and theoretical Compton profiles of silver are thought to originate from a background of bremsstrahlung radiation, associated with the Compton recoil electrons and photoelectrons, which is not significant in studies of lighter elements or with lower energy sources. The problem caused by the presence of the additional background in the scattering of high energy radiation from heavy elements is discussed.

Cancer Mortality Among People Living in Areas With Various Levels of Natural Background Radiation

There are many places on the earth, where natural background radiation exposures are elevated significantly above about 2.5 mSv/year. The studies of health effects on populations living in such places are crucially important for understanding the impact of low doses of ionizing radiation. This article critically reviews some recent representative literature that addresses the likelihood of radiation-induced cancer and early childhood death in regions with high natural background radiation. The comparative and Bayesian analysis of the published data shows that the linear no-threshold hypothesis does not likely explain the results of these recent studies, whereas they favor the model of threshold or hormesis. Neither cancers nor early childhood deaths positively correlate with dose rates in regions with elevated natural background radiation.

Electron Density Distribution of Wurtzite-Type Gallium Nitride by Maximum Entropy Method

The electron density distribution of wurtzite-type gallium nitride (w-GaN) was obtained by the Maximum Entropy Method (MEM) using the Synchrotron Radiation powder data. Contribution of the very minor zinc blende-type phase (z-GaN) to the observed powder data was eliminated by the modified Rietveld method. In the obtained MEM electron density distribution map, there are two kinds of Ga–N covalent bonds. The electron density at the saddle point of Ga–N bond parallel to [001] axis is 0.5 [e/A 3 ]. On the other hand, that of the other three equivalent Ga–N bonds are 0.8 [e/A 3 ]. Furthermore, it is found that the electron distribution of N atom shows asymmetric distortion. These features suggest asymmetric thermal vibrations of N atom which are restricted by Ga–N bonds.

Neutrons and Solid State Physics

Neutron wave optics neutron scattering lengths and cross-sections experimenting with neutrons neutrons and the structure of solids neutrons and magnetic structures dynamics and phase transitions in solids summary of neutron applications to condensed matter physics.

Reconstruction of the electron momentum density distribution by the Maximum Entropy Method

Abstract The application of the Maximum Entropy Algorithm to the analysis of the Compton profiles is discussed. It is shown that the reconstruction of electron momentum density may be reliably carried out. However, there are a number of technical problems which have to be overcome in order to produce trustworthy results. In particular one needs the experimental Compton profiles measured for many directions, and to have efficient computational resources. The use of various cross-checks is recommended.