K. Rećko

Projected iron moments in UFe4Al8 investigated with monochromatic polarized Mössbauer radiation

UFe4Al8 was investigated using a monochromatic, circularly polarized Mossbauer source. The measurements are sensitive to the sign of the hyperfine magnetic field. The iron magnetic moment component along the direction of the applied magnetic field was determined. Quantitative analysis shows that the configuration of iron moments is not collinear and, on average, only about one fourth of the atomic iron moment contributes to the total magnetization in a randomly oriented powder exposed to a field of 1 T at a temperature of 12 K. The presence of ferromagnetic clusters in an otherwise antiferromagnetic ordering is discussed. The conclusions regarding the magnetic structure are compared with the results of powder neutron diffraction experiments. In the course of the interpretation of latter, the Debye temperature D = 421(13) K was determined.

Magnetism of the UFexAl12−x alloys

Abstract Polycrystalline samples of the ternary intermetallic alloys UFe x Al 12− x with x in the range 〈3.6–5〉 were studied. The alloys crystallize in a ThMn 12 -type structure, belonging to the space group I 4/ mmm (no. 139). Studies of the powder samples by X-ray and neutron diffraction techniques confirmed the phase homogeneity of all these materials with an iron concentration, x , below 4. The samples with a higher iron content contain a small addition of extra phases. Monochromatic circularly polarised Mossbauer source (MCPMS) measurements, carried out at 12 K in an external magnetic field of 1 T, were interpreted in terms of magnetic domains in otherwise randomly oriented powders. Magnetic moments of iron are arranged in spin-canted magnetic structures. The canting angles ( α ) were estimated. The neutron data obtained for the sample with x =4.4 indicate that the two transition temperatures, observed previously by means of the magnetisation measurements, originate in different temperatures of the ordering of iron and uranium sublattices. Our data indicate that the range of iron concentration within which two transition temperatures are observed starts with UFe 4 Al 8 alloys. However, the mechanisms underlying these transitions are different for various compositions and require further investigations. It is shown that even in the case of the magnetic domains formation of the sample the MCPMS technique can bring very valuable results.

The Magnetic Properties of GaFeO_3 by Neutron Diffraction and Mössbauer Spectroscopy

The multiferroic GaFeO3 synthesized by a sol gel method and conventional solid state reaction show remarkably di erent magnetic transition temperatures. Both samples have orthorhombic crystal structure Pc21n. Results of a new analysis of neutron di raction and Mössbauer data are presented. Full agreement between parameters was achieved using the Mössbauer data as the constraints in the re nement of neutron di raction data at low and at high temperature. Magnetic moments are reported.

The method of invariants in 57Fe Mössbauer spectroscopy on selected examples

The method of invariants is an effective tool for decomposition of Mossbauer spectra into components resulting from electric quadrupole and magnetic dipole interactions. The invariants can be next analysed to obtain all possible values of hyperfine interaction parameters. In some cases it is possible to determine the parameters with satisfactory precision. The method was applied to not fully ordered UFe4Al8. Obtained results indicate that the dominant component of the electric field gradient is negative and is oriented almost perpendicularly to the direction of the hyperfine magnetic field. Another example shows that the method of invariants results in substantial reduction of the number of calculated components in modeling of hyperfine interactions in γ-FeMn.

Magnetism of UFe4−xAl8+x (x=〈−0.4, 0.4〉) intermetallics

Abstract Results of X-ray, neutron diffraction, magnetization and Mossbauer measurements with circularly polarised beam on polycrystalline samples of the ternary intermetallic alloys UFe 4− x Al 8+ x with x in the range (−0.4, 0.4) are presented. The alloys crystallise in the structure of ThMn 12 type, belonging to the space group I4/mmm (no. 139). X-ray and neutron diffraction confirmed the phase homogeneity of all samples but that one with 4− x =4.4. Monochromatic Circularly Polarised Mossbauer Source (MCPMS) measurements reveal spin-canted magnetic structures in randomly oriented powders exposed to a field of 1 T at T=12 K. Magnetization measurements in an external field of 0.01 T allowed disclosing ferromagnetic-like behaviour vs. temperature and quite dissimilar magnetization and hysteresis curves against the fields in the alloys of interest.

Magnetic and structural properties of Fe3−xCrxAl0.5Si0.5

Results of X-ray, neutron, magnetization and M?ssbauer measurements on polycrystalline samples of Fe3?xCrxAl0.5Si0.5 (x = 0, 0.125, 0.250, 0.375 and 0.5) alloys are presented. The alloys crystallize in the structure of DO3 type, and their unit cell volumes are practically independent of the chromium concentration. X-ray and neutron diffraction confirmed the phase homogeneity of all the samples. Neutron and Mossbauer measurements disclosed that Cr atoms occupy preferentially B-sites, while D sites are almost entirely occupied by Al and Si. The total magnetisation as well as the site and individual magnetic moments ?Fe(A,C), ?Fe(B) and ?Cr(B,D) have been found to be linearly dependent on the chromium concentration.

Debye temperatures and magnetic structures of UFexAl12−x (3.6⩽x⩽5) intermetallic alloys

Abstract Uranium ternary compounds UFexAl12−x crystallize in a body-centred tetragonal structure ThMn12 (I 4/mmm No.139). The neutron powder diffraction, magnetization measurements as well as Mossbauer investigations clearly indicate the magnetic ordering within the iron sites. The rearrangement of iron magnetic moments from uncompensated antiferromagnetic system in UFexAl12−x with x

Phase composition and magnetism of sol–gel synthesized Ga–Fe–O nanograins

ABSTRACT We have succeeded in synthesizing orthorhombic Ga(1−x)Fe(1+x)O3 (−0.05 x0.5), hexagonal GayFe(2−y)O3 (0y1.8) and cubic Ga(1+z)Fe(2−z)O4 (−0.1z0.8) nanograins of gallium ferrites using conventional precursors and an organic environment of Pechini scenario under atmospheric-pressure conditions (SG method). Phase composition and homogeneity were analyzed using X-ray diffraction. Small angle neutron scattering disclosed ellipsoidal particle shapes of gallium iron oxides (GFO) crystallizing in orthorhombic (o-GFO) and hexagonal (h-GFO) symmetry and parallelepiped shapes of Ga(1+y)Fe(2−y)O4 (c-GFO) grains. Despite local agglomeration among the magnetic grains, the scanning electron microscopy and transmission electron microscopy images point to faced-elliptical shapes. The Mössbauer spectroscopy with magnetization measurements was carried out in the temperature range of 5–295 K. The analysis of gallium ferrites magnetism demonstrates that iron atoms locate with various probabilities in crystallographic positions and the spontaneous magnetization preserves up to room temperature (RT).