K. Wochowski

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.

57Fe Mössbauer effect in UFexAl12−x system

Abstract 57 Fe Mossbauer effect examinations were carried out on UFe x Al 12− x alloys with x = 4, 5 and 6 in the temperature range 14–370 K. The spectra for x =5 and 6 can be fitted with the Fe atoms distributed over the 8( f ), 8( j ) and 8( i ) positions, whereas for x =4 the Fe atoms are located at the 8( f ) sites only, in agreement with earlier results. The hyperfine fields corresponding to the three sites ( f, j, i ) are 54%, 43% and 3% for x =5 and 58%, 35% and 7% for x =6 respectively. The Curie points determined from H hf ( T ) plots are 162, 265 and 350 K for x =4, 5 and 6 respectively, in fair agreement with the results of magnetic examinations.

MAGNETIC PROPERTIES OF UFEXCO10-XSI2 INTERMETALLICS

Abstract UFe 10 Si 2 and UCo 10 Si 2 form solid solution with the ThMn 12 type of tetragonal structure. The lattice parameter a follows the Vegard law and parameter c does not. The increase of cobalt content depresses a saturation magnetization at 4.2 K almost linearly, whereas the remanence for all samples is very low. The highest Curie point equal to 750 K is observed for UFe 6 Co 4 Si 2 . The annealing process does not affect the Curie point of these materials, however increases slightly the saturation magnetization. Two samples from the U(Fe, Co, Al) 10 Si 2 system have been investigated too.

Structure, magnetic properties and 57Fe Mössbauer effect in UFe10Si1.75Mo0.25 intermetallic compound

Abstract The structure, magnetic properties and 57 Fe Mossbauer effect of the intermetallic compound UFe 10 Si 1.75 Mo 0.25 have been examined. This compound is ferromagnetic below 547 K (Mossbauer effect) and 565 K (susceptibility) with saturation magnetic moment μ s = 17μ B /formula unit (magnetization). The crystallographic and Mossbauer effect examination have shown that the Mo atom occupies the 8(i) position. Moreover, a fraction of the Si atoms also occupy this site in contrast to UFe 10 Si 2 in which the Si atoms are distributed between 8(f) and 8(j) sites. The magnetic properties are discussed in comparison with both UFe 10 Si 2 and UFe 10 Mo 2 .

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.

Magnetic properties and Mössbauer effect studies of Ce1−xScxFe4 Al8 system

The investigations of the magnetic and electrical properties as well as the 57Fe Mossbauer effect are presented for Ce1−xScxFe4Al8 solid solution with 0<x<1 in the temperature range 4–300 K. Magnetic susceptibility follows a Curie–Weiss law above 200 K. The effective magnetic moment decreases with the Sc content. At temperatures lower than 130 K all compounds studied are antiferromagnets. The Neel temperature, TN is not affected by substitution of Sc for Ce. TN has no reflection in any anomaly in ρ(T). The Mossbauer spectra at temperatures lower than TN exhibit one Zeeman sextet related to the Fe nucleus at the 8(f) position. The hyperfine parameters Hhf, IS, QS have been estimated as a function of Sc concentration. The increasing of Sc content diminishes Hhf on the Fe nucleus. The calculations of electron-density distribution along the 111 direction in elemental crystallographic cell indicate a remarkable increase of electron charge at the Fe sites with the Sc content increasing. The 40–49° cone angles of the Fe sublattices at 12 K have been estimated from Mossbauer spectra analysis.

Magnetic anisotropy of UCo10Si2

Abstract Magnetic properties of UCo 10 Si 2 have been studied on aligned powders. Results are compared with properties of the isostructural compound YCo 10 Si 2 . The Curie temperature ( T C = 550K) and the molecular magnetic moment (μ m = 8.6 μ B ) were found to be considerably lower in the U compound compared with YCo 10 Si 2 (750 K and 11.0 μ B respectively). However, UCo 10 Si 2 has a strong uniaxial magnetic anisotropy (the first anistrophy constant K 1 = 3.3MJ m −3 at 4.2 K), whereas YCo 10 Si 2 has anisotrophy of the cone type. If the difference in anisotropy is attributed to the U sublattice, this might point to a magnetic state of U in UCo 10 Si 2 like in the Fe isostructural analogue UFe 10 Si 2 .

Low-temperature electrical resistance of the U(Cu,Ni)4Al8 system and magnetic and electrical properties of ScCu4+xAl8−x

The electrical resistance measurements of the UCu4−xNixAl8 were performed in the temperature range 30 mK–4.2 K. Also the samples without Ni but with various Al/Cu ratio were investigated. All these compounds exhibit strong decrease in R(T) plot below 1 K which results from a presence of CuAl2 as impurity. The ScCu4+xAl8−x alloys exist as the single-phase materials for the composition range 0 ⩽ x ⩽ 2.15. At low temperatures the alloys are weakly paramagnetic whereas at higher temperature weakly diamagnetic properties are observed. Their electrical resistivity is low and weakly temperature dependent. No anomalies in both χ(T) and ϱ(T) plots can be detected.

Magnetic properties of UMnxAl12−x alloys

Abstract The UMn x Al 12− x system exists in the single-phase form for 3⩽ x ⩽7. The magnetic properties were examined in the temperature range 4.2–1000 K and under magnetic fields up to 14 T at low temperature (LT). For x > 3 the samples are paramagnetic with θ p and μ eff equal to −110-0 K and 2.42–4.25 μ B respectively, in agreement with previous magnetic and neutron diffraction examinations. For x =3 the magnetic susceptibility shows a sharp maximum at about 30–40 K. The magnetization vs. magnetic field plot at LT shows a linear behaviour below about 2.0 T and then a sudden change in slope, reaching a kind of saturation. The results are discussed in terms of the MnMn separation and possible magnetic structure.

Crystal structure of single crystal and electrical resistivity of UMn5Al7

Abstract The crystal structure of UMn 5 Al 7 determined from single crystal refinement is tetragonal of the ThMn 12 -type with space group I4/mmm and with the room temperature lattice parameters: a=8.7870(10) A and c=5.0850(10) A . The U atoms are located in the 2(a) sites, whereas the Mn atoms are in the 8(f) and 8(j) sites. Electrical resistivity increases weakly with increasing temperature and is strongly sample dependent.