V.I. Krylov

Magnetic behaviour of UCoAl1-xSnx

The 119Sn Mössbauer spectroscopy has been applied to investigate the magnetic behaviour of the UCoAll-xSnx compounds, in which the graduring transition from paramagnetism (UCoAl) to ferromagnetism (UCoSn) reflects the reduced 5f-ligand hybridization. The influence the Al−Sn local surrounding of U atoms on the U magnetic moments has been observed for the concentrations near to the onset of magnetic ordering.

Mössbauer-effect study of competitive exchange interactions in UNi1−xCoxGa solid solutions

Abstract Results of Mossbauer spectroscopy of 119 Sn atoms substituted on Ga sites in the UNi 1− x Co x Ga solid solutions are presented and discussed together with previously reported magnetization data in terms of evolution of magnetism between the antiferromagnet UNiGa and ferromagnet UCoGa. The Mossbauer spectra were analyzed by considering the hyperfine field transferred to the 119 Sn nucleus from the six nearest U magnetic moments and a weak signal due to anisotropic polarization of Sn 5p states. The low-temperature spectra for UNiGa and UCoGa are consistent with their ground-state magnetic structures. The nonmagnetic component observed for UNiGa persists in the spectra for x up to 0.4 which indicates the presence of antiferromagnetically coupled U moments. The nonmagnetic component emerging in the spectra for x around 0.77 is tentatively attributed to clusters of frustrated U magnetic moments surrounding the 119 Sn nuclei.

Magnetization study of the UNi1−xCoxGa solid solutions

Abstract Results of a crystal-structure and magnetization study of UNi 1− x Co x Ga solid solutions are presented. All compounds over the entire concentration range have been found to crystallize in the hexagonal ZrNiAl-type structure. The concentration dependences of the lattice parameters a and c are linear on intervals x x >0.7 but they considerably change their slope around x =0.6–0.7, which is probably due to preferential occupation of the two non-equivalent transition-metal sites by the Ni and Co atoms. The lattice volume, however, does not show such an anomaly. Magnetization data are interpreted in terms of a sudden conversion of antiferromagnetism in UNiGa to ferromagnetism induced by substituting 2% of Co atoms for Ni without apparent change of the magnetic moment and magnetic ordering temperature. All compounds with x ≥0.02 show clear attributes of ferromagnets. Whereas the magnetic moment more or less monotonously decreases with increasing Co concentration the values of Curie temperature pass through a broad minimum in the intermediate Ni–Co concentration region. Since the high-field differential susceptibility exhibits a pronounced maximum in this concentration region, microscopic magnetic inhomogeneities (freezing of antiferromagnetically coupled regions) due to substitutional disorder in the Ni–Co sublattice may be a possible mechanism responsible for the non-monotonous evolution of T C values.

Influence of vacancies on the magnetic structure of UCuGe

Abstract The local magnetic moment configurations in the UCu x Ge ( x =0.90, 0.95 and 0.98) and UCuGe y ( y =0.90 and 0.95) non-stoichiometric intermetallic compounds were studied by means of Mossbauer spectroscopy in the temperature range 10–100 K. The 119 Sn nuclei in Sn atoms doped on the Ge sites were exploited as a local probe. The vacancies on the Ge sites in UCuGe y seem to have no considerable influence on the original magnetic structure of UCuGe. On the other hand, two ‘new’ magnetic moment configurations, which coexist at low temperatures with the original UCuGe magnetic structure, were observed in the Cu-deficient UCu x Ge compounds. The changes of the mutual orientation of the U magnetic moments in the ‘new’ (perturbed) magnetic structure is due to the vacancy-induced variations of the effective exchange fields, which determines also the temperature evolution of magnetization. The observed anomalous temperature dependence of the hyperfine magnetic field on the 119 Sn nuclei is qualitatively interpreted within the model of ‘melting’ of frustrated spins.

Competition between ferromagnetic and antiferromagnetic exchange interactions in the quasibinary U(Ge1-xSnx)2 compounds

Abstract The evolution of the magnetic ordering temperature T M and the low-temperature magnetic structure in the quasibinary compounds U(Ge 1− x Sn x ) 2 for 0.01≤ x ≤1 were studied by means of 119 Sn Mossbauer spectroscopy. These compounds are solid solutions between the ferromagnet UGe 2 ( T C =52 K) and the antiferromagnet USn 2 ( T N =75 K), both being collinear. The boundary compounds possess different easy-magnetization directions, the a - and c -axis, respectively. The minimum ordering temperature ( T min ≈36 K) has been found for x =0.58. The complex Mossbauer spectra observed for lower Sn concentrations could be attributed to the appearance of two non-collinear spin configurations characterized by a canting angle between pairs of U magnetic moments of 60 and 120°, respectively. When increasing the Sn concentration beyond x =0.58, the Mossbauer spectrum resembles that observed for USn 2 which is accompanied by a rapid increase of T M .

Magnetic hyperfine fields on119Sn nuclei in uranium compounds

Abstract119Sn Mössbauer spectroscopy studies were performed on 12 uranium intermetallic compounds in order to investigate correlations between the formation of the magnetic moment on the U atom and the magnetic hyperfine field transferred to 119Sn nuclei in magnetically ordered materials. The measured hyperfine fields (Hhf) are related to the values of the ordered U magnetic moments (UB) by UoHhf/n=A UU. The parameter A varies between 0.73 (UGa2) and 1.55 (UGe2). It seems to correlate with the extent of the hybridization of the 5f states with the conduction electron states.

Hyperfine fields for119Sn in laves phases Rt2 (R=rare earth: T=Fe, Co, Ni)

The hyperfine fields acting on119Sn nuclei in the RT2 (R=Sm−Lu; T=Fe, Co, Ni) have been measured by Mössbauer spectroscopy. It has been found that the hyperfine fields acting on119Sn nuclei in the RT2 compounds are changed at some electronic occupation of the 4f shell of the R-component. The occupation of 4f shell is varied with the T-component. The sharp change of the hyperfine fields are connected with the change of the electronic band structure.

On the conversion of HFF measurements to constant volume

The procedure for converting the hyperfine field (HFF) to constant volume is considered. It is shown that the equation invariably used for this purpose is valid as a first approximation only in those cases when the corrections are small. A new equation is suggested which is valid in the general case.