F.M. Mulder

155Gd Mössbauer effect and magnetic properties of novel RT2B2C compounds with TNi, Co

Abstract We have prepared the compounds GdNi 2 B 2 C and GdCo 2 B 2 C and investigated their magnetic properties by means of 155 Gd Mossbauer spectroscopy. From the temperature dependence of the hyperfine field, we derived that these two compounds give rise to magnetic ordering at 5.5±0.5 K and 21±1 K respectively. Large values of the electric field gradient V zz were found, reminiscent of compounds of the related ThCr 2 Si 2 type. A quantitative explanation of the origin of these field gradients is presented by ab initio band structure calculations. The experimental values of V zz have been used to estimate values for the second-order crystal field parameter in GdNi 2 B 2 C ( A 2 0 =−428 Ka 0 −2 ) and GdCo 2 B 2 C ( A 2 0 =−346 Ka 0 −2 ).

155Gd mössbauer effect in AlB2-type GdGa2-xAlx compounds

Abstract We have investigated the effect of Al substitution on the lattice constants and 135 Gd Mossbauer spectra of GdGa 2 . Marked changes were observed in the values of the electric field gradient derived from the quadrupole splitting of the spectra, changing from positive to negative for x > 0.5 in GdGa 2 -x Al x . This was ascribed to the concomitant change in nearest-neighbour configuration of the Gd atoms in this series, as deduced from the jump-like change in lattice parameters around x =0.5. The variation in V zz across the series GdGa 2-x Al x is discussed in relation to the second-order crystal field parameter A 2 0 which shows a similar variation and sign reversal.

155Gd Mössbauer effect studies and magnetic properties of GdFe12−xMox(Ny) compounds

Abstract Structural and magnetic properties of the GdFe 12− x Mo x compounds having the ThMn 12 type of structure were studied. Rare-earth nitrides of the composition GdFe 12− x Mo x N y were prepared by the reaction of GdFe 12− x Mo x with nitrogen gas at 475°C. The crystal structure and lattice constants have been determined for all compounds. The lattice expansion of the nitrogenated compounds is mainly along the a -axis, the Curie temperatures are increased by about 110 K. We investigated the effect of interstitial nitrogen on the magnetic properties and the 155 Gd Mossbauer spectrum of GdFe 12− x Mo x . Values of the electric field gradient V zz derived from the quadrupole splitting of the spectra were strongly negative, with V zz = −21.3 × 10 21 V m −2 and V zz = − 19.9 × 10 21 V m −2 for x = 1.5 and 3.0, respectively. Nitrogenation is shown to lead to a sign reversal of the 4f as well as of the 3d sublattice anisotropies.

155Gd Mössbauer effect and magnetic properties of ternary rare earth compounds of the type RT2Ge2 (T3d, 4d)

Abstract We investigated the magnetic properties and the 155 Gd Mossbauer spectra of rare earth ternaries of the type RT 2 Ge 2 (T3d, 4d). The electric field gradient V zz derived from the quadrupole splitting of the Mossbauer spectra gives rise to a sign change when passing across the 3d and 4d series. This behaviour was explained in terms of decreasing hybridization between the Gd valence electron states and the d electron states of the T atoms, as already reported previously for the corresponding silicides. The sign of V zz was shown to be strongly correlated with the macroscopic magnetic anisotropy behaviour of the series of compounds RT 2 Ge 2 .

The electric field gradient at Gd nuclei in intermetallic compounds with CaCu5 related structures

Abstract 155Gd Mossbauer spectroscopy was used to measure the electric field gradient (EFG) at the Gd nucleus in GdT5 compounds (T=Co, Ni, Cu and Rh) and GdT3B2 compounds (T=Rh, Ru and Co). The crystal structure of these compounds is of the CaCu5 type. Using the results of self-consistent band structure calculations of the valence electron contributions to the EFG, it is shown that the main contribution, comes from the asphericity of the Gd 6p valence electron charge density.

155Gd Mössbauer effect in AlB2-type compounds

Abstract We have investigated the magnetic properties and 155 Gd Mossbauer spectra of several AlB 2 -type rare earth compounds comprising GdGa 2 , GdGaAl, GdSi 1.67 , GdGe 1.55 , GdCuGe, Gd 0.1 Er 0.9 B 2 , and the Ni 2 In-type compound GdCuSi. From the quadrupolar splitting we derived values for the electric field gradient V zz at the nuclear Gd site. The values of V zz were found to vary widely in sign and magnitude, but were strongly correlated with the nearest Gd neighbour distance in the c direction.

155Gd Mössbauer effect and magnetic properties of aluminium- and gallium-substituted GdCu5 and GdNi5

Abstract We have investigated the effect of aluminium and gallium substitution on the 155 Gd Mossbauer spectra of GdCu 5 and GdNi 5 . No marked changes were observed in the values of the electric field gradient derived from the quadrupole splitting of the spectra. This was ascribed to the absence of a strongly preferred substitution of aluminium and gallium into one of the two available non-rare earth sites in GdNi 5 and GdCu 5 . Additional evidence for this was obtained from X-ray diffraction. Further analysis of our data has shown that the earlier reported phase transition in GdCu 5 at low temperatures is probably non-existent.

Size-evolution towards metallic behavior in nano-sized gold and platinum clusters as revealed by 197Au Mössbauer spectroscopy

Abstract Four types of molecular Au 55 clusters, with different types of ligands, have been studied using 197 Au Mossbauer spectroscopy. Emission Mossbauer measurements were also performed on the larger Pt309 cluster, in which 197 Au was produced by neutron activation. There is a large influence of the ligands on the charge densities at the surface atoms of the metal atom cluster cores. For the Au 55 clusters, the inner core atoms have a different charge density at the nucleus than the bulk metal, and it depends also slightly on the type of ligands on it. For the Pt 309 compound, the inner core atoms have the same charge density at the nucleus as in the corresponding bulk metal. We, therefore, conclude that the inner core of the Pt 309 cluster is metallic with respect to the electronic charge densities, while this is not yet the case for the Au 55 cluster.

Magnetic interaction and crystal field in ErCu2Si2

Abstract 166Er Mossbauer and inelastic neutron scattering measurements were performed on the tetragonal intermetallic compound ErCu2Si2 (TN = 1.65(5) K). From the inelastic neutron scattering results a crystal field diagram has been determined. The 166Er Mossbauer spectra show direct relaxation between the split levels of the magnetic ground state doublet of the 4 I 15 2 multiplet.

155Gd Mössbauer effect and magnetic properties of GdNi2Sb2, GdCu2Sb2 and GdAl2Ga2

Abstract We have investigated the magnetic properties and the 155 Gd Mossbauer spectra of the ThCr 2 Si 2 -type compounds GdNi 2 Sb 2 , GdCu 2 Sb 2 and GdAl 2 Ga 2 . These three compounds were found to order antiferromagnetically, with T N =6.5 K, 15.8 K, and 42.4 K respectively. The electric field gradient V 22 derived from the quadrupole splitting of the Mossbauer spectra gives rise to a sign change at the end of the T3d series in GdT 2 Sb 2 , as was observed previously also for the corresponding compounds with Si and Ge. This behaviour was explained in terms of decreasing hybridization between the Gd valence electron states and the d electron states of the T atoms.