No description available.

No description available.

No description available.

No description available.

Abstract not available

A combination of 169Tm-Mo??ssbauer spectroscopy and inelastic neutron scattering (INS) has been used to investigate the crystal field (CF) interaction at the rare earth site for the tetragonal series RT2Si2 (where R = rare earth) and the orthorhombic series RNiAl4. Single phase specimens were prepared in an argon arc furnace and characterised using x-ray powder diffraction, specific heat and magnetic measurements. For the RT2Si2 series previous investigations were extended to include T = Mn and Cr whose sub-lattices are antiferromagnetic well above room temperature. However, the rare earth sub-lattices were confirmed to order close to liquid helium temperature. With the assistance of the lattice electric field gradient (EFG) for isostructural GdCr2Si2 (determined elsewhere using 155Gd-Mo??ssbauer spectroscopy) and the within-rank CF parameter ratios for HoCr2Si2 (determined elsewhere using INS), the experimental 169Tm quadrupole interaction data were analysed to arrive at CF parameters for the Tm3+ site in TmCr2Si2. The final CF parameters match well with the trend observed for other members of the series RT2Si2 (T = Fe, Co, Ni, Cu). CF schemes were also determined for Tm3+ in TmMn2Si2 and Er3+ in both ErCr2Si2 and ErMn2Si2. For the RNiAl4 series, TmNiAl4 was determined to be antiferromagnetic below TN = 4.7 K with the 169Tm-Mo??ssbauer spectra retaining a magnetic appearance up to 80 K due to the effect of slow spin-lattice relaxation. The relaxation data are consistent with a well-isolated ground state doublet (or pseudo-doublet) whose eigenfunctions have high components of mJ = ??6 states and with the relaxation proceeding via an excited state at 350 K. Because of the relaxation effect, the 169Tm-Mo??ssbauer data could not be interpreted in terms of CF parameters in the usual way. Instead, the INS spectra recorded for ErNiAl4 were analysed using a novel semi-empirical approach that coupled rank 2 CF parameters converted from the EFG tensor for isostructural GdCr2Si2 (determined elsewhere using 155Gd-Mo??ssbauer spectroscopy) with simple point-charge model calculations of the rank 4 and 6 within-rank CF parameter ratios. The first full set of CF parameters for this series was then determined for the Er3+ site in ErNiAl4.

Thesis (Ph. D.)--University of Hong Kong, 2008.

Thesis (Ph. D.)--University of Missouri-Columbia, 2004. / Typescript. Vita. Includes bibliographical references (leaves 118-122). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2004. / Typescript. Vita. Includes bibliographical references (leaves 118-122). Also available on the Internet.

Ce travail s’inscrit dans le cadre général de la recherche de nouveaux matériaux nanocomposites durs/doux dont le domaine d’application concerne les aimants permanents performants. Ces nanocomposites présentent des propriétés magnétiques intéressantes grâce à leur forte rémanence combinée à une grande coercivité. Ceci permet d’augmenter considérablement l’énergie spécifique de l’aimant. Dans ce contexte, nous avons élaboré et étudié de nouveaux nanocomposites à base de phases dures hors équilibre Sm(Fe,Si)9C et Sm(Fe,Ga)9C et des phases douces _-Fe et FeCo. Pour les nanocomposites intrinsèques, SmFeSiC /_-FeFe, la formation de la phase douce est contrôléeà partir des conditions d’élaboration de la phase dure en agissant sur l’excès de samarium. La microscopie électronique à transmission montre des nanograins de forme pseudo-sphérique et de diamètre moyen de l’ordre de 20 nm. La valeur maximale du produit d’énergie spécifique (BH) max est obtenue pour l’échantillon avec environ 15% de _-Fe, elle est égale à 11.7 MGOe (93.6kJ/m3). Pour les nanocomposites extrinsèques, les phases dures (Sm-Fe-Si-C) et douces (FeCo) ont été élaborées séparément puis mélangées dans des conditions appropriées. Un record d’aimantation à saturation de 235 emu/g a été enregistré pour la composition Fe55Co45 élaboré par la méthode polyol. Les nanocomposites SmFeSiC /FeCo et SmFeGaC/FeCo, ont révélé une amélioration de l’aimantation avec le taux de la phase douce mais avec une diminution du champ coercitif. L’énergie spécifique atteint des valeurs maximales de l’ordre de 13.5 MGOe (108 m3) et 12.4 MGOe(99.2 kJ/m3), pour les échantillons SmFeGaC/20%FeCo et SmFeSiC/10%FeCo, respectivement. D’autre part, nous avons étudié la structure locale des alliages Pr2(Co,Fe)7, un autre cas de phase dure, par spectroscopie EXAFS et nous avons montré que le site préférentiel du Fe dans ces composés est le site 12k / This work focuses on the synthesis and the study of new hard / soft nanocomposites. These nanocomposites exhibit interesting magnetic properties thanks to their high remanence combined with a high coercivity Hc. This allows a huge increase of the specific energy of the magnet. In this context, we have developed and studied new nanocomposites based on the out of equilibrium hard phases Sm (Fe, Si) 9C and Sm (Fe, Ga) 9C and soft phases _-Fe and FeCo. For the intrinsic nanocomposites SmFeSiC /_-Fe, we have shown by transmission electron microscopy, an average grain size of about 20 nm for the studied samples. The maximum value of (BH) max is obtained for the nanocomposite with about 15 % of _-Fe, it is equal to 11.7 MGOe (93.6 kJ/m3). For the extrinsic nanocomposites SmFeSiC/FeCo et SmFeGaC/FeCo, we have shown that the energy product (BH) max reaches a maximum and then decreases with increasing the rate of FeCo. The maximum values obtained are about 13.5 MGOe (108 kJ/m3) and 12.4 MGOe (99.2 kJ/m3) for SmFeGaC / 20% FeCo and SmFeSiC/ 10% FeCo samples, respectively. Finally, we have investigated the local structure of Pr2 (Co,Fe) 7 alloys by EXAFS spectroscopy and we have shown that the preferential site Fe in these compounds is the 12k site