Study of rare-earth iron compounds and related nitrides and hydrides

Date of Completion

January 1998

Keywords

Physics, Electricity and Magnetism|Physics, Condensed Matter

Degree

Ph.D.

Abstract

This work presents x-ray structure analysis, magnetization measurements, and nuclear magnetic resonance (NMR) study of the $\rm R\sb2Fe\sb{17}$ (R = rare earth) system, its interstitial compounds $\rm R\sb2Fe\sb{17}Z\sb{x}$ (Z = N or H, 0 $<$ x $<$ 5), and Ga substitutional compounds $\rm R\sb2Fe\sb{17-y}Ga\sb{y}$ (0 $<$ y $<$ 8). The nitrogenation was carried out by using nitrogen gas or ammonia. With the nitrogen gas approach, it was found that the system was separated into nitrided and unnitrided regions, and that a strong interaction existed between interstitial nitrogen and lattice. NMR measurements of $\sp{89}$Y suggest that nitrogen predominately occupies 2 out of 3 octahedral sites in $\rm Y\sb2Fe\sb{17}N\sb{2.6}.$ In the case of $\rm Sm\sb2Fe\sb{17}N\sb{2.6},$ both lattice expansion and electronic interaction between nitrogen and Sm are responsible for the enhancement of magnetization and formation of room temperature uniaxial anisotropy. With an ammonia nitrogenation approach, pronounced differences such as nitrogenation temperature, duration, nitrogen content and occupancy have been explored, together with some similarities, such as lattice expansion and phase separation. These observations suggest that the nitrided portion in the ammonia prepared samples has a higher nitrogen content.^ The hydrogenation process is reversible in both $\rm R\sb2Fe\sb{17}$ hydrides and $\rm Y\sb2Fe\sb{17}$ nitri-hydrides, and the filling of both octahedral and tetrahedral sites has been suggested by the experiments, even though the loading of hydrogen into nitrides must be carried out at a higher temperature and a longer duration. In addition, the influence of hydrogen on the hyperfine field of $\sp{89}$Y is different. For hydrides, the center of gravity of the $\sp{89}$Y peak shifts down with hydrogenation, and no $\sp{89}$Y satellite peaks appear, but with nitri-hydrides, the NMR spectrum of $\sp{89}$Y behaves as if additional nitrogen were added into the lattice.^ Room temperature uniaxial anisotropy is found for $\rm Sm\sb2Fe\sb{17-y}Ga\sb{y}$ with Ga content in the range of 3 to 4, in spite of the fact that Ga substitution reduces the basal plane anisotropy of the Fe sublattice. The experimental results reveal that the Ga occupancy is selective. The different occupation pattern of Ga between the hexagonal $\rm Y\sb2Fe\sb{17-x}Ga\sb{x}$ and rhombohedral $\rm Sm\sb2Fe\sb{17-x}Ga\sb{x}$ is related to the difference in lattice structure. ^

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