Magnetic studies of novel mixed rare earth-iron rich R2/T17 intermetallics

The modern families of high performance permanent magnets are based on combinations of transition metals (Fe and Co) and rare earths. In general, the transition metal provides high Curie temperature and saturation moment, while the rare earth is responsible for a large uniaxial anisotropy resulting in good coercivity in the final magnet. R₂TM₁₇ (R = rare earth, TM = transition metal) crystallize either in the hexagonal Th ₂Ni₁₇-type structure with heavy rare earth elements or in the rhombohedral Th₂Zn₁₇-type structure with light rare earth elements. The hexagonal structure supports appreciable deviation from the ideal 2:17 stoichiometry and in practice is realized only in disordered (DH) phase.; In this thesis we present our results on the mixed rare earth 2:17 systems that reveal the existence of a disorder in the rhombohedral phase (DR). This novel disordered rhombohedral (DR) compounds derived from the CaCu₅ structure, with the same unit cell as the ordered rhombohedral (OR) 2:17 structure, but with excess Fe dumbbells replacing some of the rare earth atoms holds a promise of realizing better high density permanent magnets. Mixed rare earth 2:17 compounds favored the DR structure when Fe was partially replace with Si in appropriate amounts. These compounds with the DR structure, as inferred from the ND refinements, possessed high Curie temperatures. Co and Si substitution for Fe resulted in the compounds favoring two phases of either the OR and DH or DR and DH. The compounds refined with the DR and DH structures for the ND data possessed high Curie temperatures.