~(59)Co NMR study in Co-Fe alloys/Co magnetite composites

摘要

Iron-cobalt compounds containing 20-30 at. percent Fe with unusual crystallographic structure have been observed in metal-oxide composite synthesized in an aqueous media at 120-140 deg C. The oxide is a cobalt-containing magnetite. The metallic component is found in two crystallographic structures depending on the preparation conditions: a b.c.e. (#alpha#-Fe) structure and an #alpha#-Mn, also called Re_(24)Ti_5, structure (space group I-43m). The #alpha#-Mn-like CoFe alloy is encountered for the first time and transforms into the b.c.e. structure during annealing at temperatures as low as 160 deg C. In the b.c.e. structure itself, the cobalt and iron atoms present a chemical short-range order totally different from the regular B2 phase of the ordered bulk CoFe compound. This particular order is similar to the one encountered in high vacuum co-deposited thin films. For annealing temperatures larger than 400 deg C, the metallic component loses some Fe and transforms into nearly pure f.c.c. cobalt. This study shows that chemical as well as physical low temperature preparation techniques favor original CoFe ordered phases not achievable by conventional metallurgy.