Co-59 NMR study in Co-Fe alloys/Co magnetite composites

Iron-cobalt compounds containing 20-30 at.% Fe with unusual crystallographi c structure have been observed in metal-oxide composite synthesized in an a queous media at 120-140 degreesC. The oxide is a cobalt-containing magnetit e. The metallic component is found in two crystallographic structures depen ding on the preparation conditions: a b.c.c. (alpha -Fe) structure and an c t-Mn, also called Re,,Ti,, structure (space group I-43m). The alpha -Mn-lik e CoFe alloy is encountered for the first time and transforms into the b.c. c. structure during annealing at temperatures as low as 160 degreesC. In th e b.c.c. structure itself, the cobalt and iron atoms present a chemical sho rt-range order totally different from the regular B2 phase of the ordered b ulk CoFe compound. This particular order is similar to the one encountered in high vacuum co-deposited thin films. For annealing temperatures larger t han 400 degreesC, the metallic component loses some Fe and transforms into nearly pure f.c.c. cobalt. This study shows that chemical as well as physic al low temperature preparation techniques favor original CoFe ordered phase s not achievable by conventional metallurgy. (C) 2001 Editions scientifique s et medicales Elsevier SAS. All rights reserved.