Phase relation, crystal structure, and magnetic properties of La-Co-Si alloys.

Abstract

The phase relation of the La-Co-Si ternary system in the La-poor region (\ensuremath{\le}25 at. % La) was determined by means of x-ray powder diffraction. In this region there exist four ternary compounds: ${\mathrm{LaCoSi}}_{3}$, ${\mathrm{LaCoSi}}_{2}$, ${\mathrm{LaCo}}_{2}$${\mathrm{Si}}_{2}$, and ${\mathrm{LaCo}}_{13\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$. The cubic ${\mathrm{LaCo}}_{13\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ intermetallic compound with ${\mathrm{NaZn}}_{13}$-type structure could be changed to a tetragonal one with ${\mathrm{NaZn}}_{13}$-derivative structure by vacuum annealing after arc melting. The tetragonal ${\mathrm{LaCo}}_{13\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ compounds can exist stably in the composition range between x=2.0 and 5.0. The space group of the tetragonal ${\mathrm{LaCo}}_{13\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ intermetallic compound is I4/mcm. The crystal structure of the tetragonal phase was refined by the Rietveld method. Si atoms show a strong preference for occupying a specific crystallographic position in the structure; i.e., atomic ordering occurs. Magnetic measurements indicate that the tetragonal ${\mathrm{LaCo}}_{13\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ intermetallic compounds are ferromagnetic with Curie temperature higher than 900 K and anisotropy fields of about 10 kOe. The saturation magnetic moment per Co atom in the tetragonal ${\mathrm{LaCo}}_{13\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ compounds decreases with Si content. The composition dependence of the saturation magnetic moment per Co atom can be satisfactorily explained by the rigid-band model.