Synthesis and surface modified hard magnetic properties in Co0.5Pt0.5 nanocrystallites from a rheological liquid precursor

Abstract

Small crystallites of a metastable phase Co0.5Pt0.5 are precipitated by heating a rheological liquid precursor of cobalt–hydrazine complex and platinum chloride H2PtCl6·xH2O in polymer molecules of poly(vinylpyrrolidone) (PVP) in ethylene glycol. The hydrazine co-reduces nascent atoms from the Co2+ and Pt4+ that recombine and grow as Co0.5Pt0.5. The PVP molecules cap a growing Co0.5Pt0.5 as it achieves a critical size so that it stops growing further in given conditions. X-ray diffraction pattern of a recovered powder reveals a crystalline Co0.5Pt0.5 phase (average crystallite size D∼8nm) of a well-known Fm3m-fcc crystal structure with the lattice parameter a=0.3916nm (density ρ=14.09g/cm3). A more ordered L10 phase (ρ=15.91g/cm3) transforms (D≥25nm) upon annealing the powder at temperature lesser than 700°C (in vacuum). At room temperature, the virgin crystallites bear only a small saturation magnetization Ms=5.54emu/g (D=8nm) of a soft magnet and it hardly grows on bigger sizes (D≤31nm) in a canted ferromagnetic structure. A rectangular hysteresis loop is markedly expanded on an optimally annealed L10 phase at 800°C for 60min, showing a surface modified coercivity Hc=7.781kOe with remnant ratio Mr/Ms=0.5564, and Ms=39.75emu/g. Crystallites self-assembled in an acicular shape tailor large Hc from ideal single domains and high magnetocrystalline anisotropy of a hard magnet L10 phase.