Local Atomic Order, Morphology, and Magnetic Properties in Silica‐Coated FeCo Nanoparticles Synthesized by Coprecipitation Technique

Abstract

Nanoparticles (NPs) of pure Fe25Co75 are sintered by Fe and Co coprecipitation followed by SiO2 deposition by tetraethoxysilane hydrolysis to form silica‐coated Fe25Co75 NPs ((Fe25Co75)x(SiO2)100–x (90 ≤ x ≤ 10, wt%)). Study of the morphology and phase composition of (Fe25Co75)x(SiO2)100–x nanopowders evidences the formation of agglomerations of NPs with individual medium diameter ≈20–50 nm containing partially oxidized cores of α‐FeCo bcc alloy covered with amorphous SiO2. Mössbauer spectroscopy clearly reveals that coverage with SiO2 shells promotes up to four‐time enhancement of resistance of FeCo cores to undesirable formation of surface Fe(Co) hydroxides. Temperature dependencies of magnetization recorded under field‐cooled (FC) and zero‐field‐cooled (ZFC) protocols for uncoated Fe25Co75 and (Fe25Co75)70(SiO2)30 nanopowders reveal that some part of NPs being in single‐domain state form superferromagnetic long‐range ordering.