Mechano-Synthesis, Characterization, and Magnetic Properties of Nanoparticles of Cobalt Ferrite, CoFe₂O₄

TLDR

Cobalt ferrite nanoparticles were produced by mechanically milling LDH‑CO3. X‑ray diffraction shows that 5‑h milling yields pure CoFe₂O₄ nanoparticles, while longer milling enlarges particles, raises the blocking temperature, and eventually generates cobalt metal, and Mössbauer and magnetization data indicate the 5‑h sample consists of <10‑nm particles with a 200‑K blocking temperature, and magnetic measurements reveal size‑dependent saturation magnetization, loop squareness, and surface‑moment randomness.

Abstract

Cobalt ferrite, CoFe2O4, nanoparticles have been obtained from pre-prepared layered double hydroxide carbonate, LDH-CO3, by mechanical milling. X-ray powder diffraction shows the only product of the milling for 5 h of the LDH-CO3 is cobalt ferrite nanoparticles. Longer-term milling induces particle growth characterized by sharpening of the Bragg peaks and an increase of the blocking temperature, while prolonged milling results in the formation of some cobalt metal. Mössbauer spectra and temperature dependence of the magnetization of the 5-h milled sample suggest that it consists of nanoparticles of size less than 10 nm with blocking temperature of 200 K, in good agreement with microscopy showing an average size of 6 nm. The magnetic properties exhibit a strong dependence on the particle size as a result of an unusual cation distribution and of surface effect. The saturation magnetization at 5 K and the squareness of the hysteresis loops increase with the average particle size. The difference between zero-field-cooled and field-cooled hysteresis loops is correlated with the presence of randomness of the moments at the surface.