Synthesis and characterization of ultrafine nickel(II)-bearing ferrites (NixFe3 –xO4, x= 0.14–1.0)

Abstract

Ultrafine Ni^II-bearing ferrites (UNFs) with 14–100% Ni²⁺-substitution (x in Ni_xFe_{3 –x}O₄) for Fe²⁺ in magnetite have been synthesized by hydrolysis of Ni²⁺, Fe²⁺ and Fe³⁺ ions at 60 °C, followed by aging of the co-precipitates at 60 °C and heating to 300 °C. The aged co-precipitates (ACPs) obtained at an Ni^II/Fe_total mole ratio r= 0.05–0.30 (x= 0.14–0.69) in the starting solution gave the relatively sharp peaks of the spinel structure in the XRD patterns. At r > 0.40 (x 0.85), the spinel peaks were broader as r increased. The IR spectra of the ACPs showed that, with increasing the r value to r 0.30, the ACP approached an amorphous phase in which OH^– groups and/or coordinated water remained, indicating that the Fe^II ions enhance the dehydration of the ACP. The ACPs were the precursors of the UNFs, and were transformed into the UNFs by heating to 300 °C. The Ni²⁺-substitution of the UNFs was corroborated by a linear decrease in the lattice constant with an increase in the Ni²⁺ content of the product. The average crystallite sizes ranged from 8 to 17 nm. The transition from a magnetically split pattern to a doublet in the room-temperature Mössbauer spectra of the UNFs corresponded to a decrease in the average crystallite size, indicating superparamagnetic behaviour of ultrafine ferromagnetic particles. The UNFs gave a room-temperature saturation magnetization (M_s) much lower than the theoretical M_s values.