Hydrothermal synthesis of nickel doped cobalt ferrite nanoparticles: optical and magnetic properties

Nickel-doped cobalt ferrite \([{\text {Co}}_{1- x }\hbox {Ni}_{ x }\hbox {Fe}_{2}\hbox {O}_{4} \,(0\le x\le 1)]\) nanoparticles are synthesized by means of hydrothermal method. The structural, morphological and microstructural characterization revealed crystallite size was roughly spherical for lower nickel concentration while for higher ones in diamond shape consisting of nanosized grains. The optical band-gap (\(E_{g}\)) values decreased with the \({\text {Ni}}^{2+}\) ions (x) concentration being 2.94 and 2.51 eV for \(x=0\) and \(x=1\), respectively. The presence of nickel in the cobalt ferrite structure affected the magnetic properties. For instance, the saturation magnetization, \(M_{s}\) and remanent magnetization, \(M_{r}\) decreased from 369 to 256 emu cm⁻³ and 131–45 emu cm⁻³ for \(x=0\) and \(x=1\), respectively. The \(M_{s}\) data was discussed in term of the three-sublattice of non-collinear spin (canted spin) structure proposed by Yafet and Kittel model. On the other hand the coercivity, \(H_{c}\) from 890 Oe to 1590 Oe for \(x=0\) and \(x=0.6\), and sharply dropped to 50 Oe for \(x=1.\) The enhanced coercivity was discussed in terms of particles size, defects and residual strain which may act as pinning centers. The cubic magnetocrystalline constant, \(K_{1}\) determined by using the “law of approach” to saturation decreases with \({\text {Ni}}^{2+}\) ions (x) concentration being \(4.9\times 10^{6}\) and \(2.8\times 10^{6}\) erg cm⁻³ for \(x=0\) and \(x=1\), respectively. These results were discussed in terms of the inter-particle interactions induced by the presence of \({\text {Ni}}^{2+}\) ions at the octahedral sites which affected the strength of L–S coupling.