Random site occupancy induced disordered Néel-type collinear spin alignment in heterovalent Zn²⁺–Ti⁴⁺ ion substituted CoFe₂O₄

Abstract

CoFe₂O₄, cobalt ferrite (CFO) nano-particles with composition CoZn_xTi_xFe_2-2xO₄ (0 ≤ x ≤ 0.4) were synthesized by sol–gel autocombustion method. The effect of Zn2+–Ti4+ substitution on the structural, magnetic and frequency dependent permeability properties of the CFO nano-particles were investigated by X-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy, vibrating sample magnetometry, transmission electron microscopy and permeability analysis. The Rietveld refinement of XRD patterns confirm the single spinel phase and the crystallite size is found in the range of 22–32 nm. Cation distribution was estimated by refining the XRD pattern by Rietveld method, and shows Zn²⁺ ions at the tetrahedral A-sites, and Co²⁺ and Ti⁴⁺ ions at octahedral B-sites. The saturation magnetization (Ms) increased from 58 to 75 emu g−1 for up to x = 0.2 and then decreased, while the coercivity decreased continuously with Zn²⁺–Ti⁴⁺ substitution. Two distinct composition ranges with Zn²⁺–Ti⁴⁺ substitution are identified for which Ms variation with x is explained by the Néel and Yafet–Kittel models. The room temperature Mössbauer spectra are analyzed in detail for probing the magnetic properties of Fe based Zn²⁺–Ti⁴⁺ substituted CFO. The effect of Zn²⁺–Ti⁴⁺ substitution on various Mössbauer parameters, viz. hyperfine field distribution, isomer shift, quadrupole splitting, and line width, has also been studied. The variation of nuclear magnetic fields at the A and B sites is explained on the basis of A–B and B–B supertransferred hyperfine interactions. The CFO nanoparticle is considered to possess a fully inverse spinel structure with a Néel-type collinear spin alignment, whereas the Zn²⁺–Ti⁴⁺ substitution in CFO is found to be structurally and magnetically disordered due to the nearly random distribution of cations and the canted spin arrangement. This study also demonstrates that one can tailor the magnetic properties of CFO particles by optimizing the Zn²⁺–Ti⁴⁺ substitution. The increase in the permeability, saturation magnetization and lower loss factor makes the synthesized materials suitable for applications in microwave devices and deflection yokes.