Random site occupancy induced disordered N ´ eel- type collinear spin alignment in heterovalent Zn 2+ – Ti 4+ ion substituted CoFe 2 O 4 S. M. Patange, a S. S. Desai, a S. S. Meena, b S. M. Yusuf b and Sagar E. Shirsath * c CoFe 2 O 4 , cobalt ferrite (CFO) nano-particles with composition CoZn x Ti x Fe 22x O 4 (0 # x # 0.4) were synthesized by sol–gel autocombustion method. The effect of Zn 2+ –Ti 4+ substitution on the structural, magnetic and frequency dependent permeability properties of the CFO nano-particles were investigated by X-ray diffraction, 57 Fe M ¨ ossbauer 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 Zn 2+ ions at the tetrahedral A- sites, and Co 2+ and Ti 4+ ions at octahedral B-sites. The saturation magnetization (M s ) increased from 58 to 75 emu g 1 for up to x ¼ 0.2 and then decreased, while the coercivity decreased continuously with Zn 2+ –Ti 4+ substitution. Two distinct composition ranges with Zn 2+ –Ti 4+ substitution are identified for which M s variation with x is explained by the N ´ eel and Yafet–Kittel models. The room temperature M ¨ ossbauer spectra are analyzed in detail for probing the magnetic properties of Fe based Zn 2+ –Ti 4+ substituted CFO. The effect of Zn 2+ –Ti 4+ substitution on various M ¨ ossbauer 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 aN ´ eel-type collinear spin alignment, whereas the Zn 2+ –Ti 4+ 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 Zn 2+ –Ti 4+ 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. 1. Introduction The spinel ferrite crystal structure has the space group Fd3m (F 4 1/ d  3 2/m , no. 227 in the International Tables) which consists of 56 atoms; 32 are oxygen anions assuming a close packed cubic structure, and the remaining are metal cations residing on 8 of the 64 available tetrahedral (A) sites and 16 of the 32 available octahedral (B) sites. The size and valence state of the cation species determine the lling of these sites and strongly inu- ence the material's magnetic and electrical properties. Cobalt ferrite (CFO) crystallizes in an inverse spinel struc- ture. The inverse spinel form of CFO is represented generally as A 3+(TET) [B 2+ ,B 3+ ] (OCT) O 4 where the tetrahedral A-sites are occu- pied by Fe 3+ cations and octahedral B-sites are occupied by Fe 3+ ions and the divalent Co 2+ ions respectively. 1 In CFO, the magnetic interactions between these metal ions, limited only to the nearest neighbors, are antiferromagnetic in nature and their magnitudes are given by exchange integrals. For example, the exchange integral J AB describes the nature and strength of the interaction between any two nearest A and B ions. Similarly, J AA , J BB and J AB are the exchange integrals showing the strength of any A–A, B–B and A–B nearest neighbor interactions respec- tively. Generally, when the A and B sub-lattices are occupied by the magnetic ions of the same type, the relative strengths are given by |J AB | [ |J BB |>|J AA |: thus, the moments on A- and B- sites are aligned anti-parallel to each other and this impels the two A–A and B–B moments to be parallel despite the anti- ferromagnetic exchange interactions between them, so these moments are in frustrated state. One can in principle alter the relative strengths of all these exchange integrals by changing the type of the magnetic ions on A and B sites. 2 CFO is widely a Materials Research Laboratory, Shrikrishna Mahavidyalaya, Gunjoti, 413613, MS, India b Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India c Spin Device Technology Center, Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan. E-mail: smpatange@rediffmail.com; Fax: +91-2475-250091; Tel: +91-94233472 Cite this: RSC Adv. , 2015, 5, 91482 Received 24th May 2015 Accepted 21st October 2015 DOI: 10.1039/c5ra21522f www.rsc.org/advances 91482 | RSC Adv., 2015, 5, 91482–91492 This journal is © The Royal Society of Chemistry 2015 RSC Advances PAPER