J Supercond Nov Magn DOI 10.1007/s10948-016-3442-1 ORIGINAL PAPER Influence of Co 2+ Substitution on Cation Distribution and on Different Properties of NiFe 2 O 4 Nanoparticles Seema Joshi 1 · Manoj Kumar 1 Received: 30 December 2015 / Accepted: 29 January 2016 © Springer Science+Business Media New York 2016 Abstract Ni 1−x Co x Fe 2 O 4 nanoparticles with x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 (named NC0, NC10, NC20, NC30, NC40, and NC50, respectively) were synthesized by wet chemical co-precipitation method. The prepared nanoparti- cles were crystallized in the cubic spinel structure of space group Fd3m with a narrow size distribution from 13 to 24 nm. The saturation magnetization was strongly influ- enced with Co 2+ concentrations. The cation distribution, the spin canting, and the presence of Fe 2+ ions along with Fe 3+ ions were responsible for the variation in saturation magnetization. Cation distribution estimated from satura- tion magnetization suggested the mixed spinel structure of Ni 1−x Co x Fe 2 O 4 system. The calculated g values from electron spin resonance spectra were consistent with the variation of saturation magnetization. UV–vis diffuse spec- tra indicated that Ni 1−x Co x Fe 2 O 4 samples were indirect band gap materials and band gap decreased with increas- ing Co 2+ concentration. Dielectric constant and dielectric loss showed frequency-dependent dispersion along with enhancement dielectric constant with increasing Co 2+ con- centration. The complex impedance analysis confirmed that the conduction process predominantly takes place through grain boundaries. Keywords Ferrite nanoparticles · Magnetism · Optical properties · Dielectric properties  Manoj Kumar manoj.chauhan@jiit.ac.in 1 Department of Physics and Materials Science and Engineering, Jaypee Institute of Information Technology, Noida-201307, India 1 Introduction Ferrites are most widely used magnetic materials with a wide range of applications ranging from infrared to microwave region and from low to high permeability. These materials are exploited for their applications in electronic devices, ferrofluids, magnetic drug delivery, microwave devices, medical diagnostics, humidity sensors, and high- density information storage [1–4]. The ferrites are used in many magnetic devices due to their low electrical con- ductivity as compared to other magnetic materials [5–7]. Drastic change in properties has been observed in ferrites in the nanoregion as compared to their bulk counterparts (like superparamagnetism and spin canting) [8–10]. The strong change in properties of ferrites in nanorange leads to the development of many techniques for the synthesis of nano- materials. Some of these methods are mechanical milling, solid-state route, co-precipitation, hydrothermal reaction, microemulsion method, and sol–gel technique [11–16]. The structural, magnetic, and electrical properties of ferrite are strongly influenced by their composition and microstruc- ture and hence depend on the synthesis route and synthesis conditions [17]. The co-precipitation method is widely used because the crystallite size can be easily controlled by con- trolling the sintering temperature. Large pH values in the range 10–12 are used for high production yields and pH values also help in controlling the particle size. Moreover, coating with oleic acid also prevents the growth of nanopar- ticles. Oleic acid is an organic molecule with a polar head and non-polar tail. The polar head of oleic acid molecules stucks on the surface of the nanoparticles and prevents their further growth. Hence, the advantages of this method are the production of ferrite nanoparticles with controlled size, high yield, low sintering temperature, and low cost.