Current Perspectives Structural and magnetic characterization of co-precipitated Ni x Zn 1 À x Fe 2 O 4 ferrite nanoparticles Ch. Srinivas a,n , B.V. Tirupanyam b , S.S. Meena c , S.M. Yusuf c , Ch. Seshu Babu a , K.S. Ramakrishna d , D.M. Potukuchi e , D.L. Sastry f,n a Department of Physics, Sasi Institute of Technology and Engineering, Tadepalligudem 534101, India b Department of Physics, Government College (Autonomous), Rajamahendravaram 533103, India c Solid State Physics Division, Bhabha Atomic Research Center, Mumbai 400085, India d Department of Physics, Srinivasa Institute of Engineering and Technology, Amalapuram 533222, India e Department of Physics, University College of Engineering, Jawaharlal Nehru Technological University, Kakinada 533003, India f Department of Physics, Andhra University, Visakhapatnam 530003, India article info Article history: Received 1 November 2015 Accepted 20 January 2016 Available online 21 January 2016 Keywords: Thermodynamic solubility Superparamagnetism Core–shell interactions Ferrite nanoparticles abstract A series of Ni x Zn 1Àx Fe 2 O 4 (x ¼0.5, 0.6 and 0.7) ferrite nanoparticles have been synthesized using a co- precipitation technique, in order to understand the doping effect of nickel on their structural and magnetic properties. XRD and FTIR studies reveal the formation of spinel phase of ferrite samples. Substitution of nickel has promoted the growth of crystallite size (D), resulting the decrease of lattice strain (η). It was also observed that the lattice parameter (a) increases with the increase of Ni 2 þ ion concentration. All particles exhibit superparamagnetism at room temperature. The hyperfine interaction increases with the increase of nickel substitution, which can be assumed to the decrease of core–shell interactions present in the nanoparticles. The Mössbauer studies witness the existence of Fe 3 þ ions and absence of Fe 2 þ ions in the present systems. These superparamagnetic nanoparticles are supposed to be potential candidates for biomedical applications. The results are interpreted in terms of microstructure, cation redistribution and possible core–shell interactions. & 2016 Elsevier B.V. All rights reserved. 1. Introduction Spinel ferrites are an important group of ferrimagnetic mate- rials that are used for diverse and fascinating applications. The ferrites prepared using a ceramic method are generally in- homogeneous and prone to higher eddy current losses [1] in- cluding other disadvantages like hysteresis loss, residual loss and domain wall resonance, etc. [2]. In order to circumvent these drawbacks many researchers have paid attention preparing the ferrites by varying the concentrations of different dopants in the spinel structure. Unfortunately the drawbacks are not completely solved, because these are in turn are highly sensitive to micro- structure like grain size, pores, etc. [3]. However to control the microstructure and to overcome the drawbacks, an alternate ap- proach lies in synthesizing the ferrites at nanoscale. The revisit of ferrites at nanoscale opens the doors to exhibit novel properties like quantum confinement, fine particles effect which include superparamagnetism, single domain structure, spin-glassy beha- vior, core–shell interactions, etc. [4]. Understanding these unique physical properties, surely preludes the potential use of ferrites in wide range of applications in electronics and biomedical [5,6]. The most attention paid fine-particle effect is superparamagnetism, because the ferrite nanoparticles with superparamagnetic beha- vior can be used for biomedical applications like targeted drug delivary, cancer treatment by hyperthermia, etc. [7]. Among ferrites, Ni–Zn ferrite is of special interest due to its remarkable properties such as high resistivity, high saturation magnetization, low eddy current losses, mechanical hardness, etc. [8]. The functionalization of Ni–Zn nanoparticles lay down a way even for biomedical applications. Ni–Zn ferrite can be considered as a mixed ferrite of Ni and Zn-ferrites with formula unit (Zn 1 Àx 2 þ Fe x 3 þ ) tetra [Ni x 2 þ Fe 2 Àx 3 þ ] octa O 4 [9]. The introduction of Ni 2 þ (magnetic) for Zn 2 þ (non-magnetic) modifies the cation distribution, microstructure, grain size, magnetic, electrical prop- erties, etc. Spinel ferrite nanoparticles are generally prepared using var- ious methods. Among these methods co-precipitation method is widely used for the preparation of ferrites for its overriding Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials http://dx.doi.org/10.1016/j.jmmm.2016.01.060 0304-8853/& 2016 Elsevier B.V. All rights reserved. n Corresponding authors. E-mail addresses: srinivas.chintoju75@gmail.com (Ch. Srinivas), dl_sastry@rediffmail.com (D.L. Sastry). Journal of Magnetism and Magnetic Materials 407 (2016) 135–141