Electrical transport properties of nanocrystalline nonstoichiometric nickel ferrite at and above room temperature S. Choudhury a , M. Sinha b , M.K. Mandal a , S.K. Pradhan b , A.K. Meikap a,n a Department of Physics, National Institute of Technology, Durgapur 713209, West Bengal, India b Department of Physics, University of Burdwan, Golapbag, Burdwan 713104, West Bengal, India article info Article history: Received 12 October 2014 Accepted 18 October 2014 Available online 22 October 2014 Keywords: A. Nano-ferrite B. Ball-milling D. Electrical conductivity D. Dielectric properties abstract The electrical transport properties of chemically prepared nanocrystalline nonstoichiometric nickel fer- rite followed by high energy ball milling have been reported. The dc conductivity of the samples shows semiconducting behavior. The adiabatic small polaron model is most suitable for analyzing the dc con- ductivity at higher temperature, whereas at lower temperature Schnakenberg acoustical one phonon assisted hopping model is suitable. The frequency dependent conductivity has been described by power law s / (f)∝f s and a maximum has been observed in the temperature behavior of ‘s’. The dielectric prop- erties of the samples have been explained in terms of electric modulus vector. Both dc and ac activation energy have been measured for different samples. The metallic electrode and semiconductor junction formed Schottky diode and the diode parameters have been extracted from the capacitance–voltage characteristics. & Elsevier B.V. All rights reserved. 1. Introduction The widespread application of transition metal oxides, parti- cularly spinel ferrites as low and high frequency devices has in- creased their importance during the last few decades [1–3]. These ferrites play a very useful role in many technological and magnetic applications due to their high electrical resistivity and sufficiently low dielectric losses over a wide range of frequency. A consider- able amount of work has been carried out on nickel ferrite due to their possible applications in Ferro fluids [4], radar absorbing coatings, biomedical materials [5–8] and gas sensors for gases like liquefied petroleum gas [9]. The general molecular formula of spinel ferrites can be written as (A 2 þ )[B 2 3 þ ]O 4 2  where A 2 þ is a divalent cation and B 3 þ is a trivalent cation occupying tetrahedral (A) and octahedral (B) interstitial positions of the fcc lattice formed by O 2  ions. The structural, electrical, magnetic and me- chanical properties of nanocrystalline spinel ferrite are depended on their composition and microstructure which are influenced strongly by the preparation method. Therefore, various approaches are being adopted for preparation of spinel ferrites in order to obtain the desired properties [10–14]. During the last few decades, extensive studies on structural properties and magnetic properties have been performed on nickel ferrite prepared by different techniques [15–22]. However, the dc conductivity, ac conductivity and dielectric properties on the nanocrystalline nickel ferrite have been reported very few [23–26]. The electrical conductivity and thermoelectric power of some substituted nickel ferrite have been studied by Purushotham et al. [23]. Synthesis of single phase nano-sheets of spinel nickel ferrite has also carried out by Gun- jakar et al. [24] and also reported the high temperature electrical resistivity. Ponpandian et al. [25] and Mahalakashmi et al. [26] reported the ac conductivity and dielectric properties of nickel ferrite. But the systematic study on the electrical conductivity, ac conductivity, dielectric properties and capacitance–voltage char- acteristic of nanocrystalline nickel ferrite is not yet been reported. We have adopted the co-precipitation followed by mechanical milling method for the preparation of nickel ferrite nano particles. Co-precipitation method [27,28] is one of the important chemical methods for preparation of nano ferrites and mechanical treat- ment was found to be a useful activation method leading to an enhanced chemical reactivity of nanostructure spinel ferrites [29,30]. The aim of the present work is to investigate the dc, ac conductivity, dielectric properties and capacitance–voltage char- acteristics of nickel ferrite of unmilled and milled samples. In this paper we report the detailed electrical transport prop- erties like dc and ac conductivity, dielectric properties and voltage dependent capacitance of nanocrystalline nickel ferrite in the temperature range 298 rT r523 K and explain those with the suitable theories. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B http://dx.doi.org/10.1016/j.physb.2014.10.018 0921-4526/& Elsevier B.V. All rights reserved. n Corresponding author. Fax: þ91 343 2547375. E-mail address: meikapnitd@yahoo.com (A.K. Meikap). Physica B 457 (2015) 225–231