Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Research articles Dielectric, impedance, AC conductivity and low-temperature magnetic studies of Ce and Sm co-substituted nanocrystalline cobalt ferrite Syed Ismail Ahmad a, ⁎ , Abdul Rauf b , Tasneem Mohammed c , Amal Bahafi c , D. Ravi Kumar d , Madireddy Buchi Suresh e a General Science Department, Physics Division, Ibn Sina National College, Jeddah, Saudi Arabia b Department of Physics, College of Science, Jazan University, Jazan, Saudi Arabia c General Sciences Department, Chemistry Division, Ibn Sina National College, Jeddah, Saudi Arabia d Department of Chemistry, University College of Science, Osmania University, Hyderabad 500007, Telangana, India e International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad, Telangana, India ARTICLE INFO Keywords: Cobalt ferrite Dielectric constant Impedance spectroscopy AC conductivity ZFC/FC ABSTRACT In this study, we report frequency and temperature dependent dielectric, electrochemical impedance and low temperature magnetic properties of Sol-gel auto-combustion synthesized rare-earth (Ce and Sm) co-substituted cobalt ferrite, CoCe x Sm y Fe 2-x-y O 4 (x = y = 0.00, 0.1, 0.12 and 0.25). The surface morphology and particle size as obtained by XRD studies was confirmed by HRTEM measurements for a representative sample. The dielectric properties such as dielectric constant, loss tangent and AC conductivity have been measured using LCR meter, variation of dielectric parameters with substituent concentrations, frequency range from 0.1 to 10 MHz at room temperature (RT) and temperatures 100–400 °C have been well supported by Maxwell-Wagner interfacial po- larization. The AC conductivity increased with applied frequency suggesting that the conduction mechanism is owing to small polaron hopping between cations. Impedance studies shows that, the addition of RE (Ce and Sm) cause a decrease in the Z′ value, with increasing frequency which can be attributed to the space charge polar- ization effect in the material. Low temperature magnetic measurements in ZFC-FC mode and magnetic isotherms at 20 and 310 K reveals the ferromagnetic nature of RE (Ce, Sm) substituted cobalt ferrite nano particles. The coercive field has been increased at low temperature 20 K compared to that at 310 K, the samples shows super paramagnetism beyond room temperature. 1. Introduction Nano cobalt ferrite is being studied extensively by many research groups due to their novel properties which are different from their bulk counterparts. With the substitution of transitional metal and large size lanthanides, the morphological, structural, dielectric, optical, magnetic electrical properties of them can be optimized for their applications in magnetic ferro fluids, high density recording media, bio-medical ap- plications such as MRI contrast agents, hyperthermia at radio fre- quency, magnetic drug delivery, sensors and frequency absorbing ma- terials [1–5]. The dielectric, magnetic and electric properties of nano cobalt ferrites largely depends on microstructure, particle size, tem- perature and synthesis method and magnitude and type of dopants. Rare earth (RE) substituted cobalt ferrite shows an impurity phase at higher concentration of substituent. The magnetic properties of these ferrites are greatly affected with increasing RE concentration due to change in crystallite size and anisotropy, shows improved absorption capacity for dyes and in perpendicular recording media [6–8]. The RE (Y, Gd, Dy) substituted cobalt ferrite synthesized by co-precipitation method have shown high specification absorption rate (SAR) for sam- ples having higher saturation magnetization and coercivity [9,10]. Rare earth elements which consists of unpaired 4f electrons when substituted in spinel ferrites, a 3d-4f coupling is created which affects many physical parameters such as particle size, micro-structure, surface strain and they in turn affect many physical properties [11,12]. The electrical, magnetic and dielectric properties of cobalt ferrite play a very important role in designing the electronic, magnetic, microwave, and electrochemical devices. The RE ions substitution in spinel nano cobalt ferrites affects the microstructure which is an important factor that influences the AC electrical conductivity and impedance. The https://doi.org/10.1016/j.jmmm.2019.165666 Received 9 May 2019; Received in revised form 25 July 2019; Accepted 27 July 2019 ⁎ Corresponding author at: General Sciences Department – Physics Division, Ibn Sina National College for Medical Studies, P.O. Box No. 31906, Jeddah 21418, Saudi Arabia. E-mail addresses: dr.syedismailahmad@gmail.com, sahmed@ibnsina.edu.sa (S.I. Ahmad). Journal of Magnetism and Magnetic Materials 492 (2019) 165666 Available online 02 August 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved. T