Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Infuence of Co 4+ -Ca 2+ substitution on structural, microstructure, magnetic, electrical and impedance characteristics of M-type barium–strontium hexagonal ferrites Charmi D. Patel a , Preksha N. Dhruv a , Sher Singh Meena b,∗∗ , Charanjeet Singh c,∗∗∗ , Srikanti Kavita d , Mohamed Ellouze e , Rajshree B. Jotania a,∗ a Department of Physics, Electronics and Space Sciences, University School of Science, Gujarat University, Ahmedabad, 380 009, India b Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai, 400 085, MS, India c Department of Electronics and Communication Engineering, Lovely Professional University, Jalandhar, 144 411, Punjab, India d International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), IIT Chennai Research Park, Chennai, 600 113, India e Physics Department, Faculty of Science of Sfax, University of Sfax, B. P. 1171 - 3000, Sfax, Tunisia ARTICLE INFO Keywords: M-type hexagonal ferrites Sol-gel method Hysteresis loops Impedance analysis Electric modulus ABSTRACT Co 4+ -Ca 2+ substituted M-type barium strontium hexagonal ferrites with chemical composition Ba 0.25 Sr 0.75 Co x Ca x Fe 12-2x O 19 (x = 0.0, 0.2, 0.4, 0.8, 1.2, 1.6 and 2.0) were prepared by sol-gel auto combustion technique and sintered at 1150 °C for 5 h. The structural, magnetic, electrical and impedance properties of prepared samples were characterized using FTIR, XRD, SEM, VSM, Mössbauer spectroscopy and impedance spectroscopy. The FTIR spectra displayed two absorption bands in a wave-number range from 600 to 550 cm −1 and 450 to 400 cm −1 that confrm the formation of hexaferrite. XRD analysis of x = 0.0 and x = 0.2 com- positions show the formation of the majority of hexaferrite M-phase, while other samples show M-phase with other phases in the prepared compositions. The substitution of Co–Ca reduced the coercivity (H C ) from 3135.45 Oe (x = 0.0) to 48.28 Oe (x = 0.8) and saturation magnetization (M S ) decreased from 65.97 emu/g (x = 0.0) to 42.24 emu/g (x = 2.0). Mössbauer spectroscopic analysis showed that each sublattice has fve sextets that can be attributed to the Fe +3 ions in the high spin state and few Fe 3+ ions were converted into Fe 2+ ions for com- positions x ≥ 0.8. Single semicircle curves were observed in Nyquist plots, attributed to the contributions of grain boundaries. Impedance and Nyquist plots depicted non-Debye relaxation in the compositions. AC con- ductivity of all samples increases with the increase in frequency. The microstructure accompanied by grain and grain boundaries infuenced the electrical and impedance properties. 1. Introduction The investigation on hexaferrites has expanded the research sphere to considerable level on the account of their important applications such as microwave absorber, permanent magnet, in magnetic recording media, data storage, electrical/electronic components and, electric motors. A special interest is appreciated in hexaferrites among all fer- rites due to their large resistivity, high saturation magnetization, large coercivity, high Curie temperature, etc. [1–5]. Hexaferrites are cate- gorized into six groups: M, W, X, Y, Z and U according to their the crystal structure and chemical compositions. The crystal structure of M type hexaferrite contains 38 oxygen (O 2− ) and 24 ferric (Fe +3 ) ions, which are inserted in various fve sites, including three octahedral sites (12k, 4f 2 , and 2a), one tetrahedral (4f 1 ) and one trigonal bipyramidal (2b) site [6]. M-type hexaferrite belongs to the family of ferrimagnetic oxides, which have diferent properties such as high corrosion resistance and chemical stability, low cost of production, superior electrical resistance, large dielectric and magnetic loss etc. The plane anisotropy in ferrite causes natural resonance to occur in the GHz range so these materials are operated especially in microwave absorber application [7–9]. M type hexaferrites have high structural stability and high resistivity as compared to other hexaferrites (W, X, Y, U and Z). The ferrimagnetism of M-type hexaferrite material carries with magnetic loss, the resonance https://doi.org/10.1016/j.ceramint.2020.05.326 Received 26 January 2020; Received in revised form 17 May 2020; Accepted 31 May 2020 ∗ Corresponding author. ∗∗ Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: ssingh@barc.gov.in (S.S. Meena), rcharanjeet@gmail.com (C. Singh), rbjotania@gmail.com (R.B. Jotania). Ceramics International 46 (2020) 24816–24830 Available online 09 June 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved. T