Impacts of Ni–Co substitution on the structural, magnetic and dielectric properties of magnesium nano-ferrites fabricated by micro-emulsion method Rajjab Ali a , Azhar Mahmood a , Muhammad Azhar Khan b,⇑ , Adeel Hussain Chughtai c , Muhammad Shahid d , Imran Shakir e , Muhammad Farooq Warsi a,⇑ a Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan b Department of Physics, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan c Institute of Chemical Sciences, Bahauddin Zakaryia University, Multan 6100, Pakistan d BK 21 Physics Research Division, Institute of Basic Sciences, SKKU Advanced Institute of Nanotechnology, Department of Energy Science, Sungkyunkwan University, Suwon 440-746, South Korea e Deanship of Scientific Research, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia article info Article history: Received 7 July 2013 Accepted 16 August 2013 Available online 3 September 2013 Keywords: Nano-ferrites Magnetic properties Dielectric properties GHz frequency abstract Magnesium ferrite (MgFe 2 O 4 ) nano-structures doped with Ni–Co at magnesium and iron sites respec- tively were fabricated by the micro-emulsion method and characterized by the X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), vibrating sample magnetometer (VSM) and dielectric measurements. The analysis of XRD patterns confirm the single phase spinel structure and the crystallite size calculated by Scherer’s formula lies in the range of 15–26 nm. The spectral studies elucidate the char- acteristic feature of spinel phase. The real and imaginary parts of dielectric constant and dielectric loss exhibit peaking behavior. The dielectric properties have been explained on the basis of Debye-type relax- ation phenomenon in accordance with Koop’s phenomenological theory. The increase in the tendency of saturation magnetization is consistent with the enhancement of crystallinity. The crystallite size is small enough to obtain considerable signal to noise ratio in the recording media. The optimized dielectric and magnetic parameters suggest that the material with composition Mg 0.6 Ni 0.4 Co 0.4 Fe 1.6 O 4 may be used for applications in recording media and microwave devices. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Nano-ferrites have shown a growing interest in recent years due to their specific electromagnetic properties. Spinel ferrites are currently used as key materials for the advancements in information storage devices and ferro-fluid technology [1]. The dielectric properties of nano-ferrites are influenced when the crystallite size approaches the critical value, below which every crystallite is treated as single domain. Hence this realizes that the physical and chemical properties of these ferrites in the nano-regime are in general dramatically different from those of the bulk counter parts and that the nano-materials exhibit new cross over phenomena. The modern data storage requirement needs smaller crystallite size [2]. Magnesium ferrite is one of the most important ferrites. The doped magnesium nano-ferrites found considerable applications in transformers, ac power dis- tributors, magnetic recording as a magnetic heads, and magnetic sensors [3]. The basis for variety of applications is related to the verity of transition metal cations, which can be incorporated in to the spinel lattice of the parent magnetic structure. The origin of magnetic properties of these nano-ferrites is spin magnetic moment of the unpaired 3d electrons of the transition element occupied by the super exchange interaction through oxygen ions separation. The magnetic properties such as saturation magneti- zation are strongly dependent on the distribution of cations, type of dopants and the crystallite size [4]. The influence of metal ion substitution such as Zn [5], Cd [6], Al [7], Cu [8], Cr [9] and Ge [10] on various structural, magnetic, electrical and dielectric properties of MgFe 2 O 4 have been reported in the literature. In the present work, we have investigated the effects of Ni 2+ –Co 2+ substitution for Mg 2+ and Fe 3+ in magnesium based spinel ferrites (Mg 1x Ni x Co y Fe 2y O 4 ) synthesized by micro-emul- sion method. The results show that the coercivity of all the substituted samples was found to be a few hundred oersteds which is one of the necessary conditions for recording media [11]. 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.08.114 ⇑ Corresponding author. Tel.: +92 345 5411391. E-mail addresses: farooqsi_warsi@yahoo.com, farooq.warsi@iub.edu.pk (M.F. Warsi). Journal of Alloys and Compounds 584 (2014) 363–368 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom