Temperature and composition dependence of magnetic properties of cobalt–chromium co-substituted magnesium ferrite nanomaterials Muhammad Javed Iqbal a,n , Zahoor Ahmad a , Turgut Meydan b , Yevgen Melikhov b a Surface and Solid State Chemistry Laboratory, Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan b Wolfson Center for Magnetics, School of Engineering, Cardiff University, Cardiff CF24 3AA, UK article info Article history: Received 29 November 2011 Received in revised form 11 June 2012 Available online 1 July 2012 Keywords: Ferrites Microemulsion method M¨ ossbauer analysis Law of approach Magnetic anisotropy abstract The temperature and composition dependence of magnetic properties of Co–Cr co-substituted magnesium ferrite, Mg 1x Co x Cr x Fe 2x O 4 (x ¼0.0–0.5), prepared by novel polyethylene glycol assisted microemulsion method, are studied. The synthesized materials are characterized by the M ¨ ossbauer spectrometer and standard magnetic measurements. Major hysteresis loops are measured up to the magnetic field of 50 kOe at 300, 200 and 100 K. The high field regimes of these loops are modeled using the Law of Approach to saturation to determine the first-order cubic anisotropy coefficient and saturation magnetization. Both the saturation magnetization and the anisotropy coefficient are observed to increase with the decrease in temperature for all Co–Cr co-substitution levels. Also, both the saturation magnetization and the anisotropy coefficient achieved maximum value at x ¼0.3 and x ¼0.2, respectively. Explanation of the observed behavior is proposed in terms of the site occupancy of the co-substituent, Co 2 þ and Cr 3 þ in the cubic spinel lattice. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Magnetic spinel ferrites are of a great interest in fundamental science, especially for addressing the fundamental relationships between magnetic properties and their lattice structure. This is due to the fact that the ability to substitute different cations into the tetrahedral/octahedral sites of spinel lattice offers the opportunity to selectively enhance properties to match intended applications. Recently, the synthesis of magnetic materials on the nanoscale has been a field of intense study due to the novel mesoscopic properties shown by particles of quantum dimensions located in the transition region between atoms and bulk solids [1]. Spinel ferrites have been investigated in recent years for their useful magnetic properties and applications in information sto- rage systems, magnetic bulk cores, sensors/actuators, magnetic fluids, microwave absorbers and medical diagnostics. The synth- esis and magnetic structure characterization of spinel nanofer- rites have been investigated with much interest and a lot of attention has been focused on the preparation and characteriza- tion of metal oxide nanoparticles of spinel ferrites, MeFe 2 O 4 (Me ¼ Co, Mg, Mn, etc.) [2–4]. Nanoparticles of magnesium ferrite (MgFe 2 O 4 ) are the potential candidates for various applications in magnetic and microwave devices [5,6]. As many important properties of the spinel ferrites depend crucially on the exact nature of the cation distribution over the octahedral and tetrahedral sites in the spinel cubic lattice, chemical substitution can tune the properties of magnesium ferrite by altering this cation distribution. This influences the magneto-elastic properties of these materials, which can be explained in terms of the change in important magnetic proper- ties such as the magnetization characteristics and magnetocrys- talline anisotropy. For the above-said purpose, structural and magnetic characteristics of magnesium ferrite with a non-mag- netic substitution such as Zn 1 þ [7], Cd 2 þ [8], In 3 þ [9] and Ti 4 þ [10] for either individual Mg 2 þ or individual Fe 3 þ site only, have been investigated. However, doping of magnesium ferrite with a metallic binary mixture of magnetic ions to substitute iron and magnesium simultaneously has not been extensively studied. In the present study, we have investigated co-substitution of Co 2 þ –Cr 3 þ in place of some of Mg 2 þ and Fe 3 þ in magnesium ferrite and we have studied its properties at various temperatures in addition to the room temperature properties only reported before [11]. The octahedral site preference of Co 2 þ in the cubic spinel lattice of magnesium ferrite and the additional substitution of Cr 3 þ produced a more drastic change in properties in compar- ison to previously tried substitutions. 2. Experimental details Nanosized Mg 1 x Co x Cr x Fe 2 x O 4 (where x ¼ 0.0–0.5) are pre- pared by the polyethylene glycol (PEG) assisted microemulsion method. Aqueous solutions of proper compositions are mixed with aqueous solution of polyethylene glycol (PEG) in well Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.06.031 n Corresponding author. Tel.: þ92 51 90642143; fax: þ92 51 90642241. E-mail address: mjiqauchem@yahoo.com (M.J. Iqbal). Journal of Magnetism and Magnetic Materials 324 (2012) 3986–3990