Structural and magnetic properties of Sr 0.5 Co 0.5 Fe 2 O 4 nanoferrite Hafiz M.I. Abdallah n , Thomas Moyo, Itegbeyogene P. Ezekiel, Nadir S.E. Osman School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, P/Bag X5400, Durban 4000, South Africa article info Article history: Received 25 October 2013 Received in revised form 14 February 2014 Available online 28 April 2014 Keywords: Glycol-thermal Nanoferrite Hyperfine interaction Exchange bias Spin-freezing Coercivity abstract The nanoparticle Sr 0.5 Co 0.5 Fe 2 O 4 powder was produced via glycol-thermal process from high-purity metal chlorides at a low reaction temperature of 200 1C. The phase identification of the as-synthesized powder reveals cubic spinel structure with an average crystallite size of 8 nm. Room-temperature Mössbauer spectra for the as-synthesized sample and samples annealed at different temperatures show different local environments of tetrahedral and octahedral coordinated iron cations. Magnetic properties of the as-synthesized sample and samples annealed at 300, 400, 450, 500, 600, 700 and 800 1C have been investigated using a vibrating sample magnetometer at room-temperature in applied magnetic fields of up to about 1.4 T. A substantial increase in coercive field at 300 K from 0.28 kOe to 2.897 kOe was obtained for the as-synthesized and annealed sample at 800 1C. Magnetic field dependence of magnetization curves measured on a mini-cryogen free VTI system operating at a base temperature of 2 K in magnetic fields of up to 5 T have been investigated. The variation of the saturation magnetization as a function of temperature follows modified Bloch's law. Coercive field increased from about 0.28 kOe and 1.04kOe at 300K to 11.14 kOe and 10.43 kOe at 2 K for the as-synthesized sample and sample annealed at 500 1C, respectively because of spin-freezing. The effect of exchange bias and Kneller's law are used to account for the temperature dependence of coercive fields. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Spinel ferrites have been studied for both scientific and technological viewpoints. Some focused efforts have been made to enhance the properties of existing materials, develop new materials and fabricate routes leading to better performance and understanding of sample behavior [1]. Spinel ferrites show a large variety of structural, magnetic, electronic and catalytic properties depending on their method of preparation, particle size and composition [2]. They are interesting materials because of several possible applications which include permanent magnets, ferro- fluids, recording media, magneto-optic devices, telecommunica- tions, microwave components, biomedical, high-frequency applications and catalysts [2,3]. The aim of this work is to investigate the effects of heat treatments on the structure, hyper- fine interactions and magnetic behavior of a nanosized Sr 0.5 Co 0.5 - Fe 2 O 4 sample synthesized by glycol-thermal route at 300 K and at low temperatures. Evidence of unusual exchange bias effect is also presented after thermal annealing at 500 1C. 2. Experimental details The Sr 0.5 Co 0.5 Fe 2 O 4 spinel ferrite was produced by using a glycol-thermal process using metal chlorides (i.e. SrCl 2  6H 2 O (98%), CoCl 2  6H 2 O (99%) and FeCl 3  6H 2 O (99%)) without the use of any surfactants. A typical synthesis procedure is described elsewhere [4,5]. The as-synthesized sample was divided into different specimens thereafter annealed at different temperatures in continuous flow of high purity 99.999% argon gas to investigate the structure and magnetic properties. The structure parameters for the as-synthesized sample and samples annealed at 400, 500, 600, 700 and 800 1C were obtained from powder X-ray diffraction (XRD) (using type: PANalytical-EMPYREAN) with a monochro- matic beam of Co-K α radiation (λ ¼ 1.7903067 Å) in 2θ scanning between 101 and 901. A high-resolution transmission electron microscopy (HRTEM) type: Joel_JEM-2100 instrument was also used to study the microstructure of the sample after thermal annealing at 500 1C. 57 Fe Mössbauer spectra of the as-synthesized and annealed samples at 400, 450, 500, 600, 700 and 800 1C were recorded at 300 K in transmission geometry. Room-temperature magnetization measurements were also performed on the as- synthesized and annealed samples by using a vibrating sample magnetometer (VSM, type: Lakeshore 735) in applied fields of up to 1.4 T. A typical sample size of about 0.025 g was loaded into the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials http://dx.doi.org/10.1016/j.jmmm.2014.04.041 0304-8853/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Cellphone: +27728994179; Fax: +277312607795. E-mail address: hafizspin@gmail.com (H.M.I. Abdallah). Journal of Magnetism and Magnetic Materials 365 (2014) 83–87