Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Magnetic behavior of biosynthesized Co 3 O 4 nanoparticles A. Diallo a,b,d,e, ⁎ , T.B. Doyle a,b,c , B.M. Mothudi a,b , E. Manikandan a,b , V. Rajendran a,b,d , M. Maaza a,b a UNESCO-UNISA Africa Chair in Nanosciences-Nanotechnology, College of Graduate Studies, University of South Africa, Muckleneuk ridge, PO Box 392, Pretoria, South Africa b Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation,1 Old Faure road, Somerset West 7129, PO Box 722, Somerset West, Western Cape, South Africa c School of Chemistry and Physics, University of KwaZulu-Natal, Durban 4001, South Africa d Centre for Nano Science and Nanotechnology, K.S. Rangasamy College of Technology, Tiruchengode 63721, Tamil Nadu, India e Laboratoire de Photonique et de Nano-Fabrication, Faculté des sciences et Techniques, Université Cheikh Anta Diop de Dakar (UCAD), B.P. 25114, Dakar- Fann Dakar, Senegal ARTICLE INFO Keywords: Nano-biosynthesized systems Magnetization Cobalt oxide Aspalathus linearis ABSTRACT This contribution reports for the 1st time on the magnetic behavior of CO 3 O 4 nanoparticles synthesized by a “green” process using an Aspalathus linearis’ leaves natural extract. More accurately magnetic behavior of CO 3 O 4 nanoparticles successfully biosynthesized was investigated using vibrating sample magnetometer. The magnetization behavior for the samples manifests a combination of size dependent antiferromagnetic and paramagnetic behaviors, respectively, for the core and shell of the nanoparticles. 1. Introduction Magnetic nanomaterials have been extensively studied with interest from a fundamental point of view [1] as well as in technological applications such as data storage, sensors, catalysts, magnetic reso- nance imaging and biomedical applications [2–4]. Among them, CO 3 O 4 magnetic nanoparticles have attracted significance attention for their use in hypothermia and contrast agents in magnetic resonance imaging [5–7]. In the bulk state CO 3 O 4 has a spinel structure in which the Co 2+ ions occupy tetrahedral (T) sites and the Co 3+ ions occupy octahedral (O) interstitial sites in the cubic close-packed lattice [8,9]. In bulk coordination the CO 3+ ions, because of the splitting of the 3d levels by the field at the octahedral cubic sites, have no net permanent magnetic moment. The CO 2+ ions, with a moment approximately 3.9 μ B (spin-only) [10] in the (T) sites are surrounded by four nearest neighbours with opposite spins and an antiferromagnetic (AF) ordering transition is known [9] to occur below a temperature T n ≈40 K, the Néel temperature, above which in the bulk, the material becomes para- magnetic. Spin flip on one or the other sites can occur at sufficiently high applied magnetic field [11,12]. In nano-particulate CO 3 O 4 , where surface properties become significant relative to bulk properties, uncompensated surface or “shell” spins [13–15], core AF behavior, core/shell [16] and inter-particle interactions [17–20] may all con- tribute to, and determine, the overall magnetic behavior. The uncom- pensated shell spins arise from a breakdown of the AF sub-lattice pairing in the surface and tend to allow for some weak ferromagnetic ordering [21–26]. As particle size decreases in this system to below about 10 nm the saturation moment m s increases rapidly (see for example [27]). In the small particle, single domain, regime the orientation of the magnetic moment in a particle, with respect to an applied field direction, depends, inter alia, on the magnetic anisotropy energy, particle shape, inter-particle interactions and hence also on thermal activation effects. Above a so-called “blocking” temperature, T b , thermal activation overcomes the anisotropy energy, allowing for a tendency for particle/shell moment alignment with the applied field and superparamagnetic behavior results. T b depends on the time scale of measurement and decreases with both time and particle size. Typical reported values for T b and for T n in nano-particulate CO 3 O 4 , lie in ranges, respectively, of about 4 K < T b < 28 K and 20 K < T n < 33 K for particle diameters in the range 3 < d < 10 nm [24,27] and both general decrease with particle size. Below T n both field cooled (FC) and zero field cooled (ZFC) magnetization behavior manifest a non-zero coer- civity H c [27] which decreases to vanishingly small values as the particle size decreases. In summary, the magnetic behavior of the CO 3 O 4 nano-particulate system is apparently very complex, and has been stated to be “unusual http://dx.doi.org/10.1016/j.jmmm.2016.10.063 Received 4 August 2016; Received in revised form 7 October 2016; Accepted 12 October 2016 ⁎ Corresponding author at: UNESCO-UNISA Africa Chair in Nanosciences-Nanotechnology, College of Graduate Studies, University of South Africa, Muckleneuk ridge, PO Box 392, Pretoria, South Africa. E-mail addresses: abdoulayediallosn@gmail.com, diallo@tlabs.ac.za (A. Diallo). Journal of Magnetism and Magnetic Materials 424 (2017) 251–255 0304-8853/ © 2016 Elsevier B.V. All rights reserved. Available online 13 October 2016 crossmark