Synthesis and characterization of cobalt–manganese oxides J. Valencia a,c , N.P. Arias b,d , O. Giraldo b , A. Rosales-Rivera a,n a Laboratorio de Magnetismo y Materiales Avanzados, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Colombia, Sede Manizales, Manizales, Colombia b Laboratorio de Materiales Nanoestructurados y Funcionales, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Colombia, Sede Manizales, Manizales, Colombia c Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis 55455-0153, USA d Departamento de Ingenierı ´aEle´ctrica,Electro ´nica y Computacio ´n, Facultad de Ingenierı ´a y Arquitectura, Universidad Nacional de Colombia, Sede Manizales, Manizales, Colombia article info Available online 17 December 2011 Keywords: Haussmanite type materials Superparamagnetism abstract Cobalt doped/un-doped manganese oxides materials were synthesized at various doping rates by soft chemical reactions, oxidation-reduction method, which allows generating a metal-mixed oxide. The synthesized materials were characterized using several techniques including chemical analysis, X-rays diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and vibrat- ing sample magnetometer (VSM). The chemical analysis confirmed the presence of cobalt in the samples. XRD patterns reveal mainly a spinel-like structure and SEM micrographs exhibited morphol- ogy with fine aggregate of particles. TGA profiles showed weight loss due to loss of water in a first step, followed by a loss of oxygen from the lattice associated with partial reduction of Mn 4 þ to Mn 3 þ . VSM was used to measure the magnetization as a function of the applied magnetic field at temperatures T ¼50 and 300 K. Different magnetic behaviors were observed when cobalt percentage changed in the samples. These behaviors are considered to be related to the size of the particles and composition of the materials. Higher coercive field and lesser magnetization were observed for the sample with higher cobalt content. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Transition metal oxides have been considered as quite important materials because of their applications in areas such as catalysts, ion exchange, magnetic storage media and batteries materials [1–6]. Their physical properties at nanoscale are different from their bulk counterpart, that is, the reduction of their particle size down to nanometers scale, can greatly affect their chemical and physical properties and improve their potential for the applications stated above [6]. There are different preparation methods of manganese and cobalt spinel oxides materials with fine particle size. These methods include both high and low temperature procedures [7, 8]. In this paper, we report on the synthesis of cobalt doped manganese oxides by oxidation-reduction process at room temperature using two Co/ Mn stoichoimetry ratios and their chemical, structural, thermal and magnetic characterization. 2. Materials and experimental methods 2.1. Synthesis Solutions of Co 2 þ and Mn 2 þ salts were prepared in agreement with nominal molar Co/Mn relation X ¼ (0.1, 0.2). We followed the method of Luo et al. [9]; in brief, a solution of 19.6 g of Mn(CH 3 COO) 2  4H 2 O (Across Organics, 99%) and CoCl 2  6H 2 O (Carlo Erba, 99%) in 140 mL of distilled deionized water (DDW) was added slowly to a solution of 50 g of NaOH (Merck, 99.5%) in 160 mL of DDW under vigorous stirring, forming a white slurry of Co x Mn 1 x (OH) 2 . A solution of 4.8 g of KMnO 4 (Merck 99%) in 140 mL of DDW was added slowly to the white slurry under vigorous stirring, producing a brownish black suspension. It was placed to age to 40 1C for 4 days in order to obtain cobalt doped manganese oxide. The samples were labeled as CM01 and CM02 for Co/Mn ¼ 0.1 and 0.2, respectively. 2.2. Characterization Elemental analysis by atomic absorption: the global chemical composition of the synthesized materials was determined using a Perkin Elmer 3110 spectrometer. Surface area: the specific surface area was measured by nitrogen adsorption at T ¼ 77 K with Brunauer–Emett–Teller Method using Micromeritics ASAP 2020 equipment. Average Oxidation State (AOS): the average oxidation state of manganese in these materials was determined by potentio- metric titration using (NH 4 ) 2 Fe(SO 4 ) Mohr salt and sodium pyrophosphate [9]. Powder X-Ray diffraction and crystal size: structural analysis were carried out using a Rigaku Miniflex II diffractometer using CuKa radiation with an operating voltage of 30 kV and current of Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.12.051 n Corresponding author. E-mail address: arosalesr@unal.edu.co (A. Rosales-Rivera). Physica B 407 (2012) 3155–3157