Synthesis of M-type SrFe 12 O 19 by mechanosynthesis assisted by spark plasma sintering A.M. Bolarín-Miró a , F. Sánchez-De Jesús a,⇑ , C.A. Cortés-Escobedo b , S. Díaz-De la Torre b , R. Valenzuela c a Area Académica de Ciencias de la Tierra y Materiales, Universidad Autónoma del Estado de Hidalgo, Mineral de la Reforma, Hidalgo 42184, Mexico b Instituto Politécnico Nacional Centro de Inv. e Innovación Tecnológica, Distrito Federal 02250, Mexico c Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, 04510 México DF, Mexico article info Article history: Available online 26 November 2014 Keywords: Mechanochemical processing Spark plasma sintering Hexaferrite SrFe 12 O 19 Magnetic measurements Annealing treatment abstract We present a comparative study of synthesis of M-type strontium hexaferrite from strontium and iron single oxides mechanically activated by high-energy ball milling for 5 h, and assisted by two different methods: (a) conventional heat treatment and (b) Spark Plasma Sintering (SPS), both at relative low temperatures (6900 °C). Although it was found that both methods promoted the complete structural transformation of precursors to Sr-hexaferrite phase (S.G. Pmc21) for temperatures above 700 °C, higher saturation magnetization was found for SPS samples. X-ray diffraction analysis revealed that the struc- tural transformation undergoes formation of an intermediate metastable structure (Fe 2 Sr 2 O 5 ) in both methods, but with slight different kinetics. Maximum specific magnetization of 67 emu/g at 18 kOe and coercivity of 3.7 kOe were recorded from powder mixtures milled for 5 h, which were subsequently SPS-ed at 700 °C. By contrast, although magnetization values for the same milled samples after being annealed at 700 °C slightly decreased, it leads to a significant increase in the coercive field reaching 5.4 kOe. These results are explained on the basis of factors such as the complete formation of hexaferrite and the attained density of consolidated powders, in addition to particle and grain sizes also reported. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction M-type hexagonal ferrites have been extensively used as per- manent magnets, high-density magnetic recording media [1] for the last decades and are currently used in microwave devices working at frequencies in the gigahertz range [2]. Although many other hard magnetic materials have been developed after these years, the hexaferrite performance/cost ratio is still extremely favorable. The unit cell of these of ferrites consists of a spinel block (S) with two layers of four oxygen atoms with three divalent metal ions between each layer in four octahedral sites, where the cation is surrounded by six oxygen anions and two tetrahedral sites where four oxygen anions surround the cations; and a block (R) (with the stoichiometry (SrFe 6 O 11 ) 2 ) with three hexagonally packed layers of four oxygen atoms each, but one of the oxygen atoms in the center layer is replaced by a similarly sized divalent metal atom with an overlap of hexagonally and cubically packed layers [3]. Among these ferrites, strontium hexagonal ferrite, with stron- tium as divalent metal ion, SrFe 12 O 19 , possesses a special place due to its magnetic properties and particularly due to its larger magnetocrystalline anisotropy [4]. Hexaferrites can be synthesized by several processes. The conventional and oldest one is by calcination and sintering of a mixture of oxides or carbonates in a furnace at 1300 °C [5]. This process produces large particles and consumes extensive energy. Nanostructured hexaferrites can be produced by different methods such as sol–gel [6,7], hydrothermal [8], coprecipitation [9], spray- drying and microemulsion [10], conventional route [11] among others [12,13]. A particular method is mechanosynthesis; typically this method promotes formation of ferrites by mechanical activa- tion of carbonate strontium and iron oxide [13,14]. In comparison with the traditional method (solid state reaction) [15], the mecha- nochemical method has demonstrated to achieve high coercivity and magnetic saturation in these materials [11–16], nevertheless the mechanosynthesis is a potential process for mass production. Luo [14] prepared strontium hexaferrite by mechanosynthesis of a mixture of SrCO 3 and Fe 2 O 3 with subsequent annealing. They obtained an amorphous material after ball-milling for 30 h, and a SrFe 12 O 19 single phase after annealing at 900 °C for 2 h. The attained saturation magnetization reached 58.2 A m 2 /kg (58 emu/g), whereas the coercivity was 281.2 kA/m (3500 Oe) at room temperature. A similar study using the mechanosynthesis http://dx.doi.org/10.1016/j.jallcom.2014.11.124 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: fsanchez@uaeh.edu.mx (F. Sánchez-De Jesús). Journal of Alloys and Compounds 643 (2015) S226–S230 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom