Ultrafine grained high density manganese zinc ferrite produced using polyol process assisted by Spark Plasma Sintering T. Gaudisson a , Z. Beji a , F. Herbst a , S. Nowak a , S. Ammar a,n , R. Valenzuela b a ITODYS, Université Paris Diderot, Sorbonne Paris Cité, CNRS UMR-7086, 75205 Paris, France b D2MC, Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, 04510 Ciudad de Mexico, Mexico article info Article history: Received 7 June 2014 Received in revised form 7 March 2015 Accepted 12 March 2015 Available online 18 March 2015 Keywords: Mn–Zn ferrite spinel Polyol process Spark Plasma Sintering Magnetic single domain grains. abstract We report the synthesis of Mn–Zn ferrite (MZFO) nanoparticles (NPs) by the polyol process and their consolidation by Spark Plasma Sintering (SPS) technique at relatively low temperature and short time, namely 500 °C for 10 min. NPs were obtained as perfectly epitaxied aggregated nanoclusters forming a kind of spherical pseudo-single-crystals of about 40 nm in size. The results on NPs consolidation by SPS underlined the importance of this clustering on the grain growth mechanism. Grain growth proceeds by coalescing nanocrystalline aggregates into single grain of almost the same average size, thus leading to a high density ceramic. Due to magnetic exchange interactions between grains, the produced ceramic does not exhibit thermal relaxation whereas their precursor polyol-made NPs are superparamagnetic. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Manganese–zinc ferrite oxides (MZFO) play an important role in ferrimagnetic materials because of their remarkable magnetic properties. Indeed, their physical flexibility, high magnetic polar- ization in combination with a high electrical resistivity, mechan- ical hardness and chemical stability make them particularly useful for different applications in various electromagnetic fields, ranging from magnetic sensors, microwave absorbers to magnetic reading heads, among others [1]. One of the most important interests for these materials is their ability to be used as high-permeability materials for low-power losses application at relatively high fre- quencies (  1 MHz) [2,3]. In this case, the technical performances of the produced ferrites are strongly dependent to their micro- structure. Indeed, as bulk materials, most of the research related to MZFO ceramics is focused on grain-boundary engineering, since the magnetization reversal in such materials is exclusively driven by domain wall motions. The grain boundaries may represent obstacles to domain movement leading to permeability decrease and power losses increase. Obviously, considerable material en- gineering effort have been devoted to produce coarse grained MZFO, discarding automatically the processes adapted to fine grained ceramics. As a consequence, the question of the ability of nanostructured ceramics to be used as high permeability materials or power materials when their grain size is lower than the critical diameter associated to transition from a magnetic polydomain configuration to a single domain one remains open, while the solid state physics rules change at these dimensions, introducing an- other magnetization reversal mechanism, namely, coherent spins rotation. Indeed, it is commonly accepted that when the particle or grain size is reduced to a few of nanometers, the saturation magnetization decreases and the coercive field increases. The en- hancement of the randomly canted surface or interface spin frac- tion contributes to the total magnetization decrease and surface magnetic anisotropy increases [4]. Moreover, the reversal mag- netization mechanism changes from a wall displacement driven one (magnetic poly-domains) to a coherent full spins rotation monitored one (magnetic single domain), affecting drastically the dynamic magnetic behavior of the considered materials. In MZFO, the critical grain size D cr below which the grains become magnetic single domains was first measured to be about 4 mm by neutron depolarization measurements [5]. These mea- surements showed also that ceramics with grains slightly smaller than 4 mm exhibited a substantial lowering of their energy dis- sipation in the MHz frequency range [6]. Parallel to that, theore- tical calculations based on correction for the soft magnetic en- vironment using magnetostatic calculations, the modified lower and upper bound of the Brown's expression for the critical grain size, yielded values for which the observed value was found to be in between these corrected lower and upper bound [7]. Latter, Aharoni et al. did further micromagnetic calculations to deal with the soft magnetic environment [8]. Finally, Aarts et al. studied 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.2015.03.045 0304-8853/& 2015 Elsevier B.V. All rights reserved. n Correspondence to: Bâtiment Lavoisier, 15 rue Jean-Antoine de Baïf, 75205 Paris, France. Fax: þ33 1 5727 7263. E-mail address: ammarmer@univ-paris-diderot.fr (S. Ammar). Journal of Magnetism and Magnetic Materials 387 (2015) 90–95