Iron Manganites Synthesis by the Soft Chemistry Method
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Iron Manganites Synthesis by the Soft Chemistry Method Herve Coradin, Sophie Guillemet-Fritsch, Fabrice Agnoli, Philippe Tailhades and Abel Rousset Centre Interuniversitaire de Recherche et d’Ingenierie des Materiaux, UMR CNRS 5085, LCMIE, Universite Paul Sabatier, 118 route de Narbonne 31062 Toulouse, France ABSTRACT The iron manganites FexMn(3-x)O4 synthesis by soft chemistry method have been studied. The main difficulty is to obtain single phase spinel with high Mn content (0.4< x < 1.3). Oxalate precursor powders of these materials with controlled shape and nanoscopic size have been prepared. The precursors are then heat treated with a H2/H2O/N2 gas mixture at low temperature. The resulting stoechiometric spinels are metastable phases with high specific surface area and are highly reactive toward oxygen. Therefore, these oxide can be oxidized in air at low temperature in order to produce mixed valence defect manganites FexMn(3-x)O4+δ with a good reproducibility on the oxygen content. Although, some problems persist for the higher Mn contents, as the oxygen partial pressure for the reduction must be controlled precisely in order to produce the stoechiometric spinel at low temperature. The development of a low temperature reduction system, with oxygen partial pressure controlled by oxygen electrochemical pumping, is in progress. INTRODUCTION Manganese iron oxides with spinel structure has applications in magnetic recording media and electronic devices. Previous works carried on in our Laboratory have shown that it is possible to enlarge the well known properties of these materials, using metastable nanoparticles synthesized by soft chemistry at low temperature [1, 2]. Recently, the interest of these materials has greatly increased with the discovery of hard magnetic properties in metastable manganese ferrites. These properties are due to the distortion of the Mn3+ cations in the spinel structure by the Jahn-Teller effect [3]. The same bulk ferrites (without Cobalt) are well known to be soft magnetic materials. Moreover, magneto resistance effects have been found in similar spinel phases [4]. The aim of this work is to improve the synthesis of iron manganites Mn(3-x)FexO4, using thermodynamical approach and a new experimental setup. First, we describe precisely the synthesis method (“soft chemistry”) to obtain nanosized particles with controlled shape. Different characterizations of the precursor and the oxide powders are presented. We point out the difficulties to prepare stoechiometric oxides powders with high Mn content at low temperature, and present a new experimental process to overcome theses difficulties. The phase diagram of the Fe/Mn/O system [5] shows that the spinel phase can be obtained in air at temperature above 950°C. Only a few works have been published on the synthesis of this stoechiometric spinel at low temperature [3,6,7]. Probably, the difficulty to obtain the stoechiometric spinel at low temperature is due to, the need of a better control of the reduction atmosphere, the weak crystallinity of
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