Ferrimagnetism in manganese-rich gallium and aluminium spinels due to mixed valence Mn2+-Mn3+ states

Maged F Bekheet; Lukas Schlicker; Andrew Doran; Konrad Siemensmeyer; Aleksander Gurlo

doi:10.1039/c7dt04765g

Ferrimagnetism in manganese-rich gallium and aluminium spinels due to mixed valence Mn²⁺-Mn³⁺ states

Dalton Trans. 2018 Feb 20;47(8):2727-2738. doi: 10.1039/c7dt04765g.

Authors

Maged F Bekheet¹, Lukas Schlicker, Andrew Doran, Konrad Siemensmeyer, Aleksander Gurlo

Affiliation

¹ Fachgebiet Keramische Werkstoffe/Chair of Advanced Ceramic Materials, Institut für Werkstoffwissenschaften und -technologien, Technische Universität Berlin, Hardenbergstraße 40, 10623 Berlin, Germany. maged.bekheet@ceramics.tu-berlin.de.

PMID: 29411818
DOI: 10.1039/c7dt04765g

Abstract

Stoichiometric (MnGa₂O₄ and MnAl₂O₄) and Mn-rich (Mn_1.3Ga_1.7O₄ and Mn_1.4Al_1.6O₄) spinels with a small inversion degree (0.14-0.21) were obtained via a co-precipitation route followed by calcination of the as-synthesized coprecipitates at 700-1000 °C under different gas atmospheres (air, N₂ or argon). In situ synchrotron XRD at elevated temperatures reveals the conditions for synthesizing phase-pure materials. The stoichiometry of the samples is confirmed by inductively coupled plasma optical emission spectrometry as well as by structure refinement of neutron diffraction data of phase-pure specimens. XANES characterization reveals the average oxidation state of manganese to be +2.2 and 2.3 in Mn_1.3Ga_1.7O₄ and Mn_1.4Al_1.6O₄ spinels, respectively. The mixed Mn²⁺-Mn³⁺ valence states are responsible for the ferrimagnetic properties of Mn_1.3Ga_1.7O₄ and Mn_1.4Al_1.6O₄ samples below 48 and 55 K, respectively, as well as for a smaller optical bandgap when compared to stoichiometric spinels.