Stoichiometry of CoFe2O4 as a key to phase control and improved functional properties of multiferroic BaTiO3-CoFe2O4 bulk composites

Abstract

Composite multiferroics have been widely studied as materials with large magnetoelectric effect at room temperature. They have also been envisaged as materials exhibiting multicaloric effects. Although the best magnetoelectric coupling has been achieved in multiferroic bilayer thin films with 2-2-connectivity, multicaloric applications require bulk ceramic composites with large thermal mass, high density, and low resistivity. Unfortunately, high temperature sintering of multiferroic ceramics often results in the formation of secondary phases and undesirable chemical modifications of the constituents, which are detrimental to the functional properties. Thus, for the “canonical” multiferroic composites of ferroelectric BaTiO₃ and ferrimagnetic CoFe₂O₄, the formation of a secondary phase of barium hexaferrite, BaFe₁₂O₁₉, has been frequently observed, but not always explicitly reported. In this article, we present a method to suppress the formation of this phase both by sintering in nitrogen and by changing the stoichiometry of cobalt ferrite to incorporate more cobalt. The latter restricts the diffusion of the iron cations into barium titanate during sintering which is the main driving force for the formation of BaFe₁₂O₁₉. Moreover, composites with non-stoichiometric Co_1.05Fe_1.95O₄ show a three-fold improvement in magnetoelectric coefficient in comparison to composites with stoichiometric cobalt ferrite.