First-time synthesis of magnetoelectric core-shell composite via conventional solid-state reaction

In recent years, multiferroics and magnetoelectrics have demonstrated their potential for a variety of applications. However, no magnetoelectric material has been translated to a real application yet. Here, we report the first magnetoelectric in situ core-shell ceramic, where core-shell type grains develop during the sintering process. The core consists of ferrimagnetic CoFe2O4, which is surrounded by a ferroelectric shell consisting of (BiFeO3)x-(Bi1/2K1/2TiO3)1-x. We establish the core-shell nature of these grains by transmission-electron microscopy (TEM) and find an epitaxial crystallographic relation between core and shell, with a lattice mismatch of 6±0.7%. The core-shell grains exhibit exceptional magnetoelectric coupling effects, which we attribute to the epitaxial connection between the magnetic and ferroelectric phase, which also leads to exchange bias coupling as demonstrated by neutron diffraction. Apparently, ferrimagnetic CoFe2O4 cores undergo a non-centrosymmetric distortion of the crystal structure upon epitaxial strain from shell, which leads to simultaneous ferrimagnetism and piezoelectricity. We conclude that core-shell ceramics offer a number of advantages over other magnetoelectric composites, such as lower leakage current, higher density and absence of substrate clamping effects. At the same time, the material is predestined for applications, since preparation is cost-effective and only requires a single sintering step. This discovery adds a promising new perspective for the application of magnetoelectric materials.