3 - Layered multiferroic composites

Abstract

In a composite consisting of magnetostrictive and piezoelectric phases, the magnetoelectric (ME) effect is the result of a “product-property,” that is the mechanical deformation due to magnetostriction results in a dielectric polarization due to the piezoelectric effect. The early works on ME composites dealt with CoFe2O4 and BaTiO3 and were prepared by two methods: sintering and unidirectional solidification of eutectic melts. Both types of composites developed microcracks due to thermal expansion mismatch and yielded ME coefficients that were a factor 40–60 smaller than calculated values. Possible causes include (1) microcracks, defects, and impurities that result from incompatible structural and thermal properties of the two phases, leading to poor mechanical coupling and (2) a large leakage current resulting in loss of polarization. Harshe, Dougherty, and Newnham, in their pioneering work on ME composites, proposed a theoretical model for multilayer heterostructures with alternating layers of magnetostrictive and piezoelectric phases. A multilayer structure is expected to be far superior to bulk composites due to reduction in leakage current, and the structure can easily be poled electrically to further enhance the piezoelectricity. This chapter is on thick film and thin film layered composites with ferroelectric oxides and ferromagnetic or ferrimagnetic metals/alloys or oxides. Ferromagnetic/ferrimagnetic oxides, including ferrites, manganites, 3d-transition metals/alloys for the magnetic phase and lead zirconate titanate (PZT), barium titanate (BTO), or lead magnesium niobate-lead titanate (PMN-PT) for the ferroelectric phase, are considered. Recent efforts on composites with piezoelectric AlN, lanthanum-gallium-tantalate (langatate), and quartz are also addressed.