Study of the Magnetoelectric Properties of Multiferroic Thin Films and Composites for Device Applications

Abstract

This thesis presents studies on magnetic and magnetoelectric behavior of composite multiferroics, materials that are strong candidates for next generation multifunctional devices. These investigations have been carried out on matrix based composite magnetoelectric multiferroics consisting of magnetostrictive cobalt ferrite (CoFe2O4) and piezoelectric barium titanate (BaTiO3) or bismuth ferrite (BiFeO3), with the magnetostrictive phase embedded in a three dimensional matrix of the piezoelectric phase. The direct magnetoelectric (ME) effect has been investigated in 0-3 composites of BaTiO3 (BTO) and CoFe2O4 (CFO). The ME response has been found to depend non- linearly on the ratio of the magnetostrictive (CFO) content and it is proportional to the strength of the applied magnetic field. The non-linear behavior is explained in terms of the compound effect of the increasing CFO content and the number density of CFO-BTO interfaces. Epitaxial self-assembled nanocomposite thin films were grown by laser ablating targets of magnetostrictive (CoFe2O4) and piezoelectric (BiFeO3 or BaTiO3) phases, on (001) oriented substrates using a combinatorial approach. The surface morphology of the nanocomposite films reveal CFO nanopillars protruding out of a flat piezoelectric matrix. Structural properties were investigated by a four-circle diffractometer, which revealed the presence of the epitaxially grown phases and was used to find the in-plane and out-of-plane lattice parameters of the constituent phases. The magnetization hysteresis loops were measured by a SQUID magnetometer at room temperature. The highly magnetostrictive CFO phase under epitaxial strain from the piezoelectric matrix, exhibits a strong perpendicular anisotropy in the magnetic properties. Changes in the magnetic anisotropy were investigated under different strain conditions induced by (i) post growth annealing, (ii) exploiting phase transitions in the BaTiO3 substrate, and (iii) applying an electric field to the electromechanical substrates. It has been shown that by exploiting the hysteretic properties of a suitable electromechanical substrate, one can control volatile or non-volatile magnetic states by means of an electric field. These results make an important contribution towards the understanding and potential applications of magnetoelectric multiferroics.