Electric-field control of magnetism in multiferroic composites
Date of Issue2015
School of Materials Science and Engineering
Multiferroic materials have at least two of the ferroic properties, namely ferroelectricity, ferro (anitferro) magnetism and ferroelasticity.The term usually refers to materials that possess both ferroelectric and magnetic orders. An interesting and important consequence of the coexistence of multiple orders is the possible coupling between them, which is named magnetoelectric coupling. It implies that the magnetization could be modulated by an electric field and vice versa. We aim to understand the magnetoelectric coupling phenomenon in multiferroic composite systems in this project. Single-phase multiferroic materials are scarce in nature. However, by combining ferroelectric and ferro (ferri) magnetic materials together, we could design composites with multiferroic properties with much more flexibility. In such a composite, magnetoelectric coupling is achieved through interface-mediated strain effect. There are different ways to combine ferroic phases to form composites. In this project, we focus our effort on the two types of composite systems that would exhibit minimal clamping effect, such as the bulk composites and a magnetic film on a piezoelectric substrate. First, the bulk composites consisting barium titanate (BaTiO3) and cobalt ferrite (CoFe2O4) were synthesized using conventional and spark plasma sintering (SPS) techniques. Dense BaTiO3-CoFe2O4 ceramics were obtained at high temperatures in both cases. SPS has the advantage of producing samples with higher density without any secondary phases. Ferroelectric and magnetic properties of the bulk composites were investigated which showsenhanced properties as density of the composite increased. Unfortunately, due to the high leakage current and the small magnetoelectric coupling effect in bulk ceramics, we were not able to achieve electric-field control of magnetism. Subsequently, we investigated a composite where a magnetic film was deposited on a piezoelectric substrate, which had a well-defined, high-quality interface between the two phases, and was totally free of substrate clamping effect. . Magnetic CoFe2O4 films were deposited on piezoelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) single crystal substrates by pulsed laser deposition. Using an active piezoelectric substrate not only eliminates the clamping effect which is prevalent in the conventional composite film systems on passive substrates, but also facilitates effective strain coupling at the interface. Accurate crystal structure investigation confirms the strain transfer between the PMN-PT substrate and the CoFe2O4 film. When an electric field was applied to the PMN-PT substrate, the magnetic response of the CoFe2O4 film clearly altered as revealed by both their magnetic hysteresis loops and domain structures. The demonstrates magnetoelectric coupling between the substrate and the film, which can be explained using a strain transfer model. Our work has provided experimental evidence for the strain mediated magnetoelectric coupling in this composites where a magnetic CoFe2O4 film is on a ferroelectric substrate and offer valuable information for their potential applications.