A Theoretical Study On Magnetoelectric Effect Of Layered Magnetoelectric Composite

The layered magnetoelectric (ME) material is a kind of important functional material which can achieve ME effect, it is usually composed of ferroelectric and magnetostrictive material, and its connectivity is 2-2 type. Because the difference of the ferroelectric layers'thickness, we can divide the layered ME material into two kinds of different structures: one is the thick-film layered composite. After combining the prepared magnetostrictive and ferroelectric disks alternately with special glues, we can obtain this kind of structure. The thickness of both magnetostrictive disk and ferroelectric disk are about 20μm . The thick-film layered composite has some traits, such as, the structure and the experimental method are simple, and we can randomly choose different magnetostrictive and piezoelectric constituents, however, the not ideal interface coupling would restrict the ME effect in some degree; The other kind of structure is obtained by making the ferroelectric thin film grown epitaxially on the magnetostrictive substrate. The thickness of the ferroelectric thin film is only about 100nm . As far as this kind of structure is considered, the efficiency of the interface coupling is much ideal, and it is adapt to the trend of smaller volume and lower energy-consumption. both these two kinds of structures mentioned above could get giant ME effect, so they can be widely used in the devices of electric current measurement, magnetic detector and smart sensor et al. In addition, they have their own merits and some place need to improve, so both the two kinds of structures are the keystones in the field of ME composites.In present paper, we mainly consider the influence of some detailed elements on the ME effects of the two structure mentioned above. To the first kind of structure, on the foundation of Landau-Ginsberg-Devonshire thermodynamic theory, the constitutive equation of each phase and combining the appropriate boundary conditions, we respectively consider the influences of the two-dimensional isotropy stress existed in the PZT layer and the applied biased dc electric current on the transversal and longitudinal ME voltage coefficients. The calculation result shows that the decrease of compressive stress will enhance the ME coupling, this is in agreement with the experimental result qualitatively. Extending the domain to the tensile case, we find the increase of tensile stress will also improve the ME coupling; As far as the applied biased dc electric field is considered, the electric field is used to improve the piezoelectric effect, we find that it will also impair the ME effect, it offer a possible result for the phenomenon that the experimental data was usually lower than the theoretical result.Because the ferroelectric disk is thick enough, when we analyse the influence of two-dimensional isotropy stress on the ME effect,the stress could be considered to be independent of the polarization of ferroelectric film, and could be considered as constant. But to the heterostructure that make the ferroelectric thin film grown epitaxially on the magnetostrictive substrate, the strain should be constant. So we establish a model for the second kind of structure. By considering the efficient mechanical coulping at the interface, the magnetoelectric effect, especially for magnetically induced polarization of the multiferroic film, has been theoretically studied combining the modified constitutive equations with the Landau-Ginsberg-Devonshire thermodynamic theory, where the influence of residual strain, spontaneous polarization, and magnetostriction are considered. The calculation results show that large magnetic field induced polarization could be produced in ferroelectric film due to enhanced elastic coupling interaction and the ME effect peaks at the ferromagnetic transition temperature of the manganite. Comparison with data for multilayer samples reveals ideal interface coupling between the magnetostrictive phase substrate and epitaxial ferroelectric film. The calculations are in broad agreement with the available experimental results and Green function technique analysis.