Increasing Strain-Mediated Magnetoelectric Coupling in Nanocomposites with Residual Porosity

This work examines the role of nanoscale structure in the control of magnetoelectric coupling in three-dimensional multiferroic composites. Three-dimensional composites allow for a much increased interfacial surface area between the magnetostrictive and piezoelectric components, which in turn can lead to a much enhanced magnetoelectric coupling over traditional planar stack composites. Here, we investigate a composite made up of a solution-processed mesoporous material with the pores filled by atomic layer deposition with the alternate material. Tuning the thickness of the ALD-deposited layer allows for control of final composite porosity. We have found that the strength of magnetoelectric coupling depends heavily on this residual porosity, with higher porosity leading to greater magnetoelectric response. In a composite of mesoporous cobalt ferrite (CFO) filled with only 25% bismuth ferrite (BFO), we have found that an applied voltage on the composite causes a decrease in the saturation magnetization of the composite to about 50% its unpoled value, while the sample filled 100% with BFO shows little change in saturation magnetization. High-resolution X-ray diffraction confirms that the residual porosity is necessary for strain transfer to occur from the piezoelectric to the magnetostrictive component, as the porosity allows for mechanical flexing throughout the film.