Increased Magnetoelectric Coupling in Porous Nanocomposites of CoFe2O4 and BiFeO3 with Residual Porosity for Switchable Magnetic Devices

In this work, multiferroic thin-film nanocomposites were synthesized by coating the inside of mesoporous, cobalt ferrite (CoFe2O4 or CFO) with varying thicknesses of piezoelectric bismuth ferrite (BiFeO3 or BFO) grown by atomic layer deposition (ALD). Since ALD allows for precise control of the BFO layer thickness, the amount of residual porosity inside the pores can be controlled. Upon electrical poling, the piezoelectric BFO strains to be under out-of-plane tension, and since BFO is covalently bound to CFO, this tensile stress is transferred from BFO to CFO. CFO is a negative magnetostrictive material, meaning its magnetization should decrease in the direction of tension. This decrease in magnetization was observed in out-of-plane magnetometry experiments. Interestingly, the magnetization changes were found to be largest in the samples with the most residual porosity, despite the fact that they contained the smallest volume of BFO. Indeed, while the fully filled samples had a similar magnetoelectric coefficient to other dense nanostructured BFOCFO composites reported in the literature, composites with the most residual porosity showed an exceptionally large converse magnetoelectric coefficient of 1.2 x 10-6 s m-1, an order of magnitude higher than dense composites. Strain transfer was confirmed using high-resolution X-ray diffraction. Samples with more residual porosity showed larger strain changes, corroborating the magnetization data. This suggests that magnetoelectric coupling can be optimized by engineering residual porosity into multiferroic composites. Such systems have profound effects for a broad range of switchable magnetic devices, particularly in the microwave and spintronic space.