Enhanced magnetoelectric effect in core-shell particulate composites

A method for the preparation of magnetostrictive-piezoelectric particulate composites with enhanced magnetoelectric effect was developed. The composites were synthesized in situ forming a shell of barium titanate around nanoparticles of cobalt ferrite, varying the composition of the cobalt ferrite magnetostrictive phase from 20to60wt.%. Cobalt ferrite nanoparticles were obtained by coprecipitation and then added to the precursor gel of barium titanate, allowing the in situ formation of the composite and thereby restricting the contact of the ferrite particles during sintering. The samples were sintered at a temperature ranging from 1100to1250°C for 12h, followed by a plating step to be electrically poled. Additional samples were prepared by conventional mechanical milling for comparison, starting from cobalt ferrite prepared either by coprecipitation or the sol-gel technique and commercial barium titanate. Samples of same compositions prepared by different methods and sintered under the same conditions showed different behavior. For example, the in situ synthesized sample showed a piezoelectric d33 constant approximately six times larger and a magnetoelectric voltage coefficient approximately three times larger than the corresponding mechanically milled samples. The piezoelectric d33 constant decreased with the content of ferrite, achieving the maximum value of 44.6pC∕N for the in situ prepared sample with 20wt.% of ferrite sintered at 1200°C. The highest magnetoelectric effect was present in the composition of 50wt.% ferrite sintered at 1200°C, with a magnetoelectric coefficient of 1.48mV∕cmOe at room temperature.