Exploration of magnetoelectric thin-film sensors using superlattice composition spreads

Magnetoelectric materials are attractive due to the co-existence of charge polarization and magnetization. The magnetoelectric effect can be used to induce change in magnetization by applying electric field and induce electric polarization from applied magnetic field. Such materials can be used to develop novel sensors. One way to obtain magnetoelectric materials is by combining piezoelectric and piezomagnetic materials. Magnetoelectric materials have previously been pursued in bulk composites. We are exploring fabrication of artificial magnetoelectric thin films by creating superlattice structures where piezoelectric materials and piezomagnetic materials are modulated in periods of multiple unit cells. In particular, we fabricate superlattice composition spreads where one end of the spreads is a pure piezoelectric material and the other end is a pure piezomagnetic material. This technique allows us to 1) study coupling of the two properties at nanometer level and 2) systematically investigate mixing and changing of the two physical properties as a function of average composition which continuously varies across the spread. In order to fabricate a superlattice spread, a series of alternating gradient thickness deposition controlled at atomic layer level is performed for two targets with end compositions using combinatorial pulsed laser deposition. We have thus far fabricated BaTiO3, - CoFe2O4 and PbTiO3, - CoFe2O4 spreads. The lattice parameters of these materials are such that they can be grown together in a pseudo hetero-epitaxial manner. Microwave microscopy and scanning SQUID microscopy are used to determine the respective ferroic properties across the spread. We found that the composition region in the middle of the spread can often exhibit reasonable dielectric constants and magnetization simultaneously. A single composition near the middle range selected from the spread was made and patterned to further confirm the ferroic properties. We are currently studying their magnetoelectric properties.