Preparation and Magnetoelectric Behavior of Ni/BaTiO3 Heterostructures with 0-3 Connectivity

Magnetoelectric 0-3 composites consisting of small ferromagnetic Ni particles in the micrometer range embedded in a ferroelectric BaTiO3 matrix are synthesized by reductive sintering in flowing nitrogen with carbon as the oxygen getter. Phase purity of the samples Ni-x/(BaTiO3)(1 - x) (x = 0.1-0.7) is verified by powder X-ray diffraction in combination with scanning electron microscopy/energy-dispersive X-ray spectroscopy analyses and magnetization investigations. The magnetoelectric coefficient (alpha(ME)) is studied for parallel and perpendicular orientations of polarization and magnetic field. For the latter orientation, a reverse sign and a much weaker signal are found. The dependence of alpha(ME) on the magnetic direct current bias field shows a linear behavior between approximate to +/- 2 kOe and a drop to almost zero above approximate to 4 kOe. Largest values are obtained for x = 0.4. With increasing frequency of the alternating current driving field, alpha(ME) becomes larger. At low temperatures, two step-like decreases in the magnetoelectric coefficient occur, reflecting the phase transitions of BaTiO3 (tetragonal -> orthorhombic and orthorhombic -> rhombohedral). While this finding corroborates the interpretation of alpha(ME) being a product property of magnetostriction and piezoelectricity, distinct deviations between the field dependence of the integrated ME signal and the magnetostriction of nickel are found.