Origin of enhanced magnetoelectric coupling in NiFe 2 O 4 / BaTiO 3 multilayers studied by x-ray magnetic circular dichroism

NiFe2O4/BaTiO3 (NFO/BTO) multilayer heterostructures grown on (001)-SrTiO3 substrates with alternating ferroelectric BTO and ferrimagnetic NFO layers exhibit magnetoelectric (ME) coupling, which increases with the number of layers and, hence, with the number of NFO/BTO interfaces. We have studied the local electronic and magnetic states of Ni and Fe ions in the NFO/BTO multilayers with various NFO and BTO thicknesses by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. With an increasing number of layers, both Ni and Fe magnetic moments decrease. With decreasing NFO layer thickness, the average magnetic moment of Ni decreases, while the average magnetic moment of Fe ions remains unaffected, meaning that Ni ions at the interface lose ferromagnetic moments. This suggests that the enhancement of ME coupling is associated with Ni ions at the interface whose ferromagnetic behavior is weakened through hybridization with the BTO electronic states.