Unveiling and Optimizing Interface Properties of NiFe₂O₄/BaTiO₃ Heterostructures

In this study, we investigate the properties, with a special focus on the magnetic attributes, of NiFe 2 O 4 (NFO)/BaTiO 3 (BTO) multiferroic heterostructures, examining individual layer thicknesses ranging from 3 to 12 nm. X-ray diffraction reveals that as BTO thickness increases, NFO transitions from a nonstrained cubic lattice to a compressively strained tetragonal lattice. In contrast, thicker NFO layers introduce enhanced tensile stresses on the BTO layer, counteracting the compressive strain originating from the SrTiO 3 (STO) substrate. Piezoresponse force microscopy demonstrates that the polarization switching voltage escalates with increasing BTO layer thickness. Through X-ray magnetic circular dichroism measurements coupled with multiplet theory, we elucidate variations in the magnetic moments and ionic distributions within the NFO layers. Remarkably, a larger BTO thickness is associated with a chemical reduction of Fe ions in the NFO layer, indicative of increased oxygen vacancies, which are induced by the increasing compressive strain as evidenced by first-principles calculations. Thinner NFO layers showed increased tetrahedral ( T d ) site vacancies and oxygen vacancies concomitant with reduced magnetic moments, which can be optimized by either increasing NFO thickness or through air annealing at 450 °C. Combining the ionic distribution variation with in-plane lattice parameter evolution during growth, we postulated that the reduced magnetic moments originate from a 2–3 nm antiferromagnetic rock salt NiO/FeO formed at the beginning of NFO growth, while the air annealing restores the magnetism by oxidizing and arranging the NiO/FeO into spinel NFO under the synergistic effect of oxygen and heat. Air annealing at moderate temperature appears as a very efficient method to restore the magnetization of ultrathin layers of NFO, allowing us to overcome a major drawback for these compounds, which hampers their utilization in spintronics applications so far.