Design of N-Doped BaTiO₃/CoFe₂O₄ Multiferroic Heterostructures and Their Physical Properties

N-doped oxides and/or oxynitrides have recently emerged as promising materials in the field of optospintronics, blending optical, electronic, and spin-related functionalities to enable innovative device applications. In particular, the controlled incorporation of nitrogen into the crystal lattice of oxide semiconductors offers the possibility of modulating the value of their optical band gap, resulting in novel functionalities. The design of such single crystalline thin films is highly challenging. In this work, we have grown and fully characterized single crystalline N-doped oxide heterostructures, <i>i.e.</i> CoFe₂O₄/N:BaTiO₃/La_2/3Sr_1/3MnO₃/SrTiO₃(001). The aim is to develop an artificial opto-multiferroic structure. Hence, barium titanate (BaTiO₃) was chosen for its ferroelectricity and its favorable absorption spectrum, while cobalt ferrite (CoFe₂O₄) provided the additional ferrimagnetism [1]. Growth was performed in a dedicated oxygen (or nitrogen) plasma-assisted molecular beam epitaxy [2]. Then, their structural, chemical and physical properties were thoroughly studied. <i>Operando</i> electron diffraction, <i>ex situ</i> X-ray diffraction and reflectivity were used to obtain structural information on the thin films, such as epitaxial relationship between growing layers, crystalline structure, lattice parameters, film thickness and roughness. The stoichiometry and electronic structure of the layers were verified <i>in situ</i> by photoemission spectroscopy. The film microstructure was investigated via high-resolution transmission electron microscopy on CEMES-CNRS laboratory. The ferroelectric properties of the layers were probed by piezoelectric force microscopy and capacitance measurements. Finally, the fine structure and magnetic behavior of the ferrite top layers were studied at the European Synchrotron Radiation Facility on beamline ID32. The Fe and Co cation site distribution was determined by exploring the L_2,3-edges X-ray absorption (XAS) and circular dichroism (XMCD) spectra [3]. Element specific magnetic behavior was studied using field-dependent XMCD hysteresis loops at Fe and Co L₃-edges. We will show in detail the ferroelectric and ferromagnetic properties of the heterostructures as a function of nitrogen doping, correlated with a comprehensive description of the crystalline and electronic structures of the materials.<br/><br/><b>References:</b><br/>[1] N. Jedrecy <i>et al.</i> ACS Appl. Mat. & Int. vol. 10, no 33, p.28003, 2018.<br/>[2] A. Derj <i>et al.</i>, Mater. Adv., vol. 3, no 7, p. 3135, 2022.<br/>[3] P.V.B Pinho et al., Appl. Surf. Science, no 615, p.156354, 2023