Manipulation of artificial multiferroics using doping-induced plastic strain relaxation

In this work, we present a comprehensive and comparative study on the growth and magnetic properties of CoFe₂O₄ layers deposited on both undoped and self-oxidized N-doped BaTiO₃ films grown on La_2/3Sr_1/3MnO₃/SrTiO₃ (001) substrates, a prototypical magneto-electric multiferroic system. The oxide layers were grown by plasma-assisted molecular beam epitaxy. We show that N doping, even at a low level of 1 %, induces substantial changes in growth modes and strain relaxation in BaTiO₃ and consequently in the CoFe₂O₄ top layer, which exhibits enhanced magnetization and reduced magnetocrystalline anisotropy. The magnetic properties were determined from extensive element-specific magnetic X-ray dichroic measurements, which were simulated through crystal field multiplet calculations. The structural properties of the samples were investigated using grazing incidence X-ray diffraction and high-resolution electron microscopy. Our observations provide evidence that the plastic relaxation of the underlying ferroelectric N-doped BaTiO₃ layer affects the magnetic properties of the top CoFe₂O₄ layer and can serve as an additional tuning parameter for manipulating the expected properties of multiferroics.