Physical Properties Of Eu_0.5Ba_0.5TiO₃ Multiferroic Thin Films

Multiferroic materials, which simultaneously possess two or more ferroic orders and the coupling interaction between the different ferroic orders could induce new functionalities. From the fundamental research and practical application point of view, multiferroics with ferromagnetic-ferroelectric（FM-FE） orders are more attractive and highly desired. But, very few exist in nature due to the intrinsic contradiction in coexistence between the FM and FE ordering within a single phase. However, antiferromagnetic-ferroelectric（AFM-FE） materials are more commonly found. In this thesis, we have investigated the manipulation of ferroic orders as well as the magnetoelectric coupling effect by means of oxygen vacancies（VO） and strain based on Eu_0.5Ba_0.5TiO₃ with antiferromagnetic and ferroelectric properties. The main results can be summarized to two parts as follows:1. Epitaxial（001） oriented Eu_0.5Ba_0.5TiO₃-δ thin films were grown by pulsed laser deposition. To change the concentration of oxygen vacancies, the oxygen pressure during the deposition was varied and all thin films were post-annealed. It has been found that Eu_0.5Ba_0.5TiO₃-δ thin films exhibit FM-FE orders and, for the higher concentration of VO（i.e. δ > 0.04）, the ferroelectric Curie temperature is above room temperature. More than this, the magnetodielectic coupling effect has been also observed. Besides, first-principles density-functional calculations and high-resolution scanning transmission electron microscopy were used to investigate the underlying physical mechanism of the manipulation of ferroic orders in Eu_0.5Ba_0.5TiO₃-δ thin films. Generally speaking, the introduction of oxygen vacancies induced spin-polarized Ti³⁺ ions and enhanced off-centering displacement of Ti ions, mediates a ferromagnetic coupling between the local Eu4f spins and an increased ferroelectric Curie temperature respectively.2. Epitaxial（001） oriented(Eu_0.5Ba_0.5TiO₃)|_0.5:（MgO）_0.5 nanocomposite thin films were also fabricated by pulsed laser deposition to investigate the vertical strain effect on the physical properties of the nanocomposite films. It has been found that Eu_0.5Ba_0.5TiO₃ and MgO phases grow alternatively and spontaneously and form a vertically aligned columnar structure in the nanocomposite films. And the MgO nanopillars with a diameter of 5 nm are embedded in a Eu_0.5Ba_0.5TiO₃ matrix. Besides, the Eu_0.5Ba_0.5TiO₃ phase in the nanocomposite films has a vertical tensile strain of 2.62 % due to the lattice mismatch, which induced the change of ferroic orders. The ferromagnetic and ferroelectric Curie temperature are about 2.5 K and 330 K respectively.