| A cubic EuTiO3(ETO) is known to be a low temperature multiferroic material, whichperforms simultaneous G-type antiferromagnetic (AFM, TN~5.5K) and quantumparaelectric (PE, below70K) order. Below5.5K, when two ferric orders coexist, there is astrong coupling between the Eu spins and dielectric properties in the ETO bulk. In addition,the ETO thin film is transformed from AFM-PE to ferromagnetic-ferroelectric (FM-FE)behavior under large biaxial strain, ferromagnetic and ferroelectric Curie temperature are4.24K and250K, respectively. These properties mean that ETO presents the complexcharge, spin, lattice and orbital degrees of freedom, and they are strongly coupled to eachother. These behaviors cause novel magnetic, electric and many other characteristics in thebulk and film, also cause rich physical phenomena. Therefore, in this dissertation, we startfrom the deposition of ETO thin film, and investigate the physical behaviors andmechanisms of the film at different conditions, and then we study the relationshipsbetween microstructure, strain and physical propertie in this multiferroic material.In this thesis,(001) ETO thin film and (001) ETO:MgO nanocomposite film wereboth prepared on (001) SrTiO3substrate by pulsed laser deposition. X-ray diffraction(XRD) and Transimission Electron Microscopy (TEM) were used to characterize themicrostructure of the two films, the lateral and vertical strain was also investigated in detail.Then we focused on the study of the physical properties of the single-phase andnanocomposite thin film, such as magnetic, electric and optical properties. And the mainresults are listed below:1) The as-deposited (001) ETO thin film has growth-related strain, due to the lowerdeposition oxygen pressure. The as-deposited film performs FM-like magnetic behaviorunder the strain. Postannealing in a reducing atmosphere is used to relax the strain, andthen the epitaxial ETO thin film (annealed ETO film) is obtained. The annealed film showsa distinct antiferromagnetic transition at5.1K, which is the same as the magnetic properties of the bulk. A comparison of leakage current density of the as-deposited and annealed ETOfilms at room temperature shows that the leakage current density has been reduced byseven orders of magnitude as a result of post-annealing. Combined with the XPS results,the possible mechanism for such leakage is that the impurity of Eu3+contributes to morecharge carriers that in turn lead to higher leakage current density. In the differenttemperatue and bias condition, we fit and analyze the leakage current density vs. electricfield (J-E) curves of the annealed film, to obtain the different conduction mechanisms,which are strongly related to temperature and voltage polarity. It was determined that from50to150K, the dominant conduction mechanism was a space-charge-limited currentunder both negative and positive biases. From200to300K, the conduction mechanismshows Schottky emission and Fowler-Nordheim tunneling behaviors for the negative andpositive biases, respectively.2) The (001) ETO:MgO nanocomposite thin film was also prepared by PLD toinvestigate the vertical strain effect on the physical properties of the nanocomposite film.The XRD, TEM and STEM results show that the two-phase separated ETO and MgOdomains have alternately and vertically aligned, and the MgO nanopillars with diametersof~5nm are embedded in a ETO matrix. Meanwhile, the ETO phase in the ETO:MgO filmhas a vertical strain of2.56%, and then the nanocomposite film performs ferroelectricbelow250K and ferromagnetic below3.5K. From the magnetodielectric measurementsaround the ferromagnetic transition, the nanocomposite film shows a strongmagnetoelectric coupling effect. With decreasing temperature, the mechanisms of themagnetoelectric coupling change from linear EH()(10K) to bilinear EH2()(3.5K) tobiquadratic E2H2()(2K) effect.3) The optical properties of the single-phase and nanocomposite thin film werestudied. The photocarrier relaxation dynamics of strained and unstrained ETO films areinvestigated using femtosecond transient absorption spectroscopy. In both films, the decaydynamics of2p-3dt2gtransition are independent of the epitaxial strain. Remarkably, the4f-3dt2gtransition is found to be enhanced considerably in the strained film. This behavior can be linked to the strain induced band structure modulation. Then, we measured thereflection, absorption and transmission characteristics of the ETO thin film, MgO thin film,ETO:MgO nanocomposite film and LaAlO3substrate. The nanocomposite film shows thedifferent optical properties compared with the single-phase films. The relationship betweenmicrostructure and optical characteristics of the nanocomposite were simulated by theprogram CST Studio2009, to prove that there is a strong coupling between microstructure,strain and optical properties. |
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