| Orthorhombic manganites o-AMnO3 have recently received an astonishing attention due to their multiferroicity. Meanwhile, we note that most of the o-AMnO3 adopt a collinear or non-collinear antiferromagnetic order, resulting in a negligible net magnetization. Based on this fact, though it has been well reported that an external magnetic field is able to modulate the ferroelectric polarization, the net magnetization is hardly affected by the application of an electric field. Therefore, from the application point of view, it is beneficial to enhance the net magnetization in the otherwise antiferromagnetic structure. On the other hand, it is easier to apply a high electric field for a film sample than for a bulk one. Therefore, the preparation of o-AMnO3 film is very necessary to promote its application.Based on the above considerations, we focus on the study of the half-Cr-doped orthorhombic YMnO3 (YMn0.5Cr0.5O3). Firstly, we investigate the magnetic properties and cation ordering pattern of the polycrystalline sample. We then systemically study the epitaxial orientation and magnetocrystalline anisotropy of the YMn0.5Cr0.5O0.5 films. Next, we focus on the multiferroic properties and magnetoelectric coupling in the single-crystalline YMn0.5Cr0.5O3 film under in-plane compressive strain. At last, we perform a preliminary research on the relationship between the multiferroic properties and compressive strain. Specifically, there are seven chapters in this dissertation and the main conclusions of each chapter are as follows:Chapter 1. We first give a brief introduction to the crystal structure of o-AMnO3, including oxygen octahedral tilting and distortion. We then summarize a series of magnetic exchange interactions and mechanisms for ferroelectricity. Next, we give a detailed description of the magnetic model of o-AMnO3, and present the magnetoelectric phase diagram of the family. At last, we discuss the effects of epitaxial strain on the physical properties of o-AMnO3, including magnetoelectric properties and magnetocrystalline anisotropy.Chapter 2. Firstly, we introduce the preparation method and the structural characterization techniques of polycrystalline samples. Secondly, we give details of film deposition, and introduce several X-ray diffraction techniques for characterizing film structure and quality. At last, we summarize the experimental methods employed in this work for studying sample physical properties, including electrical transport, magnetic and ferroelectric properties measurements.Chapter 3. We first analyze the X-ray diffraction data of a polycrystalline YMn0.5Cr0.5O3 sample by Rietveld refinement. The results suggest that the compound has a monoclinic structure with the Mn3+/Cr3+layers alternately stacking along the (001) direction. The first-principles calculations show that the structure with layered B-site cation ordering has the lowest total energy. Moreover, it is shown that a ferrimagnetic configuration further lowers the total energy of the layered structure. At last, based upon the framework of semi-covalent exchange, we discuss the key role of the layered ordering pattern in stabilizing the ferrimagnetic state in the compound.Chapter 4. We first make a comprehensive analysis about the lattice mismatch between YMn0.5Cr0.5O3 and various single crystalline substrates. Structural characterization reveals that film orientation and quality are predominately controlled by the lattice mismatch between film and substrate. The films grown on LaAlO3 show the best crystallinity as well as the best flatness, due to the excellent lattice match between YMn0.5Cr0.5O3 and the substrate. Interestingly, we found that orientation of the films grown on SrTiO3 can be tuned by adjusting substrate temperature and post-annealing process. The YMn0.5Cr0.5O3 films are found to undergo a paramagnetic-ferrimagnetic transition at～70 K, which is consistent with the bulk sample. For the epitaxial films with different orientations, we have measured their M-H curves under various magnetic field configurations. The results indicate that the films may own a uniaxial anisotropy with the easy axis along the c-direction。Chapter 5. We first present the deposition details of YMn0.5Cr0.5O3 grown on YAlO3 substrates. Structural characterization reveals that all the films are of single crystal quality. It is found that the magnetic transition temperature increases from 75 K for the bulk sample to 120 K for the compressive-strained films. We then performed detailed pyroelectric current measurement, dielectric property analysis and P-E measurement to determine the ferroelectric properties of the present films. The results indicate that the films undergo a ferroelectric transition at 120 K, with anisotropic polarizations of Pa=0.05 μC/cm2,Pb=0 and Pc=0.17μC/cm2. Furthermore, it is demonstrated that the ferroelectric polarization can be modulated by magnetic field, indicating the existence of magnetoelectric coupling. Here, we argue that the Pa is related to the minor E-type phase, while Pc comes from the dominant bc-cycloidal phase in the present films.Chapter 6. Several (010)- and (100)-YMn0.5Cr0.5O3 films have been annealed at different conditions. Structural characterization reveals that the compressive strains in the as-grown films are reduced after annealing process. Therefore, we obtained a series of single crystal films with various compressive strains. Effects of the compressive strain on their multiferroic properties were then investigated. Magnetic measurement of (010)-films shows that the magnetic transition temperature is hardly affected by strain relaxation, while the anisotropic remnant magnetization varies significantly with compressive strain. Polarization measurement indicates that the anisotropic polarization is reduced due to the strain relaxation. Interestingly, anisotropic polarization shows a monotonous dependence on the compressive strain along the corresponding crystal axis. Further, analysis of anisotropic remnant magnetization reveals that the compressive strain along the c-axis plays an important role in modulating the magnetocrystalline anisotropy of the film. These effects can be ascribed to the modification of magnetic structure with the variation of compressive strain, suggesting that the compressive strains along the a-and oaxes predominantly stabilize the E-type and bo-cycloidal phases, respectively.Chapter 7. The entire thesis is briefly summarized and several experimental and theoretical ideas in future studies are proposed. |
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