Magneitc Low-dimensional Structure In Rare Earth Metal Oxides: Experimental And First-principles Study

Magnetic materials,as important functional materials,are of great application prospect at the area of magnetic restore,spintronic devices and next age information restore.It is a key system for the condensed matter physics and calculation material science.The abundant magnetic phases and complex magnetic phase transitions in RFeO₃?R=rare earth ions?show possibilities as magnetic switch materials.The magnetism found in?-Ga₂O₃ makes it attractive in spintronics devices.The magnetic monolayer FeCl₂ also has huge potential in developing next generation spintronics devices.Our works explored the In impurity tailoring the structure and magnetism in DyFeO₃,Ga-vacancy induced magnetism in?-Ga₂O₃ and the magnetism in monolayer FeCl₂ modulating by the strain and electric field.In chapter one,we simply introduce the magnetism and spin reorientation transitions of RFeO₃,vacancy induced magnetism in metal oxides and the properties of some magnetic single layers.We also imply our purpose and significance of our research,as well as the main systems and contents.In chapter two,we exhibit the experimental method and equipment with the theoretical foundation.The preparation of samples,the analysis of structures and the measurement of properties also been introduced.In chapter three,we gave a simple representation of the theory about the first principle calculations,including the density functional theory and the local density approximation.In chapter four,a series of DyFe_1-x In_xO₃?x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9,1?polycrystalline samples have been prepared by the conventional solid reaction methods.The crystal structures,refined by the Fullprof,are found to be orthorhombic for 0?x?0.6 and hexagonal in the case of 0.7?x?1.The lattice constants are changed by the In ions doping,and a huge distortion of the magnetic sublattice was induced.The magnetization versus temperature curves are obtained for all the samples,at the temperature between 3 K and 300 K.The pure DyFeO₃sample undergoes a spin reorientation transition at around 50 K,whereas the doped In³⁺ions significantly modify the phase transition.We reveal that such substitution retained the spin reorientation transition effect for x?0.8.We develop a formalism based on the molecular theory,giving explanation of the changes in the spin-reorientation temperature induced by the In-doping.Then,by performing density functional theory?DFT?calculation,we analyzed the magnetic phases for the hexagonal compounds.Furthermore,our findings indicate a type of?'₄-?'₂transition exists in the hexagonal Fe-In magnetic systems which directly point to a quasi-2 dimensional magnetic order.In chapter five,first principle calculations based on density functional theory were performed to study the electronic structure and magnetic properties of the?-Ga₂O₃ with the presence of cation vacancies.We investigated two kinds of Ga vacancies at different symmetry sites and the consequent structural distortion and defect states.We found that both the six-fold coordinated octahedral site and the four-fold coordinated tetrahedral site vacancies can lead to a spin polarized ground state.Furthermore,the calculation identified a relationship between the spin polarization and the charge states of the vacancies,which might be explained by a molecular orbital model consisting of the uncompensated O^2--2p dangling bonds.The calculations for the two vacancies system also indicated a potential long-range ferromagnetic order which is beneficial to the potential spintronics application.In chapter six,novel emergent spin phases in single layer FeCl₂ had been investigated.Here,first principle calculations based on density functional theory was performed to study the electronic structure and magnetic properties of the monolayer1T-FeCl₂ under strain and electric field.Based on the stable structure confirmed by the phonon calculations,we systematically investigated the magnetic ground states as a function of the strain and electric field.We proposed a phase diagram which shows a wide range and tunable temperature of the magnetic order.Our results reveal monolayer 1T-FeCl₂ as an intriguing platform for the intrinsic magnetic properties in two-dimensional systems.In chapter seven,we summarized the research in the paper.We pointed out the prospect of the magnetic order dimension in RFeO₃,the vacancy indued magnetism in?-Ga₂O₃ and the magnetism in monolayer FeCl₂.