Induced Ferromagnetic Behavior In α-Fe₂ O₃ Based On Laser Irradiation And Li⁺ Doping

α-Fe₂O₃ has been widely applied in photocatalysis, pigments, gas-sensitive materials, optical limiting, lithium ion battery and water treatment because of its perfect physical and chemical properties. Currently, little research has been reported to improve the magnetic properties ofα-Fe₂O₃ nanocrystals. And the methods used in enhancing the magntism ofα-Fe₂O₃ nanocrystals like surface modify are complex. Researching simple, environment-friendly and economical methods to improve the magnetic properties ofα-Fe₂O₃ nanocrystals is significant for broadening its practical application. Moreover, laser induced magnetism effect has potential application value in optical controled molecular magneism, optical insulation, high density information storage and optical switch. So from the view points of fundamental research and applications, studying of new photon-induced magnetism materials with perfect physical and chemical properties and understanding of its mechanism are of great importance. In this thesis, laser irradiation and Li⁺ doping were used to induce the ferromagnetism ofα-Fe₂O₃ nanocrystal, results show that both laser irradiation and Li⁺ doping can successfully change the magnetism ofα-Fe₂O₃ nanocrystals from antiferromagnetism to ferromagntism.Firstly, band gap, density of energy and optical properties ofα-Fe₂O₃ were calculated and analysed based on first principle. Fluorescence intensity dependent of 976 nm laser power was discussed. Results show that fluorescence irradiated by low power laser is so weak that it can be left out. Considering heat capacity ofα-Fe₂O₃ nanocrystals and heat diffusion, equation of temperature increment ofα-Fe₂O₃ nanocrystals and laser irradiation time was built. Result demonstrates that temperature ofα-Fe₂O₃ nanocrystals rises fastly as laser irradiaion time increases.α-Fe₂O₃ nanocrystals were irradiated by laser. Magnetic measurement of the raw sample, the irradiated sample, the irradiated sample annealed in oxygen and the irradiated sample exposed in air for seven months was carried out at room temperature using vibrating sample magnetometer. Samples before and after laser irradiation were characterized by X-ray Diffraction, Scanning Electron Microscopy, UV-Vis Absorption Spectrum, Fourier Transform Infrared Spectroscopy, Direct Current Four-pole Resistance and X-ray Photoelectron Spectroscopy. Ferromagnetism is proposed to arise from the bound magnetic polarons (BMPs) via laser irradiation-generated oxygen vacancies-bound electrons in the interior ofα-Fe₂O₃ nanocrystals. As a result ,α-Fe₂O₃ nanocrystals transformed from antiferromagnetism to ferromagnetism.Seven kinds of Li⁺ dopedα-Fe₂O₃ powders with diferent mole ratios were synthesized. Magnetic measurement of seven samples was carried out at room temperature using Vibrating Sample Magnetometer. Samples were characterized by X-ray Diffraction, Scanning Electron Microscopy, UV-Vis Absorption Spectrum, Fourier Transform Infrared Spectroscopy, Raman Spectroscopy, Direct Current Four-pole Resistance and Photoluminescence Spectroscopy. Results illuminate that Fe ions on the surface of 2 mol% Li⁺ dopedα-Fe₂O₃ power are substituted by Li ions, which generates amount of holes inα-Fe₂O₃. Holes enhance the coupling interaction of adjacent Fe ions, and thenα-Fe₂O₃ powders become ferromagnetism.