Growth and characterization of LuFe₂O₄thin film by pulsed laser deposition

x, 86 leaves : ill. ; 30 cm.PolyU Library Call No.: [THS] LG51 .H577M AP 2010 LiuRecently mixed valence material LuFe₂ O₄ has attracted great interest for its multiferroic properties originated from its electronic ferroelectricity. Unlike conventional ferroelectric material with a pervoskite structure, where the ferroelectric is driven by ion shift, the ferroelectricity in LuFe₂ O₄ is driven by the charge ordered state (CO) in the so-called Fe-O double triangle layers with mixed valence of Fe²⁺ : Fe³⁺ =1:1. Giant room temperature magnetodielectric and dielectric tunability under electric field have been reported, suggesting that LuFe₂ O₄ is promising for novel multifunctional storage element. However, all the studies have been focused on LuFe₂ O₄ single crystal or ceramic. In this thesis, the study of growth and characterization of LuFe₂ O₄ thin films are carried out and the following results have been obtained. LuFe₂ O₄ thin films have been successively grown on sapphire substrates. Substrate temperature during deposition is found to be a critical condition (at least 750℃) for the LuFe₂ O₄ phase formation, while the higher the substrate temperature the better of the crystallization. Oxygen pressure during film deposition and annealing is critical to film.s electrical and magnetic properties since optimized oxygen pressure is essential to realize the right ratio of Fe²⁺: Fe³⁺. It is also found that extra amount of Fe oxide in the target helps formation of LuFe₂ O₄ films, however, impurity phase and interfacial reactions are present in the films. X-ray diffraction and transmission electron microscopy have been used to characterize the structure and crystallization of the LuFe₂ O₄ films. Electrical properties of LuFe₂ O₄ thin film have been studied by using HP 4294A and HP 4194B RF impedance analyzers with gold inter digital electrodes coated on LuFe₂ O₄ films surface. Dielectric tunability under DC bias voltage has been found in LuFe₂ O₄ films, and the result is comparable to that found in the bulk material. Electric field induced phase transition has been explored in the LuFe₂ O₄ film, where higher electric field is needed to break the charge ordering in the LuFe₂ O₄ film than bulk. Temperature-dependent phase transition has also been found both in dielectric and resistance measurements, where at charge ordered (CO) transition temperature LuFe₂O₄film experiences insulator-to-metal transition due to the charge ordering breakdown. Magnetic properties have also been studied by vibrating sample magnetometer (VSM), where anisotropic magnetic properties of the LuFe₂ O₄ film are found. Magnetic and electrical coupling has also been found in dielectric measurement, where the dielectric tunability under DC bias voltage of the sample is suppressed by a small magnetic field.Department of Applied PhysicsM.Phil., Dept. of Applied Physics, The Hong Kong Polytechnic University, 201