Multiferroic And Photo Effect Of Oxide Films And Heterostructures

Multiferroic materials as a group member of oxide family have attracted considerable attention since they simultaneously show spontaneous electric and magnetic order in a single phase.Coupling between the magnetic and ferroelectric order parameters can lead to a magnetoelectric?ME?effect,which therefore offers new approaches to the design and application of information storage devices,sensors,spintronics and actuators.BiFeO₃?BFO?is a typical single-phase multiferroic material at room temperature and expected to be one of optimal candidates for the application because of high ferroelectric Curie temperature and high anti-ferromagnetic Néel temperature.However,several issues,including inhomogeneous spin structure,small ferroelectric polarization,high leakage current and integration with semiconductors,are still remained to solve before it can be practically applied into multiferroic devices operating at room temperature.In this thesis,we present a research of preparation of various BFO based oxide heterostructures with particular interfaces by pulsed laser epitaxy technique,and characteristics of their multiferroic and magnetoelectric coupling.In addition,ferromagnetic manganese oxide La₁-_xSr_xMnO₃?LSMO?and new type of dilute magnetic semiconductors ZnCo₂O₄?ZCO?were also investigated.The details are shown below:?1?The pure single-phase Ba and Mn doped BFO ceramics were prepared by solid state reaction combining with high-temperature sintering process.The dopping effect of Ba and Mn ions in BFO on the structure,ferroelectric and magnetic properties were explored.It is found that the Ba and Mn ions cause a distortion of BFO crystal structure,and the grain size distinctly decreases with increasing doping content.Besides,the remanent polarization and the ability to withstand voltage can be effectively adjusted.The saturation magnetization,residual magnetization and coercivity greatly increase as well.?2?The origin of ferroelectricity and ferromagnetism of the doped BFO ceramics were investigated.It is found that the enhancement of ferroelectric polarization and the decrease of the withstand voltage value should be attributed to the crystal structure distortion caused by the substitution of large ionic radius of Ba²⁺for the small ionic radius of Bi³⁺.The increased leakage current should be induced by the produce of Fe²⁺ions with the formation of oxygen vacancy.In addition,the introduction of Mn ion changes the spatial structure of lone pair electrons of Bi ions,which affects the covalent bond of Bi-O and causes the change of bond angle of Fe³⁺-O-Fe³⁺.The latter induces the spin pinning phenomenon and results in the decease of ferroelectricity.The introduction of Ba and Mn ions bring three main consequences on the magnetic property:an improvement of the exchange interaction between Fe²⁺and Fe³⁺,a tilt of FeO₆ octahedron and a change of Fe–O–Fe bond angle,which eventually enhance the BFO magnetic property.?3?Bi_0.95Ba_0.05FeO₃?BBFO1?and Bi_0.9Ba_0.1FeO₃?BBFO2?thin films were fabricated by PLD on the substrates of Si?LaAlO₃?LAO?and SrTiO₃?STO?,respectively.It is found that the films on Si substrate are polycrystalline structure,and the films on LAO and STO substrates have better quality.Such as,the epitaxial films on STO grow into single crystal.The ferroelectric polarization of the films on Si substrates increases with increasing Ba ion content.The epitaxial single crystal film shows a good ferroelectric property with rectangularity,indicating that the epitaxial growth reduces the defects and reveals its intrinsic physical property.Besides,the magnetic property of the polycrystalline film shows an isotropic behavior,whilst that of the epitaxial single crystal film is anisotropic.?4?ZCO films were fabricated by MS and PLD on the substrates of Si and ALO.The structure characterization,electric transport mechanism,origin of ferromagnetism and photo-voltage effect of the ZCO films were studied.It is found that ZCO/Si heterostructures exhibit a good rectifying,and the transport behaviors of these heterostructures can be qualitatively explained by the diffusion or recombination mechanism in the space charge region at low voltage region,and the space-charge-limited current conduction mechanism at high voltage region,respectively.The oxygen partial pressure is the key factor to determine the magnetic property of ZCO thin film.The room temperature ferromagnetism may rely on a strong ferromagnetic exchange interaction of the inerant electrons between the Co ions in ZnCo₂O₄.In addition,the photo-voltage and photo-current are proportion to laser intensity,and photo-voltage increases with decreasing temperature.Thus,photo-voltage effect mainly relies on the number of photoproduction carrier at the interface of heterostructure.?5?BBFO1/LSMO heterostructures were prepared by PLD.It is found that the BBFO thin film is epitaxially grown along[100]direction.The high remanent polarization Pr?about85?C/cm²?and quality magnetic property of the heterostructures are gained.The structure of thin film affects the type of ferroelectric domain and density of leakage,and influences the behavior of ferroelectric polarization.It is confirmed that the interface coupling is the main mechanism of the improvement of the ferroelectric and magnetic properties.?6?BBFO1/ZCO heterostructures were fabricated by PLD.It is found that each layer of the heterostructures maintains single-phase polycrystalline tissue,and the surface is smooth and dense.The transport mechanism of the heterostructures can be explained by the space-charge-limited current conduction mechanism when the applied voltage is lower than5.47V,and Schottky emission mechanism when the applied voltage is higher than 5.76V,respectively.The obvious exchange bias of the heterostructures is mainly due to the net spins caused by interface effect in about 2 nm thickness of BBFO1 thin film near the interface.The heterostructures show good photovoltaic response to the different wave-length of blue and green light.The response time of blue light is significantly faster than that of green light,and the peak value of photo-voltage is influenced by the light intensity and temperature.