| Normally, the multiferroic materials possess over one of the properties including ferroelaticity, ferroelectricity, ferromagnetism and magnetoelectric coupling effect. The magnetoelectric coupling effect is a new phenomenon produced by the coupling effect from ferromagnetic and ferroelectric. The multiferroic materials consist of either one single or multiple phases. Compared with the single-phase multiferroic materials, the multiple-phase multiferroic materials generate stronger magneto-electric coupling effect. Therefore, they are widely applied.BaTiO3 and BiFeO3 are two typical ferroelectric materials. BaTiO3, with a hexagonal crystal structure, is also a kind of photosensitive materials (applied in solar cells). BiFeO3, with a band gap value between 2.5 and 2.5 eV, can effectively absorb the visible light resulting in good photoelectric performance. Such special properties make BaTiO3 and BiFeO3 have the potential application in spin electronics devices, microcircuits, dynamic random access memory and sensors, and etc. In this MPhil project, both BaTiO3 (ferroelectric material) and BiFeO3 (weak ferromagnetic material) were used to prepare the multiferroic composite materials-BiFeO3 BaTiO3 granular films. Then, special properties of such composite materials, including the magnetoelectric coupling effect and the photoelectric property, were investigated as follows:(1) BiFeO3/BaTiO3 granular films and BaTiO3 thin films were grown on a n-type single Si(100) substrate through magnetron sputtering. Then, the BiFeO3/BaTiO3 granular films were annealed at 400,500 and 600℃ under vacuum conditions, respectively. However, the BaTiO3 thin films were only annealed at 600℃.(2) Phase identification and chemical compositions of the BiFeO3/BaTiO3 granular films and the BaTiO3 thin films were determined using X-ray diffraction (XRD) and (EDX), respectively. The results indicate that:(a) both granular films (BiFeO3/BaTiO3) and thin films (BaTiO3) crystallized well, (b) there was no new phase formed in either granular or thin films.(3) The current density-voltage (J-V) curves of BiFeO3/BaTiO3 granular films, measured by electrochemical analyzer (CHI600D), exhibited a diodelike rectifying behavior. This was probably attributed to the Schottky barrier generated between the BiFeO3/BaTiO3 granular film and the Ag electrode.(4) The photoconductivity of BiFeO3/BaTiO3 granular films, annealed at different temperatures, were examined under an incandescent lamp. The power density of the lamp was 20 mW/cm2. It was found that:(a) the photoconductivity value increased when increasing annealing temperature; (b) the photoconductivity effect was more obvious under the negative bias due to the Schottky barrier at the interface between the films and Ag electrode; and (c) after introducing a reverse bias voltage of 2 V, the dark current of the BiFeO3/BaTiO3 granular films annealed at 600℃ was 0.5μA, but the light current reached 39.8μA.(5) The effect of different annealing temperature on the photovoltaic response of BiFeO3 BaTiO3 granular films was investigated. The light source is the same as shown above. Experimental results indicate that the photovoltaic response of the samples increased along increase of the annealing temperature. Further, comparison of the photovoltaic response between BiFeO3, BaTiO3 and BiFeO3/BaTiO3 (annealed at 600℃) show that:the BiFe3/BaTiO3 granular films was the best and its open circuit voltage reached 245 mV.(6) The ferrelectric hysteresis loop of the BiFeO3/BaTiO3 granular films annealed at 600℃ were measured by the precision LC unit under light. Light produced an significant effect on the electricity hysteresis loop of the BiFeO3/BaTiO3 granular films. Although the coercive field was reduced, the remanent polarization increased and the optical storage efficiency reached 75%.(7) The magnetoelectric coupling effect of the samples was examined using the high-precision magnetometer, LC unit and the low temperature system. The results show that:both the coercive field and the remanent polarization electric field at room temperature were not changed when the magnetic field intensity was 0 T or 1.5 T; however, the electric hysteresis loop under magnetic field varied significantly at 200K.(8) The J-V curve (current density vs. voltage) of the BaTiO3 thin films in different light intensity were determined. Through calculation, the photovoltaic efficiency was found to reach the maximum 1.25% when the optical power density was 0.1 mW/cm2. |
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