Preparation And Properties Of BiFeO₃ And Its Magnetoelectric Composite Films

Multi ferromagnetic composite materials,simultaneously possessing the properties of ferroelectricity,ferromagnetism.Interaction between the ferroelectric and ferromagnetic orders will lead to the effect of magnetoelectric?ME?coupling.Which in multi ferromagnetic composite materials,electric field effect,exchange bias effect and tunneling electroresistance effect,was a very rich physical phenomenon.It includes potential applications in various functional devices such as high sensitivity magnetic sensors,polymorphic memory,transducer and so on.It is one of the fewmultiferroic materials exhibiting both ferroelectric and ferromagnetic properties at room temperature.Investigation on preparation and properties of BiFeO₃ and its magnetoelectric composite thin films,is conducive to the application of multiferromagnetic composite materials.In conclusion,the BiFeO₃ films were prepared by RF magnetron sputtering method.The Fe₄N films were prepared by DC magnetron sputtering method.The Fe₄N/BiFeO₃magnetic composite films of BiFeO₃ films with different sputtering time were prepared by combined with RF magnetron sputtering and DC magnetron sputtering method.The experimental conditions of BiFeO₃ films and Fe₄N films were prepared were analyzed by X-ray diffraction?XRD?,scanning electron microscope?SEM?and vibrating sample magnetometer?VSM?,and the effect of diferent thicknesses of BiFeO₃ layer on the exchange bias of Fe₄N/BiFeO₃ magnetoelectric composite films was discussed.The conclusions are summarized as follows:1.The effect of deposition conditions on the structural and morphology of BiFeO₃thin films.Effect of the substrate,cooling pressure,cooling atmosphere,sputtering power,sputtering pressure,substrate temperature and sputtering time on the structure,morphology and grain size of BiFeO₃ thin films was investigated.The best experimental conditions for BiFeO₃ film were found.The BiFeO₃ thin films were synthesized on single crystal STO?100?,the substrate temperature was 750?,the sputtering pressure was 2.0Pa,the sputtering power was 100W on the radio frequency target,the sputtering power was 70W on the strong magnetic target,oxygen?volume fraction was 99.99%?was used as the reaction gas,Ar gas?volume fraction was 99.99%?was used as the sputtering gas,the oxygen flow was kept in 90sccm,the Ar gas flow was kept in30sccm,the cooling atmosphere was oxygen,the cooling pressure was 4.0Pa.2.The effect of deposition conditions on the structural and magnetism performance of Fe₄N films.The Fe-N thin films were prepared by DC magnetron sputtering.The bettercrystallinity,the better soft magnetic performance and the higher saturation magnetization of the single-phase Fe₄N films were obtained when the nitrogen content to keep near 12.5%.The best experimental conditions for Fe-N film were found.The substrate temperature was 450?,the sputtering pressure was 0.8Pa,Ar/N₂=8.6sccm/60.2sccm,the sputtering power 60W,the sputtering time was 1h.3.Preparation and characterization of Fe₄N/BiFeO₃ magnetoelectric composite thin films.The Fe₄N/BiFeO₃ thin films were prepared by magnetron sputtering.When the preparation time of BiFeO₃ film was 1h,the middle layer BiFeO₃ film had a strong orientation.It was easy to induce the growth of Fe₄N along?010?.For Fe₄N thin films,the surface density was relatively attenuated and the particles grow from small to large,as the thickness of the underlying layer increases.The M-H curve indicate Fe₄N had magnetic properties.The coercive force increases first and then decreases with the increase of layer thickness.The coercive force becomes the biggest,when the preparation time of BiFeO₃ film was 1.5h.Meanwhile,In the magnetic composite film of Fe₄N/BiFeO₃,we find that there was an exchange bias.The offset first increases and then decreases with the thickness of the bottom.When the preparation time of BiFeO₃film was 1h,the offset was maximum and reached 73Oe.The phenomenon disappear of the offset,when the preparation time of BiFeO₃ film was 2h.When the anti-ferromagnetic layer was relatively thin,the magnetic hysteresis loop of ferromagnetic/anti-ferromagnetic offset to the positive direction.