Research On Characteristics And Device Application Of BaTiO₃ Thin Films Under External Field Control

In recent years,there is an increasing attention in ferroelectric and ferromagnetic materials because of their excellent physical properties and wide application prospects in non-volatile memory,pyroelectric devices and microwave voltage control devices.In particular,people are more interested in the preparation and control of ferroelectric materials.However,ferroelectric thin films are usually grown on a rigid substrate,which causes the thin film to be clamped by a rigid substrate,thereby impairing the film properties.Therefore,a high-quality free-standing film should be produced to overcome this obstacle,which can provide more freedom to change and optimize the ferroelectric behavior.Besides,strains usually can be imparted into thin films through differences in lattice parameters and thermal expansion behavior between the film and the underlying substrate,or they can arise from defects formed during film deposition.However,the strain introduced by these conventional ways is biaxial strain,thus the magnitude of the strain that can be applied is relatively small.More importantly,there is a lack of continuous tenability to change the strain state.These limitations will weak our ability to manipulate the properties and enhance performance in ferroelectrics.In this paper,the free-standing ferroelectric?ferromagnetic?film can be transferred to the flexible substrate by etching the sacrificial layer,which can be freed from the limitation of the substrate clamping.This can not only achieve mechanical strain by bending,but also can achieve the regulation by a magnetic field.A series of studies have been carried out in the paper,including the preparation of BaTiO₃monolayer and BaTiO₃-CoFe₂O₄ bilayer heterojunction,substrate transfer process and device preparation and testing.The following is the research content in detail:First,high quality single crystal BaTiO₃ and BaTiO₃-CoFe₂O₄ were prepared by pulsed laser deposition system,which was achieved by optimizing the growth parameters of BaTiO₃ and BaTiO₃-CoFe₂O₄ thin films in the SrTiO₃ substrate of?001?crystal orientation.Second,the free-standing BaTiO₃ and BaTiO₃-CoFe₂O₄ films were obtained by removing the SrTiO₃ substrate,which use the substrate transferring method.The free-standing films still maintain completeness and high epitaxial single crystal properties after separating from the substrate,providing the material foundation for the preparation of flexible devices.Third,the BaTiO₃ film was transferred to a flexible substrate to prepare a flexible device of MSM structure.The test showed that the free-standing film retained good ferroelectric properties.Besides,the different mechanical uniaxial strain is introduced by bending the substrate,which can achieve the continuous regulation of the polarization characteristics of the BaTiO₃ film.Fourth,the free-standing BaTiO₃-CoFe₂O₄ film was prepared to a device of MSM structure,and its magnetoelectric coupling characteristics controlled by the magnetic field were investigated.Compared with the as-grown film,the free-standing film exhibited better piezoelectric characteristics and enhanced magnetoelectric coupling effect in a magnetic field environment due to the elimination of the rigid substrate,which can be used to prepare for high sensitive magnetic field detectors.