Preparation And Electromagnetic Properties Of BNT Based Ferroelectric Films And Multiferroic Composite Films

With the development of electronic devices towards integration,miniaturization,intelligence and versatility,higher requirements have been put forward for the quality and performance of thin film capacitors.Ferroelectric films,a significant functional material,present a broad application prospect in MEMS（such as actuators,sensors and multistate storage devices）and energy storage owing to their excellent electrical properties（dielectric,ferroelectric and piezoelectric）.Meanwhile,ferroelectrics can be combined with magnets in nanoscale to form multiferroic composite films,which establishes conditions for the multifunctional electronic devices and is a frontier research direction in the current condensed matter physics and field of multifunctional devices.Bi_0.5Na_0.5TiO₃（BNT）based films are typical A-site complex perovskite ferroelectric films,which possess excellent ferroelectric and piezoelectric properties due to the lone pair electrons of 6s² in the electronic configuration of Bi³⁺.Moreover,BNT is easy to composite with other perovskite-structure ferromagnetic materials to form multiferroic composite films,thus expanding its application field.In this dissertation,the pulsed laser deposition（PLD）technology was used to prepare film samples,and(Bi_0.5Na_0.5)TiO₃（BNT）-Ba TiO₃（BT）system consisting of(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃（BNT-6BT）films near the morphotropic phase boundary（MPB）was used as the object of study.A series of researches and investigations have been carried out on the preparation of high quality BNT-6BT ferroelectric films and the reduction of their leakage currents and losses.From the microscopic point of view,the microstructure of BNT-6BT film was adjusted by controlling internal defects and strain mismatch of the film.High-quality columnar crystal films were grown and their polarization and leakage behavior of the films were optimized.From the macroscopic point of view,layered multiferroic composite films were constructed by introducing LSMO layer.The leakage and residual polarization of the films were reduced by the cooperation between imprint effect and interfacial effect.The ferroelectric polarization and the energy storage performance of the films were improved simultaneously.And the enhancement mechanism of the energy storage behavior was revealed.Meanwhile,the aim of enhanced room-temperature electrical and magnetic properties of the composite films were reached by adjusting grain size of BNT-6BT film.（1）The polarization magnitudes of BNT-6BT film were enhanced along with reduced leakage and loss by designing the annealing atmosphere after film preparation.The results show that point defects in annealed films led to local structural distortion,thereby affecting their electrical properties.The cation vacancy and oxygen vacancy of the films were modulated by annealing under vacuum and different atmospheres（O₂,Air,N₂）,thus optimizing their electrical properties.The formation of oxygen vacancies and cation vacancies was inhibited and concentration of oxygen vacancies was effectively reduced due to the presence of both oxidizing and reducing properties of N₂ at high temperature.The film sample annealed in N₂ possess a highε_r of 505,a dielectric tunability of 22.3%,a relative high P_max of 26.24μC/cm²,a lower tanδof 0.034 and P_r of 4.87μC/cm²,indicating that the electrical properties of the film are enhanced.（2）High quality columnar crystal BNT-6BT films were prepared on Si-based substrate by controlling strain of films.The strain mismatch was regulated by depositing BNT-6BT films with different thicknesses and the critical thickness for stress relaxation of the thin film was 450 nm.When the thickness was above 450 nm,BNT-6BT films grew highly uniform columnar crystals.Theε_r-E curve of the aged films showed a double butterfly curve under the adjustment of defect polarization of P_D,and the tunability ofε_r increased with the increase of film thickness,reaching the maximum value of 36.7%at 950 nm.Meanwhile,the highestε_r of 572,the minimum tanδof 0.032 and the maximum P_max of25.19μC/cm² were obtained in 950 nm film with a larger grain size.The results showed that the columnar crystal reduced the leakage current and loss,enhanced the polarization magnitudes and response ability of dielectric constant to external fields.（3）The design concept of enhancing ferroelectric properties and energy storage characteristics by using imprint effect was innovatively proposed.By introducing LSMO layer,a layered BNT-6BT/LSMO（t）multiferroic composite film was constructed.The interface"dead layer"was formed at the film-electrode interface in the multilayer structure,which created conditions for reducing the residual polarization and leakage current.The leakage current and residual polarization were significantly reduced via the imprint effect,which greatly enhanced the energy storage characteristics of the film.The best energy storage characteristics(W_rec=4.05 J/cm³,η=69.8%),as well as good frequency stability（500-10k Hz）and fatigue resistance（10⁸ cycles）were obtained in BNT-6BT/LSMO（400nm）composite films with the application electric field of 400 k V/cm.The results indicate that the low field energy storage characteristics of ferroelectric materials can be effectively improved by composite film.（4）The excellent room-temperature electrical and magnetic properties were obtained byadjusting grain size and interface effect of multiferroic composite films.Based on the above work,the structure,electrical and magnetic properties of BNT-6BT/LSMO composite films were optimized by adjusting the grain size with different deposition temperatures.With the increase of grain size of BNT-6BT films,the crystallinity and the oxygen vacancy concentration of the film increases slightly.The composite film with a grain size of 106.0 nm of BNT-6BT layer presented the maximumε_r of 568,the minimum tanδof 0.08,and the maximum P_max of 35.27μC/cm².Under the adjustment of interface effects,the magnetization of the composite film was much greater than that of the single LSMO film in the investigated temperature range.A maximum M_s of 239 emu/cm³ was obtained at room temperature and Curie temperature T_c increased 43K up to 356 K.The good room-temperature electromagnetic properties of this composite film indicate its potential application in multifunctional devices.