Studies On Domain Distributions And Dynamic Magnetoelectronic Coupling In BiFeO₃ Ferroelectric By Femtosecond Laser

Ferroelectric is a functional material,which possesses excellent electrical and optical properties and has a broad application prospect.As the domain switching is the microscopic physical mechanism of macroscopic nonlinear constitutive behavior in ferroelectrics,the orientation of domain directly determines the physical properties and applications of ferroelectric materials.The changes of volume fractions of different domain orientations after switching are also related to the efficiency and stability of ferroelectric materials and their devices.Therefore,determinations of domain orientation and its volume fraction(domain distribution)are essential for ferroelectric device engineering.Multiferroic,as a special class of ferroelectrics,not only has ferroelectricity,but also has ferromagnetism and other ferroelectric order parameters.The magnetoelectric coupling between these ferromagnetic order parameters in multiferroic provides new degree of freedom for the design of high-performance electronic devices.As a new excitation type of multiferroic,the electromagnon is considered to be the unique product of dynamic magnetoelectric coupling,which shows the character of a magnon giving rise to electric polarization' oscillations under the field of light.The electromagnon contains rich and interesting physical phenomena,and studying the physical properties and mechanisms of electromagnon are crucial for tracing the origin of dynamic magnetoelectric coupling in multiferroic.Femtosecond laser,showing the characteristics of ultra-short pulse,high peak power,high electric field strength and wide spectrum,is one of the most powerful new tools after development of laser science in recent years.The femtosecond laser detection as an all-optical detection method also has unique advantages of non-contact,non-destruction,high efficiency and high sensitivity,which can be directly used to study the physical properties and micro-mechanism of materials.Here,this dissertation aims at establishing detection method to realize the characterization of ferroelectric domain distribution and dynamic magnetoelectric coupling by femtosecond laser,the main research contents and results are as follows:(1)Based on strong correlation between second harmonic generation(SHG)and material symmetry and the unique advantages of femtosecond laser,an azimuth-polarization-dependent second harmonic generation(APD SHG)system is established,and the corresponding APD SHG theoretical model is also constructed.By combining the theoretical simulation and experimental results,an APD SHG method for charactering ferroelectric domain distribution is established.(2)Using the above method,the relationships between phase structure,domain structure,domain population and the APD SHG signals are studied in the typical ferroelectric thin films including rhombohedral phase(R-phase)71° domain wall BiFeO3(BFO),R-phase 109° domain wall BFO,tetragonal phase(T-phase)Pb(Zr0.2Ti0.8)O3,T-phase BaTiO3(BTO)and T-phase BFO.Then,the relations of domain distributions and SHG signals in the typical ferroelectric thin films are also established.By fitting the experimental APD SHG data with the established theoretical model,the domain distributions of the ferroelectric thin films are determined,and the volume fractions of these domain distribution are obtained.As the results of domain distributions obtained by APD SHG in the ferroelectric films are consistent with the piezoelectric force microscope photographs,the reliability of the APD SHG system are finally determined.Therefore,the domain distributions of ferroelectrics can be effectively determined by APD SHG system.(3)Taking the BTO films grown on miscut substrates with different angles as examples,the relationships between the surface-step-terrace(SST)induced strains from miscut substrates and second-order nonlinear optical coefficient are studied.The results show that different miscut angles of the substrates can bring about different SST strain states in the BTO films,which make different contributions on the second-ordernonlinear optical coefficients.Therefore,the second-order nonlinear optical coefficients can be effectively controlled by SST induced strains.(4)Based on terahertz time domain spectroscopy system,a femtosecond laser detection method for dynamic magnetoelectronic coupling effect(electromagnetic oscillator)of multiferroic nanoparticles materials is established.Taking the Nd-doped BFO(BNFO)nanoparticles as examples,the physical mechanism of electromagnon in BNFO nanoparticles is studied,and the impacts of doping induced phase transition in BNFO nanoparticles on electromagnon are discussed.The structure transformations of BNFO nanoparticles are further characterized by Raman and ferromagnetism properties.The mechanism of phase transition mediated electromagnon in BNFO nanoparticles is analyzed.The electromagnon of BNFO nanoparticles can be effectively regulated by phase changes.