Stress-regulated Mangneto-transport Properties And Ultrafast Spectroscopy Study Of Manganites Prepared By Sol-gel Technique

Perovskite manganite with the formula La_1-xA_xMnO₃?A=Sr,Ca,Ba?is a strongly correlated system with strongly coupled among charge,spin,lattice and obit.They have complex physical properties such as metal-insulator transition,ferromagnetic-paramagnetic phase transition,magnetoresistance effect,and so on.It has drawn considerable attention in recent years on theoretical research and device applications of manganite.Strongly correlation systems are sensitive to internal or external disturbances such as lattice stress,electrical field,magnetic field and light field.As the stress and interface widely exist in thin films and devices,these influences on the magnetic properties and electrical transport properties,especially magnetoresistance were firstly investigated in our study.The stress and interface were tuned by film thickness and multilayer structure.Furthermore,the dynamic processes obtained by the ultrafast spectroscopy were used to illustrate the physical mechanism of magnetoresistance effect and find the existence of non-thermodynamically stable phase in manganite at room temperature.The results we have are shown as follows:?1?The atomic flatness STO substrate with Ti-O as termination was obtained by chemical corrosion and heat treatment.The highly oriented LSMO thin film was obtained by optimizing spin coating and annealing treatment process.X-ray diffraction,atomic force microscopic morphology scan and high resolution transmission electron microscopic are used to characterize the structure properties.Taking the ultrathin?⁵ nm?thin films for example,it is epitaxial and the interface between LSMO and STO is clear and smooth.Based on this,LSMO monolayer and LSMO/LCMO multilayer films were prepared on STO?001?substrate.The samples are highly preferred orientation and have clear interface without obvious mutual diffusion.?2?The effects of stress and interface on the magnetic and electrical properties,especially the magnetoresistance,are investigated.The distortion under compressive stress in out-of-plane and dynamic Jahn-Teller distortion of MnO₆ octahedra in monolayer LSMO film will influence the ferromagnetic-paramagnetic transformation.The Curie temperature?T_C?increase as the thickness of LSMO film increases,because of the gradually relaxation of compressive stress in out-of-plane and the decrease of Jahn-Teller distortion.The metal-insulator transition temperature(T_MI)of LSMO and LCMO have a certain deviation in the multilayer film,which is mainly caused by different stress state of LSMO and LCMO single layers.The MR values of monolayer and multilayer manganese oxide films increase with magnetic field and reach the maximum values near T_MI of both LSMO and LCMO thin films.It is mainly caused by the much stronger phonon scattering and much more localized carriers forming magnetic polarons.The spin fluctuation in the system is easy to be affected near T_C,and so the maximum MR values is obtained.?3?The polaron dynamics in perovskite manganite films?LCMO,LCSMO and LSMO?were investigated using ultrafast reflectivity spectroscopy.It was founded that all the?R/R procedure in those samples can be clearly divided into three processes on the time scale:first,the polaron was released within a few hundred femtoseconds after the laser excitation;then,the isolated single polaron was regenerated through the electron-phonon interaction in less than 1ps,which is similar in three manganese oxides.Finally,from tens to hundreds picosecond,regenerated polaron formed polaron cluster through polaron-polaron interaction.Polarons in LCMO equilibrate in 10–100 ps in the temperature range of 0.6–1.4 T_C,which is much shorter than those in LCSMO?50³50 ps?and LSMO?300-850 ps?.The relaxation time constant of the last process is dominated by the ionic radius of A doping.It is mainly caused by the slight decrease of ionic radius in LCMO?⁰.99??leads to rapid increase of polaron interacting strength compared with LSMO(the ionic radius of Sr²⁺is about 1.13?),and then the time constant of polaron clusters formation was reduced and magnetoresistance increased dramatically,which is consistent with the bi-polaron theory calculation results.?4?A room temperature"hidden"state was tracked using picosecond time-resolved x-ray diffraction in La_0.7Ca_0.175Sr_0.125MnO₃ thin films near its Curie temperature of 300 K.The"hidden"state is accompanied by a unique lattice change which is different from thermodynamic lattice expansion accompanying with phonon softening from 26 GHz to 40 GHz.The"hidden"state is generated when the excitation light energy is larger than the threshold value 2 mJ/cm² and existed in less than 170 ps.Through the analysis of possible electron transition pathway,it is found that the"hidden"structure mainly originates from a photoexcited intersite charge transfer as???????,which disturbs the orbital arrangement and results in the subsequent structural reorientation.