Structure And Physical Properties Of Perovskite-type Manganite Thin Films By Polymer Assisted Deposition Method

Manganese oxides based on perovskite type structure have shown rich properties including electrical,magnetic,ferroelectric,and magnetodielectric effect due to the subtle interplays between spin,lattice,orbital,and charge.Besides of their application value,the physical mechanism behind the fantastic performances also attracted the attention of many researchers.In this paper,the influence of ionic radius of R element and bixial epitaxial strain on the structure and properties of R₂Ni/CoMnO₆ films are investigated.The related studies can help us to understand the internal interaction of such strongly correlated system.In addition,the perovskite manganese oxides also received great attention in the field of electrochemistry,owing to their high catalytic activities,good durability,structural and chemical flexibility by manipulating compositions,and economic impact on the oxide materials.Similar to those of many oxides,the electronic transport property of perovskite manganese oxides is poor,which hinders their practical application as an electrocatalyst.In this paper,the conductivity and electronic state of LaMnO₃ are studied aiming to develop a simple and effective modulation strategy to tune the catalytic activities of LaMnO₃ and the related perovskite catalyst.The detailed researchs include:In chapter one,the basic concepts of perovskite are introduced,including crystal structure,electronic structure,and internal interaction etc.Next,we also introduce the physical and chemical properties and research progress of perovskite manganese oxide.Finally,the hot and difficult problem existing in this kind of material has been summarized.In chapter two,we have fabricated R₂CoMnO₆（RNMO,R= La,Pr,Nd,Sm,Y）thin films on（001）LaAlO₃（LAO）substrates via a polymer assisted deposition（PAD）method.The structural and morphological characteristics show that the RNMO films were epitaxially grown on（001）LAO substrates successfully.It is shown that,with the lanthanide ionic radius decreasing,the positions of Raman mode shift toward the low frequency and the Curie temperature Tc decreases.The results indicate that the intensities of Raman peaks and magnetic interaction are weaking with lanthanide ionic radius decreasing.It is suggested that the A-site ionic radius plays an important role in the structure and magnetic property of RNMO.Moreover,polarized Raman spectra and magnetic analyses show that ordered P21/n or R3（R3m）phase and disorder Pbnm phase coexist in all the RNMO films,which lead to the appearance of two magnetic transition temperatures.All the Tc values of the RNMO films are lower than those corresponding values of the RNMO bulks,indicating the ferromagnetic interaction weakening.It is suggested that the ferromagnetic interaction is affected by the strain existing in the film through affecting the B（B’）06 octahedra and the cation disorder related to the ionic radius of Re,which can be used as an effective way to improve themagnetic properties of the film and obtain the higher Tc functional materials.In chapter three,the R₂CoMnO₆（RCMO,R=Pr,Nd,Sm）films on（001）LAO substrates were fabricated by a simple polymer assisted deposition method.XRD results reveal that all films are a single phase with a highly preferential c-axis orientation.The study of the magnetic properties indicates that all the films are B-site ordering and show a single FM transition at Curie temperature（Tc）with decreasing temperature.Meanwhile,an anomalous phonon softening closely related to the spin-phonon coupling is observed at around Tc in the temperature-dependent Raman spectra.With the decreases of R-site ionic radius（rR）and/or the in-plane biaxial compressive strain,both the turning point TR of the temperature dependent △ω（T）and Curie temperature Tc decrease,revealing the existence of spin-phonon coupling and its intensity decreasing in the order of Pr2CoMnO6>Nd2CoMnO6>Sm2CoMnO6 accompanied with the Co-O-Mn bond angle decreasing and the Co（Mn）-O bond length elongating.It is suggested that the existence of the biaxial compressive strain may be helpful to form the B-site ordered phase and suppress the formation of the disordered phase by decreasing the oxygen vacancies.Both the chemical strain and the biaxial compressive strain are cooperating on the magnetic interaction and spin-phonon coupling in the system,which can be an effective way to adjust the internal interaction of spin-spin and spin-lattice in film.In chapter four,the double perovskite Y₂NiMnO₆（YNMO）epitaxial thin films with different strain states induced by substrates and film thickness have been obtained by a simple PAD method.The X-ray diffraction and Raman scattering observations indicate that the YNMO films were obtained successfully.All films show ferromagnetic behavior,and the Tc of the films is lower than that of the YNMO bulk due to the strain in films.The Tc increases as film thickness increasing,which is ascribed to the reducing of the biaxial tensile strain in film.Besides,the magnetic properties of YNMO films show a dependence on substrate type,which can be associated with the difference in strain relaxation and substrate effect.All the above results suggest that the magnetic properties of the film can be adjusted by the strain state in film,which can be controlled by film thickness and substrate type.In chapter five,we have fabricated LaMnO₃（LMO）thin films on SrTiO₃（STO）and LAO substrates by a simple PAD method.All results reveal that the annealing condition and orientation of substrate have remarkable effect on the structure and properties of LMO film.With the annealing temperature increasing,the resistivity of the film decreases and Tim shifts to a higher temperature.The maximum resistivity is highly orientation dependence with the resistivity in ascending order of（100）<（110）<（111）.Furthermore,the TIM of LMO film increases with the substrate orientation changing from（111）to（100）.The lattice distortion can be tuned by different annealing condition and the orientation of the substrate,which can be effective ways to adjust the electrical and magnetic property of manganites.