Preparation Of La_0.7Ca_0.3Mn_1-xCo_xO₃/Ag_δ Composite Materials And Study Of Their Low-field Anisotropic Magnetoresistanc

With the rapid development of the information industry,the demand for high-precision magnetic field sensor is dramtically increasing,thus the anisotropic magnetoresistance（AMR）enhancement of magnetically sensitive materials for linear sensors has become an urgent problem.Due to the coupling among charge,lattice,spin,and orbit degrees of freedom,ABO₃ perovskite manganites exhibit unique electrical and magnetic transport behavior and magnetoresistance effects,particularly the intense AMR effects excited by external magnetic fields in different directions.These effects can be understood via spin-orbit coupling（SOC）,grain boundary tunneling scattering,double exchange（DE）interaction,Jahn-Teller（J-T）distortion,and transport property modeling.Currently,such materials exhibit poor AMR performance at low magnetic fields,and research on the anisotropic transport properties of La_0.7Ca_0.3Mn_1-xCo_xO₃ thin films is not yet systematic.Therefore,the research on enhancing AMR performance has important scientific significance and practical value.In this thesis,we first synthesized La_0.7Ca_0.3Mn_1-xCo_xO₃ polycrystalline ceramics with excellent AMR values using sol-gel technique.Subsequently,the corresponding polycrystalline films were prepared by chemical spin-coating process.The dependence of anisotropic transport properties with sintering process,substrate,and lattice distortion was investigated.Finally,the La_0.7Ca_0.3Mn_1-xCo_xO₃/Ag_δ/La_0.7Ca_0.3Mn_1-xCo_xO₃ sandwich composite film with significantly improved AMR was obtained by combining ion sputtering technology.The specific results are as follows:（1）In La_0.7Ca_0.3Mn_1-xCo_xO₃（0≤x≤0.06）polycrystalline ceramics,the peak resistivityρgradually increases with increasing Co introduction,and the metal-insulation transition temperature T_MI shifts toward lower temperatures with weaker double-exchange interactions.Refinement of the XRD results reveal that the ceramic samples with Co introduction still possess a chalcogenide structure with high crystallinity,and the calculation of the single-electron bandwidth indicates that a planar mode J-T distortion is produced.The maximum AMR value of-46.75%for this series was found at x=0.04 after applying 1 T magnetic field,which is related to the J-T distortion and the SOC effect under different directions of the magnetic field.（2）La_0.7Ca_0.3Mn_0.96Co_0.04O₃/La Al O₃ films were grown by chemical spin-coating process with varying sintering temperature.A higher sintering temperature can make the evaporation rate of the wet films,resulting in an inverse proportional relationship between the sintering temperature and the thickness of the film.At a sintering temperature of 1300℃,the high-quality La_0.7Ca_0.3Mn_0.96Co_0.04O₃ film showed a large AMR value of-70.71%.Next,different substrates were selected to prepare La_0.7Ca_0.3Mn_0.96Co_0.04O₃ films at 1300°C.The La_0.7Ca_0.3Mn_0.96Co_0.04O₃films grown on Sr Ti O₃（STO）substrates showed small lattice mismatch with STO and the smallest average roughness,good grain boundary connectivity,and the AMR performance of polycrystalline film materials was further improved to-80.07%.Then,La_0.7Ca_0.3Mn_1-xCo_xO₃（0≤x≤0.06）polycrystalline films were prepared,by adjusting the chemical pressure of the B-site to change their transport properties.La_0.7Ca_0.3Mn_1-xCo_xO₃/STO films with different Co contents were prepared at 1300°C sintering temperature.The best AMR properties were obtained at x=0.04,which is consistent with the polycrystalline ceramic results.This could potentially indicate that the anisotropy of the polycrystalline structure is the main factor dominating the transport process among this series of materials.（3）A sandwich-structure La_0.7Ca_0.3Mn_0.96Co_0.04O₃/Ag_δ/La_0.7Ca_0.3Mn_0.96Co_0.04O₃ composite film was prepared using ion sputtering technology according to the optimal process conditions and composition obtained from the thin film.The AMR of this composite film reached the maximum value-105.54%of this thesis,which is 25.47%higher than the AMR of the sample without Ag composite.This is due to the fact that the Ag composite phase with high conductivity fills the interstices of the manganese oxide film,reducing the defects while building more conductive channels at the intergranular positions,increasing the probability of carrier leap at the interface,and significantly enhancing transport properties,resulting in improved AMR performance.Meanwhile,the rapid sintering process produced Ag⁺,which formed substitutional,highly mismatched parts of Ag⁺substitutes La³⁺and Ca²⁺,forming J-T distortion with stronger anisotropy and further enhancing the SOC effect.We ultimately achieved an AMR performance of-105.54%,and the correlation between the preparation process of the films,the introduction of the Co-regulated La_0.7Ca_0.3Mn_1-xCo_xO₃（0≤x≤0.06）component at the B-site,and the oxide/metal/oxide structure construction and the anisotropic transport process was revealed.This provides an alternative material for constructing AMR linear sensing devices and provides insight into the construction of other manganese oxide/metal/manganese oxide structures,laying the foundation for this field.