Study On The Structure And Properties Of Several Double Perovskite Oxide Films/Superlattices

In recent years,double perovskite oxide has received extensive research and attention in the field of multiferroic functional materials due to its flexible lattice structure and the coupling between various ordered parameters.This dissertation takes La₂NiMnO₆,La₂CoMnO₆ and Sm₂NiMnO₆ as basic research objects and optimizes the growth process of epitaxial film,finally designs the La₂NiMnO₆/La₂CoMnO₆ superlattice film and systematically studies the film in terms of growth preparation,phase structure and physical properties,the following conclusions are obtained.The ceramic targets of La₂NiMnO₆,La₂CoMnO₆,Sm₂NiMnO₆ with double perovskite structure were prepared by traditional solid-phase sintering,and the crystals were determined to be monoclinic(P2₁/n space group)through XRD structure refinement.The results of PPMS show that the three materials are all ferromagnetic,and their internal exchange effects are discussed separately.In this work,the etching process of the STO substrate is optimized,and a clearer and continuous atomic terrace has been obtained.High-quality double perovskite epitaxial films(La₂NiMnO₆,La₂CoMnO₆,and Sm₂NiMnO₆)were prepared by L-MBE technology,and the S-K growth mode and the precisely control of the number of layers was investigated by the in-situ RHEED monitoring system.HR-XRD and AFM test were used to assist the characterization of the microstructure of the epitaxial films.The magnetic results show that the three epitaxial films are all good ferromagnets,and the magnetism comes from the super-exchange between B-site ions and Mn⁴⁺ions,and their respective ferromagnetic Curie points are confirmed.The La₂NiMnO₆/La₂CoMnO₆ superlattice film was designed and prepared using L-MBE technology.The additional diffraction patterns of RHEED proved the film with a double perovskite structure.Combined with the analysis for the changes of the number on laser pulses and oscillation intensity,the critical thickness was estimated due to the clamping force of the substrate during the lattice growth.The good epitaxial characteristics of film growth was further confirmed by the testing of AFM and XRD.In the results of TEM,the film thickness is 46.3nm and the lattice distortion caused by stress release is observed at 2.7nm,which is basically consistent with the analysis of RHEED.The superlattice film basically retains the magnetic properties of the parent material.Its ZFC curve has two Curie temperature points,but an abnormal peak appears at 55K and a weak ferromagnetism is at room temperature,which is due to the distortion of the oxygen octahedron in double perovskite superlattice,changing the original ferromagnetic exchange effect.A weak ferroelectricity was clearly confirmed by the P-E hysteresis loop and the test of the voltage curve and writing domain by PFM.The origin of the ferroelectricity is attributed to the change of the magnetic order in the crystal lattice.The distortion of the oxygen octahedron may induce a magnetic frustration system.A large magnetic coupling effect was obtained at room temperature through the magnetoelectric system.