The Regulation Of Oxygen Content And Stress On The Magnetoelectric Properties Of LaMnO_3+δ Films And Its Mechanism

In recent decades,the research of perovskite-structured manganese oxides has greatly promoted the development of spintronics materials and devices.With the continuous progress of social information and the increasing demand for multi-functionalization of materials and devices,it is more urgent to control the magnetoelectric properties of perovskite-structured manganese oxides.Because this system is a highly coupled system with multiple degrees of freedom such as spin,charge,orbit and lattice,the controllable control of magnetoelectric properties and the study of related physical mechanism of manganese oxide becomes more complicated.To realize the regulation of physical properties of manganese oxides and to understand the corresponding regulation mechanism has become a key scientific problem to promote the multifunctional application of oxide spintronics.In this paper,LaMnO₃（LMO）,the parent phase material of manganese oxide,was selected as the research object to systematically study the variation of the electromagnetic properties of LMO thin films under different annealing states,and the regulation mechanism of oxygen content on the magnetoelectric properties of LMO films based on the phase separation model was proposed;The magnetoelectric properties of LMO films under different Sr₃Al₂O₆（SAO）buffer layer thickness were studied systematically,and the regulating mechanism of oxygen migration and strain on the magnetoelectric properties of LMO films was proposed.This paper focuses on LMO and carries out the following work:1.The effects of annealing oxygen pressure on magnetic and electrical transport properties of LMO films were studied.At 5×10^-5 Pa,30 Pa and 5×10⁴Pa annealing oxygen pressures,LMO films exhibit three different magnetic ground states of antiferromagnetic-insulating,ferromagnetic-insulating and ferromagnetic-metal,respectively,at low temperature,and the samples anneal at ultra-low oxygen pressure(5×10^-5 Pa)exhibit exchange bias as high as-979 Oe（5K,+4 k Oe FC）,the magnetic resistance of high oxygen pressure annealing samples was significantly enhanced.It is found that the competition between ferromagnetic-metallic phase and antiferromagnetic-insulating phase induced by oxygen content changes in LMO film is the main reason for regulating its magnetic and electrical transport properties.By repeatedly annealing the 5×10^-5 Pa annealed sample at 30Pa and 5×10⁴ Pa oxygen pressures and comparing LMO films grown on La Al O₃ and Sr Ti O₃substrates,we found that the regulation of oxygen content on the magnetoelectric properties of LMO films is reversible,and the regulation law is independent of the compression and tension strain states introduced by the substrate.2.The effects of buffer layer SAO thickness(t _SAO)on crystal structure magnetism and electric transport properties of LMO thin films were studied.When t _SAO=0 nm,LMO film is subjected to the in-plane compression stress and exhibits ferromagnetic and insulating properties.When t _SAO=2 nm,LMO film is in stress relaxation state and still exhibits ferromagnetic and insulating properties,but Curie temperature（T_c）and conductivity are lower than those of t _SAO=0 nm.At t _SAO≥3 nm,LMO film is subjected to in-plane compression stress,and exhibits ferromagnetic-metallic properties and two ferromagnetic phases coexist.With the increase of t _SAO,the T_c of the two ferromagnetic phases and room-temperature magnetoresistance gradually increase.At t _SAO=20 nm,the T_c of the high-temperature ferromagnetic phase is up to 342 K,which is much higher than the reported LMO film,and the room-temperature magnetoresistance is better than la_0.7Sr_0.3Mn O₃ film.We believe that the oxygen ion migration induced by the strong oxygen affinity of Al and the stress changes induced by t _SAO changes are the main reasons for regulating the magnetic and electrical transport properties of LMO films.The high temperature ferromagnetism may be caused by the in-plane orbital occupation of e_g electron and the change of bond length and bond angle of Mn-O octahedron under tensile state.