The Modulated Magnetic Domain Structure And Magnetic Property In The La_0.67Sr_0.33MnO₃ Ferromagnetic Films By Epitaxial Strain Engineering

Perovskite-type manganese oxide represented by La_1-xSr_xMn O₃has extremely strong electron correlation and shows great potential in spin electronics.Its semimetallic behavior and room temperature ferromagnetism have broad application values in magnetic tunnel junctions,magnetic storage,etc.Among them,La_0.67Sr_0.33Mn O₃（LSMO）has become one of the hottest ferromagnetic materials due to its high Curie temperature and spin polarization rate.Nowadays,exploring and controlling novel nanoscale domain structures has become the focus of research,and a variety of domain types（such as stripe domains,magnetic bubble domains,skyrmions,etc.）provide more possibilities for the development of high-density,high-sensitivity,and low-power consumption memory.How to effectively control and understand the rules of domain structure transformation is a scientific problem that urgently needs to be solved.In this article,LSMO is selected as the research object,and the influence of hetero-substrate materials on the epitaxial stress of LSMO films was systematically studied.By changing the substrate material to change the size of the epitaxial stress,it was proved that compressive stress is beneficial to the increase of Curie temperature and the formation of in-plane domains,and the flexibility of the film was successfully achieved.The effect of hetero-substrate orientation on the magnetic anisotropy of the film was systematically studied,and it was proved that magnetic anisotropy can change the domain structure.This paper focuses on LSMO thin films and conducts the following work:1.The effect of epitaxial stress orientation on the magnetic properties and domain structure of LSMO thin films was studied.LSMO thin films were epitaxial grown on La Al O₃（LAO）,（La Al O₃）_0.3（Sr₂Al Ta O₆）_0.7（LSAT）and Sr Ti O₃（STO）substrates（called LAO/LSMO,LSAT/LSMO and STO/LSMO,respectively）.LAO/LSMO thin films exhibit labyrinth domain structure with small domain width and high magnetic domain density.The LSAT/LSMO film has a stripe domain structure,and the stripe width is larger than that of the film grown on LAO.However,STO/LSMO films exhibit feathery domain structure with in-plane domain characteristics,without obvious domain boundaries.We found that epitaxial stress is the main cause affecting the magnetism and magnetic domain structure of the films.As the epitaxial stress changes from compressive to tensile,the domain density gradually decreases,and the domain width increases.By controlling the in-plane magnetic field,it was found that the saturated field and coercive field of LAO/LSMO films with stronger compressive stress are larger,and the magnetic domain structure is more stable.The flexible LSMO film can still maintain a Curie temperature higher than room temperature and ferromagnetism,but did not show obvious out-of-plane domain structure.2.Researched the influence of stress direction on the magnetic properties and domain structure of LSMO films.LSMO films were grown on（100）,（110）and（111）oriented（La Al O₃）_0.3（Sr₂Al Ta O₆）_0.7（LSAT）substrates（called LSMO（100）,LSMO（110）and LSMO（111）respectively）.The magnetic anisotropy exhibited by thin films varies with the direction of force applied.The（100）-and LSMO（110）films show stripe domain patterns,and the magnetic anisotropy of the films shows weak uniaxial anisotropy and standard uniaxial anisotropy,respectively.After the in-plane magnetic field is saturated,the direction of the fringe domain in LSMO（100）does not rotate with the change of the magnetic field,while the fringe domain in LSMO（110）rotates continuously.LSMO（111）thin films produce randomly distributed bubble-like magnetic domains that exhibit isotropy on the plane composed of[101]and[1 2 1].With the increase of the applied magnetic field,these domains are gradually connected into bands.Under the effect of the magnetic field,the domain stability of LSMO（111）films is better than that of（100）-and LSMO（110）films.In addition,we found that with the continuous increase of thickness,the epitaxial stress was gradually released,that the random domain configuration was gradually changed into the flux closure domain configuration with clear maze-like stripe domains.These phenomena can be understood by the regulable strain anisotropy and the magnetic anisotropy.This study suggests an effective way to modify the magnetization anisotropy and magnetic domain thus spin polarization depends strongly on the crystal orientation as well as epitaxial strain.