The Study Of Magnetism And Orbital Tuning In La_0.67Sr_0.33MnO₃ Films And Related Magnetoresistance Effect In Heterojunctions

Manganese oxides produce a lot of novel physical phenomena because of thecoupling of the four freedom degrees,i.e.lattice,orbit,charge and spin,which has great potential application in the field of spintronics.Among the perovskite oxides,La_0.67Sr_0.33MnO₃（LSMO）possessing excellent properties,such as semimetal property,high Curie temperature（Tc=369K）and Colossal magnetoresistance effect（CMR）,has become the outstanding one in the numerous materials.It is worth noting that as the thickness of the film decreases,the Curie temperature of the LSMO also decreases,which is very harmful to the practical application.We study the magnetism of the film,Curie temperature and perpendicular magnetic anisotropy through tuning orbital induced by stress.In addition,we have grown LSMO/SrTiO₃/LSMO heterojunction structure to study the tunneling anisotropic magnetoresistance（TAMR）effect.The main research contents include:（1）[（LSMO）₃/（BaTiO₃）₃]_n superlattice films were grown epitaxially on SrTiO₃（001）substrates with different cycles.The relationship between high Curie temperature and orbital occupancy in LSMO film were studied using the characterization of XAS and XLD measurements.We find that regular intercalation of BaTiO₃ layers between LSMO layers effectively enhances ferromagnetic order and increases the Curie temperature above room temperature in LSMO/BaTiO₃ superlattices.This maybe originates from preferential orbital occupancy of e_g（x²-y²）in LSMO layers induced by the tensile strain from BaTiO₃ layers.The in-plane orbital occupancy is beneficial to the double change ferromagnetic coupling of film samples.（2）LSMO films with perpendicular magnetic anisotropy were grown on LaAlO₃（001）substrates by PLD to further study the intrinsic connection between the effect of orbital ordering and magnetic properties of LSMO films.The mismatch between LaAlO₃ and LSMO results in compressive stress for LSMO films along in-plane direction,which induces the tendency of perpendicular magnetic anisotropy in LSMO films.We also study the relationship between the orbital ordering and magnetic anisotropy by XAS and XLD.Our results reveal that the relationship between the preferential orbital occupancy of e_g（3z²-r²）and perpendicular magnetic anisotropy in LSMO layers is related to the larger proportion of e_g（3z²-r²）orbital beneficial to perpendicular magnetic anisotropy.（3）We also designed and prepared LSMO/SrTiO₃/LSMO magnetic tunneling junctions on SrTiO₃（001）substrates terminated with TiO₂ by PLD.The magnetotransport properties of LSMO/SrTiO₃/LSMO junctions have been analyzed as a function of temperature and magnetic field.The junction exhibit magnetoresistance value of about 35%at low temperature（4K）.The TAMR value of around 4%are found at low temperature.This may be related to the charge mismatch at the LSMO/SrTiO₃ interface and spin-orbital coupling at the interface between LSMO/SrTiO₃ layers.