Interfacial Effects Tuned Magnetotransport Properties Of La_0.67Sr_0.33MnO₃ Films And Ni/YMnO₃ Bilayers

Multifunctional oxide films show rich physical properties due to the essential coupling of several electronic and lattice degrees of freedom.Tailoring the magnetic and electric properties of functional films via interfacial effects is one of the hot topics of condensed matter physics and material science.In this dissertation,La_0.67Sr_0.33MnO₃（LSMO）epitaxial films and Ni/YMnO₃（Ni/YMO）bilayers were fabricated by magnetron sputtering.The interfacial effects（strain,exchange bias）modified magnetotransport properties of the samples（LSMO,Ni/YMO）were investigated systematically.Structure analyses confirm the epitaxial growth of the LSMO/substrate（LaAlO₃,SrTiO₃,MgO）films.A flat and smooth interface can be observed in the LSMO/SrTiO₃ films due to the weak strain（?0.95%）.However,large lattice mismatch in the LSMO/MgO systems（?8.13%）gives rise to the structure defects near the interface.Compressive strain in the LSMO/LaAlO₃ films（1.98%）can induce lattice distortion of the MnO₆ octahedron,which will strongly localize the e_g electrons through the electron–phonon coupling and lead to the insulating behaviors.Instead,both the LSMO/SrTiO₃ and LSMO/MgO films show ferromagnetic metal properties due to the weak or released strain.The temperature dependent resistivity of the LSMO/MgO films show a resistivity upturn at low temperatures,which is ascribed to the combination of weak localization effect and electron–electron interaction.In addition,the anisotropic magnetoresistance（AMR）of the LSMO/SrTiO₃ films shows typical sin²θdependence with two–fold symmetry,and four–fold symmetric AMR can be observed in the LSMO/MgO films,which is associated with the lattice distortion enhanced magnetocrystalline anisotropy.In the Ni/YMO bilayers,obvious exchange bias（EB）effect was observed at low temperatures.The exchange coupling between the interfacial Ni moments and the net magnetic moments from the YMO antiferromagnetic domain walls is the origin of the EB phenomenon.The hysteresis loops shift towards the opposite direction of the cooling field,indicating the ferromagnetic coupling of the magnetic moments at the interface.The EB field(H_EB)decreases with the increasing temperature and nearly vanishes at blocking temperature.Besides,H_EB also decreases as the Ni layer thickness increases because the EB effect is interfacial in origin.In the exchange biased Ni/YMO bilayers,H_EB plays the role of a unidirectional pinning field and gives rise to a preferential magnetization direction.Under the influence of EB effect,the magnetization vector rotation lags behind the external fields,thus leading to the phase shift of the AMR curves.