The Study And Control Of Magnetic And Electric Properties In Manganite-based Heterostructures

In the last decade,it has been reported that transition metal oxides exhibit broad applications in the fields of security,medical care,energy and so on,which is attributed to an incredible variety of fascinating functionalities,including metal-insulation transition,ferroelectricity and superconductivity.When two dissimilar oxides are grouped together,interface exchange coupling would break the delicate balance between different degrees of freedom.Therefore,oxides heterostructures would display unexpected functionalities,which play an important role for the development of functional oxides devices.In light of the rapid development of films growth technology,atomic-scale precise control of interfaces in oxides heterostructure has become feasible and provides an ideal platform for the study of interface properties.Manganites,as a member of the strongly correlated transition metal oxides,are considered the most potential materials for the next-generation spintronic devices all the time owing to their colossal magnetoresistance effect,nearly 100%spin polarization and high Curie temperature.So,we focus on the manganites as the research object.And the influence of orbital reconstruction and polar mismatch on the properties of the heterostructure is systematically studied by mean of the artificial design of the oxide heterojunction.Further,the reversible and nonvolatile manipulation of interface properties is achieved utilizing the electrical-field.This work would provide a new insight for the application of the manganites-based electric devise.The specific contents and results are as follows:（1）The superlattice consisted of A-type antiferromagnetic La Mn O₃ and G-type Sr Mn O₃was deposited on the STO substrate.The results of magnetic measurement indicates that the superlattices display exchange bias effect and vertical magnetization shift with intermediate periods.And with the vary of films thickness,the superlattices show the metal-insulator transition.X-ray linear dichroism measurement detects the orbital reconstruction.Combined all of the experiment results to detailed analyze the spin arrangement in the superlattice,it is conclusion that the novel properties in the superlattices can attribute to the competition between interface ferromagnetism and canted antiferromagnetism.Later,the magnetic anisotropy of La_0.7Sr_0.3Mn O₃/Sr Mn O₃heterostructure was investigated by replacing the A-type La Mn O₃ to the ferromagnetic La_0.7Sr_0.3Mn O₃.It is unexpected that the magnetic easy axis of superlattices gradually switches from in-pane to out-of-plane with the change of Sr Mn O₃ thickness.The synchrotron radiation measurement and first-principles calculation reveal that the dependence of the magnetic anisotropy and thickness in the superlattice is related to the orbital reconstruction.This work provides a useful route for the artificial design of functional oxide heterostrucute.（2）Ferromagnetism is detected in the non-magnetic La Ni O₃/Sr Mn O₃superlattice because of the charge rearrangement induced by the interface polar mismatch.Magnetic and electric measurement results approve that interface magnetism can only be observed in the thicker superlattice.And the results of the X-ray photoelectron energy spectroscopy and X-ray absorption spectroscopy show a significant dependence between the interface charge transfer and thickness of the superlattice.With the increase of La Ni O₃ thickness,the super-exchange interplay of Ni²⁺-Mn⁴⁺decrease and double-exchange interaction of Mn³⁺–Mn⁴⁺increases.Furthermore,X-ray magnetic circular dichroism signals of Mn and Ni ions is opposite,which means the antiferromagnetic coupling between La Ni O₃ and Sr Mn O₃ layer.Therefore,it is the reason why previous researchers could not observe ferromagnetic properties in the same superlattice.Polar mismatch not only can induce charge rearrangement in interface but also cause the structural transformation in some materials.Based on it,we design the Sr Cu O2/La_0.7Sr_0.3Mn O₃heterostructure,in which the structure of Sr Cu O₂ would vary from infinite layer to chain-type with the decrease of films in order to eliminate electrostatic potential induced by polar discontinuous.Correspondingly,the magnetic properties of superlattices have a nonlinear dependence on the thickness of the Sr Cu O2 films.And combined results of the synchrotron radiation measurement,we do explain the unusual interface coupling effect of superlattice.This work proves the importance of polar mismatch on the manipulating interface properties of systems,and provides a new route for the research on the oxides heterostructure.（3）Through optimizing the experimental conditions,the Sr Co O_3-x/La_0.7Sr_0.3Mn O₃（0<x≤0.5）bilayer were fabricated on the tensile Sr Ti O₃ substrate.For the bilayer composed of hard ferromagnetic Sr Co O_3-x（0<x≤0.25）and soft ferromagnetic La_0.7Sr_0.3Mn O₃,the exchange spring effect can be detected.And the exchange bias effect is observed in the bilayer consisted with antiferromagnetic Sr Co O_2.5 and ferromagnetic La_0.7Sr_0.3Mn O₃.Moreover,when applying the voltage on the sample surface by inion liquid,the topmost cobalt oxide can reversible transformation between hard ferromagnetic Sr Co O_3-xand antiferromagnetic Sr Co O_2.5by mean of the oxygen migration.Consequently,the electric field can reversibly and nonvolatile manipulate the exchange coupling effect（exchange spring and exchange bias effect）of bilayer.In addition,it is interesting that Sr Co O_2.5films shows brownmillerite structure.So,these fascinating properties would emerge if the perovskite La_0.7Sr_0.3Mn O₃ and brownmillerite Sr Co O_2.5（B-SCO）films group together owing to the interface asymmetry.The RHEED was used to in situ monitor the growth process of films in order to achieve atomic-level flat interface in the bilayer.In contrast to the single La_0.7Sr_0.3Mn O₃ films with in-plan magnetic easy axis,the B-SCO/La_0.7Sr_0.3Mn O₃ bilayer displays abnormal perpendicular magnetic anisotropy.The first-principles calculation and X-ray linear dichroism reveal that electrons prefer occupation of the out-of-plane 3z²-r² orbital in bilayer.Then,the reversible topotactic phase transformations of top B-SCO layer can be realized utilizing the bias voltage.Therefore,electric-field-control perpendicular magnetic anisotropy can be achieved in B-SCO/La_0.7Sr_0.3Mn O₃ bilayer.This work provides the foundation for further electric-field control of magnetic properties.