Interface Coupling In The Transition Metal Oxide Heterostructure And Its Electric Tuning

As typical strong electron-correlated system,there exists strong competitive coupling interaction between the lattice,charge,orbital and spin degrees of freedom in transition-metal oxide,which shows a series of important physical effects and phenomena,such as the giant magnetoresistance effect,high temperature superconductivity effect,and phenomena associated with charge and orbital ordering,making it a hot field for oxide electronics.The combination of two or more different transition-metal oxides to form heterointerface leads to abundant physical phenomena,such as interlayer coupling effect,quantum size effect,interface orbit/charge reconstruction effect,which is an important way to obtain new structures and new states.However,considering the matching of crystal structure and symmetry,previous studies on complex oxide superlattice/multilayer are often focused on the combination of perovskite/perovskite interface.As a kind of derivative structure of perovskite oxide,the brownmillerite structure,which owns the superstructure composed of oxygen octahedral layer and oxygen tetrahedral layer alternately stacking,holds an atomic-ordered one-dimensional oxygen vacancy channel,making it an excellent ionic conductor,oxygen separation membrane and catalyst.Specifically,some brownmillerite oxides have antiferromagnetic order,which provides a new way for interface spin structure design.In this paper,we systematically studied the epitaxial growth of brownmillerite cobalt oxide films on substrates with different orientations and materials.We designed and prepared high quality heterostructures of perovskite manganese oxide La_2/3Sr_1/3Mn O₃(LSMO)and brownmillerite cobalt oxide Sr Co O_2.5(SCO_2.5)or perovskite cobalt oxide La_0.8Sr_0.2Co O₃(LSCO),and found the abnormal interfacial magnetic anisotropy in the heterostructures.Furthermore,based on the systematical control of the topological phase structure or ionic valence state of the cobalt oxide layer in the heterostructure using the ionic liquid gating technique,we had successfully achieved the reversible electric control of the magnetic anisotropy and interface exchange coupling of the heterostructure,demonstrating a novel and efficient"electrically controlling of the magnetic property"scheme.The main innovative achievements of this paper include:1.High quality brownmillerite SCO_2.5 films were grown on Sr Ti O₃(STO)and(La Al O₃)_0.3(Sr₂Al Ta O₆)_0.7(LSAT)substrates with(001),(110)and(111)orientations by pulsed laser deposition.By means of atomic force microscopy,X-ray diffraction and reciprocal space mapping,we systematically studied the surface morphology,structural order,and the direction of superstructure for the brownmillerite films with different lattice orientation and substrate stress.Different from the single domain structure of the vertical to film surface Co O₆-Co O₄ stacking(the superstructure)direction for the(001)-oriented films,we found that for SCO_2.5 films grown on the(110)-and(111)-oriented substrates there exists double or triple domain structure with different ordered directions.The(110)-oriented films contains the double domain structure of two kinds of superstructure along either[100]or[010]direction,while the(111)oriented films,contains the triple domain structure of three kinds of superstructure along the[100],or[010]or[001]axis.In addition,the strong structural anisotropy in the[110]and[001]directions of(110)-oriented SCO_2.5 films could lead to the corresponding electronic or ionic transport anisotropy.Specifically,we found that the structural order degree of the(110)-oriented brownmillerite SCO_2.5 films could be repeatedly switched between the ordered and disordered states under the directional migration of oxygen ions driven by the external electric field,at the same time the surface morphology,crystal structure and the anisotropic electronic transport changes correspondingly.This work shows the possibility to design and develop new functional oxide electronic devices based on the selective migration of oxygen ions in the brownmillerite materials.2.High quality SCO_2.5/LSMO brownmillerite/perovskite bilayer heterostructure had been successfully fabricated on(110)-oriented LSAT substrate(LSAT(110)),and a series of novel physical effects due to the mismatch of heterointerface symmetry had been found in the heterostructure.The magnetic easy axis for LSMO/LSAT(110)monolayer is along the[001]direction,however,it turns to[110]for the LSMO layer in the heterostructure due to existence of interface coupling effect in the brownmillerite/perovskite interface.In addition,based on the ionic-liquid gating method,the magnetic easy axis of LSMO layer shows an in-plane 90°reversibly rotating as the SCO_x film transforms repeatedly between the SCO₃ phase and the SCO_2.5phase.Careful calculation showed that such change of magnetic anisotropy during the gating process is up to 3.41×10⁵ erg/cm³.By means of X-ray linear dichroism(XLD),we showed that such change in magnetic anisotropy under the gating process comes from the change of electron preferential orbit occupation on the e_g energy-level of the Mn ion at the interface:the electron preferential occupation orbit at the brownmillerite/perovskite interface for SCO_2.5/LSMO heterostructure is d_3z²-r²,while it is d_x²-y²at the perovskite/perovskite interface for SCO₃/LSMO heterostructure.This work shows a new method to design and control the spin structure of oxide film,that is,the spin state of oxide film can be greatly controlled by adjusting the phase structure of another layer in the heterostructure without affecting the structure or chemical composition of the film.3.High-quality LSCO/LSMO perovskite/perovskite bilayer heterostructures were successfully fabricated on(001)-oriented LSAT substrate(LSAT(001)),and the anomalous strong perpendicular magnetic anisotropy caused by the charge transfer effect at the cobalt oxide/manganese oxide interface in the heterostructure was observed and confirmed,while at the same time effective electric-field control of the perpendicular magnetic anisotropy was realized.The LSMO/LSAT(001)monolayer with thickness less than 8 nm usually owns an in-plane magnetic anisotropy,however,by growing a LSCO coating layer on the top of LSMO/LSAT(001)monolayer,we successfully made the LSMO layer with same thickness as the LSMO monolayer obtain an abnormal perpendicular magnetic anisotropy.X-ray absorption spectroscopy(XAS)and XLD confirmed that there exists charge transfer effect from Mn³⁺to Co³⁺at the LSCO/LSMO interface.The charge transfer process comes from the coupling of d_3z²-r²orbital of Co and Mn ions through the 2p_z orbital of O ion,resulting in the ferromagnetic exchange coupling mode in Co²⁺-Mn⁴⁺ion-pair a lower energy bonding orbit.Upon that,the occupation state of the electron orbit at Co/Mn interface changes,making the spin orientation turn to the out-of-plane direction.Furthermore,we used the ionic-liquid gating technique to precisely control the valence state of Co ions in the upper layer LSCO film,finding that the magnetic easy axis of LSMO layer can be driven reversibly between the out-of-plane direction and the in-plane direction under positive and negative electric field,and the exchange bias effect switches repeatedly between the ferromagnetic coupling mode and the antiferromagnetic coupling mode at the same time.The XAS results clearly showed that the valence state of Co ions at the interface decreases(or increases)under the positive(or negative)electric field.The change of the valence state of Co ions will affect the charge transfer process at the LSCO/LSMO interface,which will lead to the changes in the electron orbital occupation and spin orientation of the e_g electrons for Co and Mn ions at the interface,resulting in the changes of magnetic easy axis and the exchange bias effect macroscopically.This work demonstrates the feasibility of artificially designing the interface spin structure and magnetic coupling based on the electric-field control of the oxide interface charge transfer process.