First-principles Studies Of Electronic Reconstruction At The Heterointerface Of Manganites

Ever since the discovery of high temperature cuprates and colossal magnetoresistivemanganites, extensive investigations have been made on the transition metal oxides. Withthe growing growth of technique for material preparation, people are quite eager to designnew oxide devices by combining various transition metal oxides. Oxide interfaces havericher physics than semiconductor interfaces because of possible lattice reconstruction,charge reconstruction, spin reconstruction as well as orbital reconstruction. In many cases,the properties of these interfaces turn out to be much richer than those of their bulkconstituents. Recently, LaAlO3/SrTiO3interfaces have been studied extensively andhigh-mobility quasi two-dimensional electron gas or even superconductivity emerged dueto charge transfer. Comparing with Ti-based oxide interfaces, Mn-based oxide interfaces,which is the focus in our studies, may have richer physical phenomena due to spin degreeof freedom of Mn cation and therefore spin-polarized two dimensional electron gas can beexpected. In this thesis, based on extensive first-principle calculations, the magneticreconstructions of the SrMnO3(001) and (111) surface and heterointerface were studiedsystematically. Highly spin-polarized two dimensional electron gas was realized atSrMnO3-based heterointerfaces. The main results are summarized as follows in detail.1. For the pristine Mn-terminated SrMnO3(001) surface, due to the symmetry broken atthe surface, surface Mn and subsurface Mn ions show the occupancy ofd z2orbital, and asa result, spin-flip AFM, FM along the c axis and AFM in the ab plane, is energeticallyfavorable. When oxygen vacancies (OVs) concentration is25%, electrons start to occupy surface andd z2orbital of subsurface Mn2, while keep the surface and egorbital ofsubsurface Mn1still empty, leading to a ferromagnetic surface. With the increasing OVs(50%and75%),dx2y2orbital of surface Mn start to be occupied, while keeping t2ganddx2y2orbital occupation of subsurface Mn. Therefore, spin-flip AFM surface is obtained.In100%case, five3d orbitals of surface Mn are completely filled, anddx2y2orbital ofsubsurface Mn also start to be occupied, so AFM superexchange dominates. As a result,bulk like G-type AFM ground state is obtained. The epitaxial strain does not change orbitaloccupancy and spin-flip process at any OV case.2. In the nn-type LaAlO3/SrMnO3supperlattices, electrons transfer from (LaO)+layer intoSrMnO3component induced by polar electric field in LaAlO3. These transferred electronsdistribute uniformly in SrMnO3component and occupy Mn’s egorbital, inducinghalf-metallic ferromagnetism in the framework of Zener double exchange. In the pp-typeLaAlO3/SrMnO3supperlattices, the polar electric field in the LaAlO3drives holes out ofthe LaAlO3component and into SrMnO3component, which reside almost uniformly at theoxygen atoms in the superlattices. With absence of the egstates at the Mn sites, bulk-likeG-type AFM ordering were obvious. But pp-type superlattices are metallic because of holetransfer. When a magnetic field is applied to induce FM ordering, the pp-type superlatticesshow the characteristics of coexistence of electron-doping and hole-doping, suggestingunusual magnetotransport properties therein. In the np-type LaAlO3/SrMnO3supperlattices,complex magnetic orderings emerge by changing the number of MnO2layer and thicknessof LaAlO3component. With increasing LaAlO3layers, the energy difference betweenground state and FM state gets smaller and smaller, so a tendency of transition from acomplex AFM state to FM state can be expected. These results suggest that possiblecolossal magnetoresistive effects can be observed when an external magnetic field isapplied in these np-type LaAlO3/SrMnO3superlattices. Further calculations demonstrate that oxygen octahedral rotation and tilting don’t change the robust half-metallic electronicstate in nn-type LaAlO3/SrMnO3superlattices. But epitaxial strain will change the egorbital occupation, consequently, and influence ground state magnetic ordering of this kindof superlattices. For pp-type and np-type superlattices, charge transfer changessignificantly with strain due to increase (decrease) of polar field in the tensile (compressive)condition. On the other hand, oxygen octahedra rotation and tilting does not change theground-state magnetic ordering in pp-type superlattices. But oxygen rotation and tiltingstrengthen FM coupling between neighboring Mn ions in np-type superlattices.3. For the pristine Mn-terminated SrMnO3(111) surface, due to the polarcatrosphy, Mncation at surface should be Mn2+, therefore leading to different electronic phase at the(111) surfaces. Furthermore, in the n-type LaAlO3/SrMnO3(111) supperlattices,electrons transferred from LaAlO3component distribute unevenly in SrMnO3componentand occupy Mn’s egorbital, inducing half-metallic ferromagnetism in the framework ofZener double exchange. With increasing SrMnO3layers, the sum of every Mn magmonkeep a constant suggesting a fixed number of charge transferred from LaAlO3component.With egorbital occupancy, the systems show obvious MnO6octahedron rotation andtilting. For p-type superlattices, holes reside almost uniformly at the SrO3and LaO3planedrived by the polar electric field in the LaAlO3and SrMnO3component. With absence ofthe egstates at the Mn sites, bulk-like G-type AFM ordering were obvious with almostimperceptible octahedron rotation and tilting. But p-type superlattices are metallicbecause of hole transfer.With the above first-principle calculations and analysis, we have provided acomprehensive picture of electronic and magnetic reconstructions at the surface andinterface of SrMnO3-based heterostructures. These novel magnetic and electric propertiesdemonstrate their potential application in spintronic devices.