Interface Coupling And Its Effect On The Magnetic Properties Of Transition Metal Oxide Heterostructures

The rich physical effects of perovskite oxide heterostructures are the focus of condensed matter physics.There are many kinds of interface coupling effects in the heterostructure interface composed of strongly correlated materials,such as charge transfer,orbital reconstruction and interface chemical bonding,novel physical properties and abundant physical effects completely different from the monomer materials are observed,such as two-dimensional electron gas,two-dimensional ferromagnetism,the coexistence of superconductivity and magnetism,and the Rashba effect controlled by electric field.Some perovskite oxides have ordered oxygen vacancy defects,forming a new perovskite-like crystal structure,such as ABO_2.5 brownmillerite oxide,ABO₂ infinite layer oxide and so on.We find that the combination of perovskite-like structure and perovskite structure oxide to form a heterojunction with interface symmetry mismatch can introduce new interface coupling effect,which opens a new way for exploring the striking properties of strongly related oxide system and developing oxide composite functional devices.In this paper,perovskite manganese oxides and a variety of perovskite-like cobalt oxides are combined to form a heterojunction interface.The coupling effect of the interface and the strain effect on the magnetic properties of the heterojunction,especially the magnetic anisotropy,are systematically studied.The change rule of the occupied state of the interface electron orbit is found and confirmed.The main results are as follows:1.High quality LaCoO_3-?/La_2/3Sr_1/3MnO₃(LSMO)/LaCoO_3-?trialyers under different strain was prepared on(LaAl O₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7(LSAT),SrTiO₃and KTa O₃single crystal substrate.In particular,the tensile stress is uo to 2.2%which is achieved on KTa O₃ substrate by inserting buffer layer.It is found that the LSMO layer of trialyers on SrTiO₃and KTa O₃ substrates show significant perpendicular magnetic anisotropy,the perpendicular magnetic anisotropy is up to?3.9×10⁶ erg/cm³and increases with the increase of tensile stress,while the three-layer films on LSAT substrates do not show obvious magnetic orientation.The results show that the interface coupling effect overcomes the magnetoelastic coupling effect caused by tensile strain,and the latter support in-plane magnetic orientation.In contrast,the three-layer films on LSAT substrate with small strain do not show obvious perpendicular magnetic orientation.The results show that the tensile strain enhances the interface coupling effect and leads to the perpendicular magnetic anisotropy of the heterostructure.2.High quality(110)-LaCoO_3-?/La_2/3Sr_1/3MnO₃/LaCoO_3-?(LCO/LSMO/LCO)trialyers were prepared.The anomalous in-plane magnetic anisotropy was found.The correlation between 3d electron orbital reconstruction and spin orientation at the heterostructure interface and its effect on the macroscopic magnetism were elucidated.Different from the(110)oriented monolayers,the easy magnetization direction of LaCoO₃/La_2/3Sr_1/3MnO₃/LaCoO₃ trilayers is[1(?)0]axis.The experimental results of X-ray absorption spectrum and X-ray dichroism show the reconstruction of orbital energy levels between d_x2_-y2 and d_3z2_-r2 of Mn ions in LSMO layer,that the preferential state of trilayers becomes the d_x2_-y2 orbital with lower energy.The d_3z2_-r2 orbital of LSMO monolayer is lower,which is the preferential state.This orbital reconstruction effect is due to the coupling of the d_x2_-y2 orbitals of Mn and Co ions through the 2p orbitals of O,resulting in the formation of lower energy bonding levels,accompanied by the charge transfer process from Mn ions to Co ions.According to the strong coupling relationship between the orbital and spin degrees of freedom in Bruno model,the easy magnetization direction of LSMO layer will change 90°correspondingly in the plane.3.High quality(001)-LSMO/YBCO/STO heterostructure were prepared with different symmetry LSMO and YBaCo₂O_5+?(YBCO),the anomalous perpendicular magnetic anisotropy of heterostructure is systematically studied.The results of structural analysis show that YBCO is double perovskite with highly ordered arrangement of A-site cations and oxygen vacancies,forming[Ba O]layer and[YO_?]layer periodic stacked alternately along the[001]direction.Magnetic measurements show that the LSMO layer in the heterostructure has perpendicular magnetic anisotropy,and the anisotropy constant is?4×10⁶ erg/cm³,which is two orders of magnitude larger than the anisotropy produced by magneto elastic coupling.X-ray absorption spectra and density functional theory calculations confirm the orbital reconstruction at the LSMO/YBCOinterface leads to the preferential occupation of the d_3z2_-r2 orbital of e_gelectrons,resulting in the perpendicular anisotropy.We further discuss the internal physical mechanism:the coordination fields of Mn and Co are MnO₆ octahedron and CoO₅ square pyramid,respectively.They form covalent bonds through the coupling of O ions at the interface,which makes the energy of the reconstructed d_3z2_-r2 orbital lower.Furthermore,high quality(110)-YBCO/LSMO/STO heterostructure was prepared on(110)oriented SrTiO₃ substrate.The results of structural characterization show that(110)oriented YBaCo₂O_5+?is still maintained high quality A-site ordered epitaxial growth,and the ordered direction is[100]or[010]axis direction inclined 45°out of plane.In this heterostructure,the easy magnetization direction of LSMO layer is[1(?)0]axis.The anisotropy constant is10⁶ erg/cm³.This work preliminarily demonstrates the possibility of constructing artificial interface spin structure based on the design of double perovskite/perovskite heterointerface.