Research Of Interlayer Coupling And Magnetic Anisotropy Properties Of Co-and Mn-based Oxide Films Heterostructures

Due to the strong correlation among multiple degrees of freedom of spin,charge,lattice and orbit,transition metal oxides have exhibited various effects,such as high temperaturesuperconductivity,colossalmagnetoresistance,ferroelectricity,multiferroic and d base two-dimensional electron liquid,and a series of novel physical phenomena etc.,which are the research hotpots in the field of new multifunctional materials.Combining different transition metal oxides into a heterostructure will break the balance of different interactions among original materials and form new oxygen polyhedral frameworks at the interface,producing unpredictable novel physical phenomena and unique functions.Therefore,how to choose a suitable material system to construct the functional interface and regulate its performance have become the core issues in the study of oxide films heterostructures,as well as the focus of this paper.We have prepared high quality manganese oxide and cobalt oxide epitaxial films with pulsed laser deposition technology and realized special antiferromagnetic interlayer coupling.Moreover,based on the lattice symmetrical mismatch of the perovskite/brownmillerite interface,a great perpendicular magnetic anisotropy was successfully obtained,which proposed new ideas for the design and regulation of oxide heterostructure interfacial spin structure.The specific research content and innovative achievements of this paper are as follows:1.The interfacial coupling effect of the[LaMnO₃?t₁?/LaCoO₃?t₂?]₅ superlattices under tensile strain was systematically studied and an antiferromagnetic layer coupling with thickness modulation was found at the insulating ferromagnetic interface.We have deposited the[LaMnO₃?t₁?/LaCoO₃?t₂?]₅ superlattices with different thickness on the?LaAlO₃?_0.3?Sr₂AlTaO₆?_0.7?LSAT?and SrTiO₃?STO?substrates.The X-ray diffraction results show that the superlattice grown epitaxial along the substrate in-plane direction,sharing the same lattice constant with the substrate.The scanning transmission electron microscopy data indicate that the parallel dark stripes with 2a?a is the unit cell parameter?period were formed in the LaCoO₃?LCO?layer.Combining with the magnetic measurement results,we find that when the LaMnO₃?LMO?layer is thin?t₁<11uc?or the LCO layer is thick enough?t₂>11uc?,the magnetization weakens rapidly below 75K and a similar anti-ferromagnetic interlayer coupling appears.Increasing the applied magnetic field,a spin flip exists in the anti-parallel magnetic moment,performing a ferromagnetic increment below 75K.Meanwhile,we obtained a positive exchange bias of 0.04T offset in the magnetization curve by saturating the sample under a field of+1T.The existence of spin flipping and exchange bias can verify the antiferromagnetic behavior between LMO and LCO.Combined with the first principle calculation,it is confirmed that the Mn³⁺and Co³⁺aligned anti-parallel at the interface and the magnetic moments of Co³⁺oscillated in both magnitude and direction as the distance between the CoO₂layer and the interface increasing.The results indicate the possibility of regulating the spin structure by interlayer designing.2.The induced abnormal perpendicular magnetic anisotropy of the LSMO layer at the perovskite/brownmillerite interface of La_2/3Sr_1/3MnO₃/LaCoO_2.5?LSMO/LCO?heterostructures deposited on STO?001?was systematically studied.We have obtained several LSMO/LCO multilayers with different thickness by regulating the growth condition.The result of scanning transmission electron microscopy shows that the parallel dark stripes formed in the brownmillerite LaCoO₃?LCO?layer with 4a period,as well as the interface consisted with MnO₆ oxygen octahedron and CoO₄oxygen tetrahedron.The magnetic measurement results indicate that a large perpendicular magnetic anisotropy exists in the multilayers and the maximal perpendicular magnetic anisotropy energy is about 1.3J/cm³,which is more than one or two order of magnitude higher than that achieved via the conventional approaches.Combined with the first principle calculation,it can be confirmed that the symmetry mismatch at the perovskite/brownmillerite interface lead to an elongation of MnO₆oxygen octahedron,which results in the priority occupation of 3d electrons to the orbit,and shows a large perpendicular magnetic anisotropy.We further changed the oxygen pressure during the deposition.The results show that with the increase of the oxygen pressure,the LCO layer is gradually in full-oxygen and the perovskite/perovskite interface replaces the perovskite/brownmillerite interface,as well as the corresponding perpendicular magnetic anisotropy disappears.This work is the first one to use the lattice symmetrical mismatch to control the spin orientation of oxide heterostructures and propose a new concept of symmetric mismatched oxide heterostructure.3.The in-plane magnetic anisotropy of the LaCoO₃/La_0.67Sr_0.33MnO₃?LCO/LSMO?bilayers deposited on LaAlO₃?110?was studied systematically.Comparing with the oxide films along the?001?orientation,the?110?-oriented films possess a more complex crystal strcture.It will no longer keep in balance between[001]and[110]direction and easily resulting different properties.Fixing the thickness of LSMO to 8nm and changing the LCO thickness?0⁸nm?,we have prepared six different LCO/LSMO bilayers.The magnetic measurement results indicate that the pure LSMO film deposited on LaAlO₃?110?shows an easy axis along[110],while a spin reorientation appeared when the LCO thickness exceeds 1nm and the easy axis turns from[1-10]to[001]below 225K.This transformation enhances with the increase of LCO layer thickness.Our research indicates that the magnetic anisotropy is controlled not only by the lattic strain,but also by the structural distortion at the interface.