Physical Properties Study Of Ferromagnetic Metal Perovskite Oxide [111]-oriented Films

Perovskite oxides of the correlated electron have a subject of intense investigation with a prime focus on their films and heterostructures due to the coupling between spin,orbital,lattice,and charge.With the development of various thin film deposition technologies,perovskite oxides along the[001]orientation have completely different novel properties from the bulk materials due to their simple epitaxial growth because of the reduction of dimension,the rotation and distortion of oxygen octahedron and the symmetry breaking caused by the construction of heterogeneous interface.And the（111）surface is hexagonal symmetry and can form honeycomb or triangular lattice.Therefore,the films and heterostructure along the[111]direction may give rise to non-trivial topological two-dimensional electronic states,such as quantum anomalous hall effect,topological semimetal and quantum spin liquid,which provide another platform for studying the origin and modulating novel physical properties.Compared with[001]orientation,the high-quality growth of[111]-oriented films and heterostructure is a great challenge due to the existence of polar surfaces and the lack of readily lattice-matched substrates.However,the research and understanding（111）systems are not in-depth,and many experiments and studies are not perfect.For example,the quantum anomalous Hall effect in ferromagnetic SrRuO₃（111）Bilayers calculated theoretically has not been realized,and the physical mechanism of topological properties in materials is not clear.To solve the above problems,we selected two kinds of[111]-oriented perovskite oxides.One is manganese oxides with colossal magnetoresistance effect,which have attracted much attention all the time.One is the 4d transition metal oxide SrRuO₃（111）system with strong spin-orbital coupling.In this paper,SrRuO₃（111）films with high crystalline quality on different substrates were prepared by pulsed laser deposition technology.Two kinds of manganese oxide superlattices[（Sr Mn O₃）_m/（La Mn O₃）_2m]_nand[(Pr_2/3Ca_1/3Mn O₃)₁/(La_2/3Ca_1/3Mn O₃)₂]_n epitaxial on Sr Ti O₃（111）substrate were characterized,the transport properties tested.The effect of A-site cation ordering on its physical properties was studied.By characterizing SrRuO₃（111）films grown on different substrates,it is found that the SrRuO₃（111）films grown on KTa O₃（111）substrates have different crystal structures and abnormal transport properties.Firstly,for the[111]-oriented manganese oxides,we change the order degree of A-site cation by fabricating the superlattice structure.It is found that the ferromagnetism and electrical transport properties of superlattice and disorder alloy-thin films are primarily independent of the order degree of A-site cation,as evidenced by their identical critical temperature of phase transition(Curie temperatureand metal-insulator transition temperature).For La_2/3Sr_1/3Mn O₃ system,theandare independence of the order degree of A-site cation（La/Sr）,thus the ordered arrangement of A-site La³⁺and Sr²⁺cations do not alter the distribution of Mn³⁺and Mn⁴⁺.As a result,the ferromagnetism is independent of the degree of ordering of the A-site cations,which may provide experimental evidence of the spontaneous nature of the EPS.Compared with the influence of the order degree of A-site cation,the oxygen vacancy has a greater impact on its physical properties.For(La_2/3Pr_1/3)_2/3Ca_1/3Mn O₃system,the superlattice samples do not improve their transport properties compared with disordered alloy films,but the thermal hysteresis of the superlattice is significantly suppressed,which may be closely related to the reduction of the length scale of electronic phase separation.For the[111]-oriented SrRuO₃ thin films,it is found that the structure of epitaxial SrRuO₃films grown on KTa O₃（111）substrate changes from Pbnm orthorhombic to R3?(8trigonal structure((6^-(6^-（6^-rotation using the Glazer symbol）due to tensile strain using X-ray diffraction,reciprocal space mapping and scanning transmission electron microscopy.And its magnetic easy axis has been changed and tends to be in-plane.By measuring the planar Hall effect and the abnormal angular-dependent magnetoresistance,it is found that the SrRuO₃ film grown on KTa O₃（111）substrate has six-fold symmetry and there may be ferromagnetic Weyl fermion.By measuring and analyzing the anomalous Hall effect of SrRuO₃ film,it is found that there is two anomalous Hall effect in the system.One anomalous Hall effect has a small coercive field and high mobility,which may be related to its intrinsic anomalous Hall effect,that is,to the Berry curvature in the system.The other anomalous Hall effect with a large coercive field may be related to the magnetization.In addition,the anomalous Hall resistivity of SrRuO₃/KTa O₃（111）films is divergent with temperature at low temperature,which is completely different from the bulk SrRuO₃ and SrRuO₃（001）films.