Study On The Reversal Mechanism Of Oxide Synthetic Antiferromagnets By Polarized Neutron Reflectometry

Abundant emerging physical phenomena have been found in transition metal oxide(TMO)systems,such as high temperature superconductivity,metal insulation transition,giant magnetoresistance effect,ferroelectricity,multiferroicity,etc.,making TMOs popular areas of condensed matter physics and materials science research.The novel phenomena and properties are owing to the interaction of multiple degrees of freedom,such as charge,spin,orbit,and lattice.These complex but intriguing physical mechanisms have attracted wide concern from researchers.Benefiting from the development of film growth technology,the atomic level control of the epitaxial growth process has been possible,which facilitates the design and fabricaiton of complex oxide heterostructures and superlattices and greatly expands the physical properties of TMOs.Plenty of novel physical mechanisms have been found at epitaxial heterointerfaces,such as interfacial charge transfer,structural proximity effect,and interface orbital reconstruction.In this article,we mainly study the interfacial effect and the interlayer exchange coupling in superlattices composed of manganites and ruthenates.The "dead layer" effect of manganite thin films is a main challenging for there wide application in spintronics devices,that is,the reduction of dimension will induce to the degeneration of magnetic properties of the manganite thin films.Researchers have paid great effort to overcome the "dead layer" effect of the La0.67Ca0.33MnO3 film,which is the optimized doped composition.Inspiringly,the "dead layer" effect of La0.67Ca0.33MnO3 film was found to be fully suppressed by interfacing with ruthenates.In this paper,the study on the interfacial effect of the manganite/ruthenate superlattices is extended to La1-xCaxMnO3 ultra-thin films with other compositions and altering fround states.In addition,this thesis also revealed the layer-resolved magnetic reversal mechanism of the La2/3Ca1/3MnO3/CaRu1/2Ti1/2O3 synthetic antiferromagnet by polarized neutron reflectivity,which is a powerful tool to characterize the magnetic depth profiles in oxide superlattices.Finally,we also designed bilayer structures based on the melting and reentry process of the charge-ordered state in the La0.67Ca0.33MnO3 to realize the controllable exchange bias effect,which is likely to expands the functions of manganites in magnetic storage devices.This thesis includes five chapters.Chapter 1:The relevant research background and the latest research progress of the research content of this article are introduced.First,the crystal structure,symmetry and of the orbital splitting of perovskite oxides are introduced.Then,we introduced the abundant physical phenomena at the oxide heterointerfaces,including the structural proximity and the interfacial charge transfer effect.Besides,we also described the theoretical model of the interlayer exchange coupling mechanism and the latest progress of synthetic antiferromagnets.Finally,we introduced the polarized neutron reflectometry,which is a powerful tool to characterize the depth-resolved microscopic magnetic structure in oxide films and superlattices.Chapter 2:We mainly introduced the materials and methods involved in this thesis,including the preparation of polycrystalline target,the fabrication of epitaxial film,X-ray diffraction technique,high-resolution transmission electron microscope and low-temperature magnetoelectric property measurement method,etc.Chapter 3:The interfacial enhanced ferromagnetism in Maganite/Ruthenate system is regarded as a promising path to broaden the potential of oxide-based electronic device applications.Here,we systematically studied the physical properties of La1-xCaxMnO3/SrRuO3 superlattices and compared them with the La1-xCaxMnO3 thin films and bulk compounds.The La1-xCaxMnO3/SrRuO3 superlattices exhibit significant enhancement of high Curie temperature(TC)beyond the corresponding thin films and bulks.Based on these results,we construct an extended phase diagram of La1-xCaxMnO3 under interfacial engineering.We considered the interfacial charge transfer and structural proximity effects as the origin of the interface-induced high-TC.The structural characterizations revealed a pronounced increment of B-O-B bond angle,which could be the main driving force for the high-TC in the superlattices.Our work inspires a deeper understanding of the collective effects of interfacial charge transfer and structural proximity on the physical properties of oxide heterostructures.Chapter 4:All-oxide-based synthetic antiferromagnets(SAFs)are attracting intense research interest due to their superior tunability and great potentials for antiferromagnetic spintronic devices.In this work,using La2/3Ca1/3MnO3/CaRu1/2Ti1/2O3(LCMO/CRTO)superlattice as a model SAF,we investigated the layer-resolved magnetic reversal mechanism by polarized neutron reflectivity(PNR).We found that the reversal of LCMO layer moments is mediated by nucleation,expansion,and shrinkage of a magnetic soliton.This unique magnetic reversal process creates a reversed magnetic configuration of the SAF after a simple field cycling.Therefore,it can enable a vertical data transfer from the bottom to the top of the superlattice.The physical origin of this intriguing magnetic reversal process could be attributed to the cooperation of the surface spin-flop effect and enhanced uniaxial magnetic anisotropy of the bottom LCMO layer.This work may pave a way to utilize all-oxide-based SAF for 3-dimensional spintronic devices with vertical data transfer and high-density data storage.Chapter 5:The recent research work shows that uniaxial strain induces the charge-ordered phase of the La2/3Ca1/3MnO3 epitaxial film.The essence of the charge-ordered phase is a CE-type antiferromagnetic phase,which can melt or re-enter with the evolution of external magnetic field.We utilized the controllable phase transition process of the charge-ordered La2/3Ca1/3MnO3 film as the pinning layer to realize the controllable exchange bias effect.Due to the important role that exchange bias plays in magnetic storage devices,our work is likely to exend the funtions of the bilayered ferromagnetic-antiferromagnetic film applied in magnetic devices.