| In strongly correlated complex oxides,charge,spin,orbital,and lattice degrees of freedom co-exist and interplay cooperatively,and the correlation between these degrees of freedom and related couplings generate a rich spectrum of competing phases and physical responses,including high temperature superconductivity,metal-insulator transitions,colossal magnetoresistance,(anti-)ferromagnetism,(anti-)ferroelectricity,piezoelectricity,and more recently multiferroics properties.This has led to extensive studies of both bulks and thin films,with the aim of increasing our understanding of the fundamental nature of existing materials systems,so that we might be able to better control and design novel materials for applications.In recent years,creating atomically sharp interfaces in epitaxial complex oxide heterostructures has become possible.The result is new opportunities to realize model systems in which exchange interactions can be tailored on an atomic length scale,yielding novel magnetic properties and perhaps even new applications.In addition to the usual exchange interactions found in complex oxides,interfaces enable new effects that include charge transfer,structural coupling,and orbital reconstructions.The optimally doped manganites,e.g.,La0.67Ca0.33MnO3,is of great potential for the spintronic devices owing to its half-metal character.However,the dimensionality reduction induced ferromagnetism deterioration of the manganite film can give rise to the so-called magnetic or electric "dead layer",which is believed to impede the development of the spintronic devices.In this thesis,we fabricate high-quality epitaxial heterostructures composed of Lao.67Cao.33MnO3 and ruthenates,which can effectively suppress the "dead layer" at the manganite interfaces.When the SrRuO3 is chosen as ruthenate layer,the ferromagnetism is remarkably enhanced.Moreover,La0.67Ca0.33MnO3/Ca(Ru,Ti)O3 and La0.67Ca0.33MnO3/(Ca,Sr)RuO3 exhibit clear antiferromagnetic interlayer exchange coupling,which can be modified by the external stimulus.These intriguing phenomena at the manganite/ruthenate interfaces greatly enrich the functionalities of all-oxide heterostructures.The thesis includes seven chapters.Chapter 1:We first introduce the basic structure of the perovskite oxides.Then,we mainly illustrate the interfacial reconstructions in strongly correlated oxide heterostructures,including the structure coupling,charge transfer,spin/orbital reconstructions.Furthermore,we exhibit the modifications of these interfacial effects on the magnetic properties at oxide interfaces.Meanwhile,we also introduce the modulated magnetotransport properties via chemical doping and the representative interfacial asymmetry in the oxide heterostructures.Finally,we summarize the research progress about the synthetic antiferromagnets.Specifically,the antiferromagnetic interlayer exchange coupling behaviors in the oxide heterostructures as well as their applications for the future spintronic devices are emphatically introduced.Chapter 2:We simply introduce the methods to prepare and characterize the epitaxial heterostructures,including the preparation of polycrystalline targets,pulse laser deposition,high-resolution x-ray diffraction,high-resolution cross-sectional scan transmission electron microscopy,and the low-temperature transport and magnetic measurements(VSM,SQUID and PPMS).Chapter 3:The ultrahigh-Tc ferromagnetism and unambiguous antiferromagnetic interlayer exchange coupling in La0.67Ca0.33MnO3/SrRuO3 heterostructures are presented.Moreover,the result obtained from the high-resolution STEM demonstrates that there is an intrinsic interface structural asymmetry existing in the La0.67Ca0.33MnO3/SrRuO3 heterostructures,which highly relies on the stacking sequence.The ferromagnetism,which is also asymmetric,in these heterostructures are closely related to the interface structural asymmetry.This result indicates that the interfacial cation intermixing can effectively enhance the interfacial effects,including the charge transfer,epitaxial strain,octahedral coupling,which are able to improve the TC of La0.67Ca0.33MnO3 layers.Chapter 4:We systemically studied the antiferromagnetic interlayer exchange coupling in the La0.67Ca0.33MnO3/CaRu1-xTixO3(0?x?0.5)heterostructures.Strong antiferromagnetic interlayer exchange coupling with layer-resolved magnetization reversal is observed in La0.67Ca0.33MnO3/CaRu1-xTixO3 heterostructures except for x=0.Upon changing the Ti doping level and the growth orientation,we can effectively tune the antiferromagnetic interlayer exchange coupling behaviors.As x increases,the interlayer exchange coupling field Hex first increases and then decreases,while the coercivity He decreases monotonously.Moreover,the samples grown on NGO(110)substrates possess the larger Hex and smaller He as compared with those grown on NGO(001)substrates.Chapter 5:The antiferromagnetic interlayer exchange coupling in the La0.67Ca0.33MnO3/Ca1-xSrxRuO3(0?x?1)superlattices is systemically investigated.An unambiguous antiferromagnetic interlayer exchange coupling is observed in La0.67Ca0.33MnO3/Ca1-xSrxRuO3(0<x?1)superlattices,which is capable to be modulated by the Sr doping.The interlayer exchange coupling strength increases monotonously with increasing x.In the meantime,the Tc of the superlattices is significantly elevated.Unfortunately,the layer-resolved magnetization reversal which has been observed in La0.67Ca0.33MnO3/Ca(RuTi)O3 superlattices disappears.Based on these features,we construct a "hybrid" structure,which can exhibit both high TC and layer-resolved magnetization reversal at the same time.Chapter 6:The evolution of the ferromagnetism in the La0.67Ca0.33MnO3/CaRuO3 superlattices via chemical doping in the nonmagnetic CaRuO3 interlayer is examine.We demonstrate that the Ti,Sr,and La doping in the CaRuO3 can effectively modulate the interfacial octahedral coupling as well as the interfacial charge transfer in the superlattices,thus leading to significant variation of the ferromagnetism of the ultrathin La0.67Ca0.33MnO3 layers.Our result indicates that the interfacial charge transfer is not only controlled by the band alignment of the constituents,the carrier itinerancy of the electron donor should also plays a role.Chapter 7:An ultrahigh-TC ferromagnetism is achieved in ultrathin La0.67Ca0.33MnO3/CaRuO3 superlattices.The Tc can be maintained near 265 K even with the La0.67Ca0.33MnO3 layer down to three unit-cells.When the La0.67Ca0.33MnO3 is only 1 unit-cell,the La0.67Ca0.33MnO3/CaRuO3 superlattices can also maintain the ferromagnetism with the TC of 13 5 K.More intriguingly,as the thickness of the CaRuO3 layer is reduced to 1 unit-cell,the magnetic easy-axis of the La0.67Ca0.33MnO3 layers abruptly rotates from the[010]to the[100]axis. |
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