Interface Effects In La_2/3Ca_1/3MnO₃/CaRu_1-xTi_xO₃ Multilayers And The Discovery Of Layer-Resolved Antiferromangenetic Interlayer Coupling

Oxide interface is one of the hottest issues in condensed matter physics.The complex entanglement of lattice,charge,spin and orbital degrees of freedom at the interfaces can lead to a wide spectrum of electronic states,which are often inaccessible in their bulk counterparts.Among those extensive researches in the past decades,the interfaces of colossal-magnetoresistive manganites have attracted great interests.Novel phases can always be expected since the competition of multiple interactions in manganites is quite sensitive to the interfacial structural rearrangements and electronic reconstructions.Moreover,the interface engineering has been demonstrated to be an effective approach to modify the functionalities of manganites.Ferromagnetic manganites have potential applications in spintronic devices for their half-metallic nature.However,dimension reduction inevitably results in magnetism deterioration at the interface,namely the "dead layer" effect,which has hampered the further developments of manganite-based devices.Therefore,it is highly desirable to stabilize the ferromagnetism in ultrathin manganite films by modifying the interface properties.On the other hand,the ubiquitous "dead layer"effect has also limited the research on synthetic antiferromagnets composed of correlated-oxides.In this thesis,based on the control over the ferromagnetism in ultrathin manganite layers,we observe for the first time the layer-resolved antiferromagnetic interlayer coupling in La_2/3Ca_1/3MnO₃/CaRu_1/2Ti_1/2O₃ multilayers,which greatly enriches the functionalities of all-oxide heterostructures.The thesis includes six chapters.Chapter 1:We summarize the common interfacial reconstructions at correlated-oxide interfaces,including the structure rearrangements,charge transfer and spin/orbital reconstructions.Then we focus on the creation and manipulation of novel magnetic states at oxide interfaces.Finally,we recall the research status about the synthetic antiferromagnets composed of simple metals and complex oxides.Chapter 2:We simply introduce the methods to preparer and characterize those multilayers,including the sintering of polycrystalline targets,pulsed laser deposition,X-ray diffraction and reciprocal space mapping,cross-sectional scanning transmission electron microscopy,as well as the low-temperature transport and magnetic measurements.Chapter 3:By designing CaRuO₃/La_2/3Ca_1/3MnO₃/CaRuO₃ sandwiches,we succeed in stabilizing the ferromagnetism in ultrathin La_2/3Ca_1/3MnO₃ layers owing to the interfacial charge-transfer enhanced double-exchange interactions.Particularly,the Curie temperature?TC?can be maintained near 250 K even with the La_2/3Ca_1/3MnO₃ layer down to four unit-cells?1.6 nm?.Chapter 4:We achieve a nontrivial control over the ferromagnetism in CaRu_1-xTi_xO₃/La_2/3Ca_1/3MnO₃/CaRu_1-xTi_xO₃?0?x?0.8?trilayers by modifying the interface properties,including the charge transfer magnitude and orbital occupations.The Tc can be continuously tuned in a wide temperature range,from 262 K at x=0 to 186 K at x=0.8,where the centered LCMO layer is fixed at 3.2 nm.Chapter 5:We for the first time observe the layer-resolved antiferromagnetic interlayer coupling in all-oxide La_2/3Ca_1/3MnO₃/CaRu_1/2Ti_1/2O₃ multilayers.The magnetizations of the interior and outer La_2/3Ca_1/3MnO₃ layers switch separately because of the different interlayer interactions received from the nearest magnetic layers.This is manifested by the discrete magnetization plateaus in the magnetic hysteresis loops.Moreover,the strong uniaxial magnetic anisotropy guarantees the sharp magnetic switching for each La_2/3Ca_1/3MnO₃ sublayer,which provides great advantages for their device applications.Chapter 6:The coercive field?HC?and interlayer coupling field?Hex?in La_2/3Ca_1/3MnO₃/CaRu_1/2Ti_1/2O₃ multilayers can be tuned by growing on NdGaO₃ substrates with different orientations.The HC is greatly enhanced in the?001?-oriented films compared to the?110?films.When the La_2/3Ca_1/3MnO₃ layers are antiferromagnetically coupled,a larger Hex is observed in the?110?films.We ascribe such differences to the strain-related interface effects.