| Strongly correlated transition metal oxides,as a new type of electronic functional materials,since strong and robust couplings between the internal multi-dimensional degrees of freedom(spin,charge,orbit,lattice),endowing them with rich physical connotations.As a family of high-spin semi-metallic magnetic materials,colossal magnetoresistive manganese oxides have received extensive attention from researchers for their great potentials in spintronic devices.Especially a series of heteroepitaxial manganites-based multilayer films,superlattices,and other low-dimensional all-oxide systems has also been precisely constructed with the help of interface engineering and advanced experimental technologies,which provide a great platform for controlling the physical properties and cleavage micro-mechanism in low-dimensionality.Thanks to the stronger entanglement of the multi-dimensional freedom parameters in space-time at proximity interfaces,abundant novel physical properties,and quantum states differed from the bulk materials will emerge at heterointerfaces,such as high-temperature superconductivity,two-dimensional electron gas,multiferroic coupling,etc.,Besides,its rich interfacial magnetism also brings infinite possibilities for the artificial modulation of the spin-magnetic textures among complex oxides and inspires the development of multi-field stimulations,as well as provides more enlightenment for dissecting physical connotations.However,the coexisted excellent magnetic and electrical properties of the manganite-based systems have been sharply suppressed owing to the limited thickness and the degradation of the boundary or surface for long-standing,which hinders the actual applications.Inspired by the research upsurge of two-dimensional ferromagnets and the recent dilemma for the practical applications of oxide-based functional materials,we have systematically studied the interfacial magnetism and physical mechanisms based on low-dimensional manganite/ruthenate epitaxial system.We have made some interesting progress in the enhancement of ferromagnetic order,in-plane spin-reorientation,and evolution of spin orders,interlayer exchange couplings.Therefore,the modulation of spin-orders based on such low-dimensional interfaces will greatly promote the implementation of spintronic devices to the post-Moore era.This dissertation includes the followed seven chapters.Chapter 1:We first briefly introduce the crystal structure and basic magnetic physics of perovskite oxides.Then,we elaborate on interface magnetism controlled by the multi-degree-of-freedom.Subsequently,we briefly review the recent research progress of low-dimensional material systems.Afterward,we summarize the main contents of this chapter through the discussion of the above aspects.We also list our research plans which focus on the low-dimensional manganite/ruthenate system.Chapter 2:We briefly introduce the fabrication and characterization methods of heteroepitaxial films,including the preparation of polycrystalline targets,thin-film deposition techniques,X-ray diffraction,high-resolution transmission electron microscopy,absorption spectrum,and photoelectron energy spectrum,as well as low-temperature magnetic-electrical properties measurements.Chapter 3:Based on previous studies,we realize a high-TC ferromagnetic phase(?291 K)in La0.67Ca0.33MnO3/SrRuO3 ultrathin superlattices assisted by the interfacial charge transfer and structural modification.Even the La0.67Ca0.33MnO3 thickness is only 4 u.c,the TC around 285 K still can be maintained and it highly depends on the stacking sequences of the bilayer.Through the followed STEM and XPS characterization as well as theory calculations,it confirms that such a high-temperature ferromagnetic phase which is about 30 K higher than that of the bulk TC,is mainly determined by the interfacial charge transfer at the two sharp and asymmetric interfaces(MnO2/SrO and La0.67Ca0.33O/RuO2),and the associated polar-distortion.Eventually,the two collaborative interface effects at atomic scale which rarely appears at the same time jointly stabilize the high-TC ferromagnetic phase in the atomically-thin La0.67Ca0.33MnO3.Chapter 4:We can continuously modulate the long-range ferromagnetism of the confined[La0.67Ca0.33MnO3(l-6 u.c)/SrRuO3(2 u.c)]N superlattices via increasing the stacking period of the LCMO/SRO bilayer.The TC of atomically thin superlattices can be continuously enhanced by increasing N(1 to 15)and ultimately exhibits saturated ferromagnetism,with the largest modulation range?133 K.Moreover,the saturated TC can be still stabilized around room temperature even in series superlattices with only 1-3 u.c-thick La0.67Ca0.33MnO3 layers.The further results indicate that both the metallicity and thickness of the ruthenate significantly affect the modulation range of the ferromagnetic order.We suggest that this tunable ferromagnetism at the confined system might be governed by carrier-assisted RKKY-like interlayer ferromagnetic coupling.Chapter 5:In this chapter,the stacking period N-driven lateral spin-reorientation is systematically studied.The magnetic easy-axis in[La0.67Ca0.33MnO3(6 u.c)/SrRuO3(2 u.c)]N superlattices can be gradually switched from[010]to[100]-axis as changing the N from 1 to 15,leading a large reversal KMAE around-1.83x105 erg/cm3.With the help of the XAS spectrum and microstructural characterizations,we find that the continuously switched in-plane magnetic easy-axis is determined by the stacking period-driven competition between strain and orbital engineering.For N<3,the magnetic easy-axis prefers the tensile-strained[010]direction.However,as increased N,the enhanced hybridization between Ru 4dxz/dyz orbital and Mn 3dx2-y2 orbital will compete with the strain effect.Finally,the increased Mn 3dx2-y2 orbital polarization along the[100]-axis leads to easy magnetic-axis reconstruction.Chapter 6:In this chapter,we meticulously analyze the evolution and extended applications of the interlayer exchange couplings in the LSMO-based asymmetric magnetic structures via changing the interval thickness and temperature.In these asymmetric magnetic structures(CaRu0.5Ti0.5O3/La0.67Sr0.33MnO3/CaRu0.5Ti0.5O3/La0.67Sr0.33Mn0.95Ru0.05O3/CaRu0.5Ti0.5O3)which exhibit determined magnetization reversal sequence,the exchange coupling behavior can continuously evolve from ferromagnetic coupling to antiferromagnetic coupling,then weak ferromagnetic coupling or decoupled ferromagnetic behavior.And a larger range of step-like magnetization can be observed.Besides,control of the system temperature also can efficiently modulate the exchange strength between the two asymmetric ferromagnetic layers.Chapter 7:We briefly summarize the main research contents and innovation points,as well as present the prospects of synthetic antiferromagnet devices,Si-based devices,and their electro-and magnetic-catalytic performance based on the current findings of interfacial magnetism in low-dimensional manganites/ruthenates. |
PDF Full Text Request