Layer-resolved Magnetoresistance Of Manganite/ruthenate-based Synthetic Antiferromagnets And Multiple Strain Control Of Manganite Thin Films

Owing to the strong coupling of lattice,charge,spin and orbital degrees of freedom,perovskite-type transition metal oxides exhibit a plethora of emergent phenomena and intriguing physical properties.Among them,manganites have attracted much attention due to their properties of colossal magnetoresistance,charge/orbital ordering,halfmetal feature,and phase separation.The proximal energies between various phases make them susceptible to external perturbations,which provides us an opportunity to tune their physical properties on demand.Recently,our group has made a major breakthrough in the synthesis of manganite/ruthenate-based synthetic antiferromagnets.Atomically precise La0.67Ca0.33MnO3/CaRu1-xTixO3 and La0.7Sr0.3MnO3/SrRu1-xTixO3 synthetic antiferromagnets exhibit layer-resolved magnetization reversal.Engineering and exploiting their tunability will surely lead to new physics and applications for synthetic antiferromagnetic spintronics.In order to study the magneto-transport properties of manganite thin films under multiple strains and understand the relationship between the magnetoresistive behavior and the interlayer exchange coupling of all-oxide-based synthetic antiferromagnets,high-quality manganite thin films and La0.67Ca0.33MnO3/CaRu1-xTixO3 antiferromagnetic multilayers are synthesized by pulse laser deposition technology.The microscopic structural characterizations,macroscopic magnetic and magneto-transport measurements are performed.This dissertation includes six chapters:Chapter 1:We introduce the background of perovskite oxide thin films in detail,including the structural distortions,interfacial phenomenons and funtional devices.Then,we summarize the development of antiferromagnetic spintronics and the modulation of interlayer exchange coupling.Finally,we briefly introduce the main mechanism of magnetoresistance effect.Chapter 2:We briefly introduce the experimental methods and characterization methods,including the preparation of targets,pulsed laser deposition technology,x-ray diffraction,high-resolution scanning transmission electron microscopy,micro-and nano-scale technology,and the methods for low-temperature magnetoelectric properties.Chapter 3:We study the structural evolution of Pr0.5Sr0.5MnO3/(LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7(110)films.As the film thickness increases,the evolution of structural relaxation modes and magnetic/magneto-transport properties are characterized.Due to the monoclinic distortion of Pr0.5Sr0.5MnO3 unit cells,the evolution of anisotropic strain along the two orthogonal in-plane directions is quite different.Using magnetic force microscopy,we further demonstrate the phase distribution and annihilation dynamics of domain order.Ferromagnetic metallic domains are regularly aligned along the[111]and[111]crystal directions,forming 71 ° or 109° domains,which is consistent with the structural domains revealed by the reciprocal space mappings.Chapter 4:For the ferromagnetic metal La0.67Ca0.33MnO3,we show that tensile uniaxial strain enhances the cooperative Jahn-Teller distortion and promotes selective orbital occupancy,inducing the antiferromagnetic insulating phase in La0.67Ca0.33MnO3 thin films.On the other hand,hydrostatic pressure can suppress the Jahn-Teller distortion to tune it back to a hidden ferromagnetic metallic phase.Moreover,using a larger uniaxial strain,the antiferromagnetic insulating state is induced in doping levels of 0≤x≤1/2,giving rise to an extended antiferromagnetic insulating phase diagram.Chapter 5:We study the magneto-transport of La0.67Ca0.33Mn03/CaRu0.5Ti0.5O3 synthetic antiferromagnets.When the current is perpendicular to the plane,the resistance abruptly increases as the relative magnetization alignment of adjacent La0.67Ca0.33MnO3 layers separated by an insulating CaRu0.5Ti0.5O3 layer changes from parallel to antiparallel,which is consistent with the two-current model.With increasing temperature,the tunneling magnetoresistance exhibits sign reversal,arising from the local-state-assisted resonant tunneling.When the current is along the in-plane direction,the current flow over the entire in-plane length of samples,and the magnetoresistance is dominated by carrier confinement effects due to the half-metal feature of the La0.67Ca0.33MnO3 layers.Chapter 6:Hydrostatic pressure is applied to the synthetic antiferromagnets of La0.67Ca0.33MnO3/CaRu0.5Ti0.5O3 and La0.67Ca0.33MnO3/CaRu0.8Ti0.2O3.We found that hydrostatic pressure promotes ferromagnetism and electrical conductivity of the manganite/ruthenate-based synthetic antiferromagnets and can increase the Curie(and Neel)temperature.As hydrostatic pressure increases,the magnetoresistance shows cusps of conductivity,arising from the interfacial states.Moreover,multilayers with antiparallel magnetic configurations become high resistance states at～2.5 GPa from low resistance states at ambient pressure,which is attributed to the competition between spin-dependent scatterings and spin selective carrier confinement effects.