Manipulation Of Magnetic And Orbital Behavior In Manganites And Its Mechanism

The development of information technology intensely demands the spintronic devices with low power consumption,high storage density,and high integration.The oxide spintronics functional materials,which are represented by manganites,show superior magnetic/electric performances,rich orbital related magnetic/electric properties,and high thermostability,attracting wide attentions.The effective manipulation of the magnetic and orbital behavior in manganites is significant to the application of manganites in the areas of information storage,etc.This dissertation proposes the main idea of manipulating the magnetic and orbital behaviors of manganites by electric field and strain engineering.The manipulation of crystal structure,electronic phase,magnetic/electric performance,and orbital occupancy in manganites and corresponding mechanism are systemically studied.Based on delicate interfacial designing,the correlation between the modulations of interfacial magnetism and orbital occupancy under electric field is studied.Attempts are made on developing the model device based on the interfacial orbital reconstruction to explore the application of manganites in information technology with low power consumption and high storage density.Utilizing ionic liquid gating to apply a gate voltage on La??1–x?Sr_xMnO₃?LSMO?,the electron could be injected into or extracted from the films under positive and negative gate voltage,respectively.The magnetic phases with different magnetic and electric properties of LSMO are able to be reversibly tuned in a nonvolatile way by ionic liquid gating,replicating the tendency in conventional magnetic/electronic phase diagram based on Sr doping.The realization of a reversible metal-insulator transition in colossal magnetoresistance materials would also further the development of four-state memories,which can be manipulated by a combination of ionic liquid gating and the application of a magnetic field.Based on the investigation of crystal and electronic structure,the novel phase is demonstrated to randomly nucleate and grow across the film,revealing that the electric manipulation is caused by the migration of oxygen ions?or vacancies?.More importantly,the orbitaloccupancy and corresponding magnetic anisotropy of these thin films could be altered by gate voltage in a reversible and quantitative manner based on the electron injection and extraction,which is demonstrated by synchrotron radiation X-ray linear dichroism.It is promising to fulfill the physical picture of electrical control of lattice,charge,spin,and orbital degrees of freedom.To realize the control of interfacial magnetic and orbital behavior,a model ferroelectric/ferromagnetic heterostructure of Ba TiO?3/La_1-xSr_xMnO₃ is prepared.The shuttle movement of Ti ion driven by ferroelectric polarization modulates interfacial Ti-O-Mn covalent bond.The orbital reconstruction accompanied with the enhancement or suppression of Ti-O-Mn covalent bond controls the Curie temperature and magnetoresistance of manganites without onerous limitations.Thus the electric-field control of magnetism by orbital reconstruction is firstly found and demonstrated in experiment.Based on the modification of the orbital overlap of Ti-O-Mn covalent bond under gate voltage,a prototype device of interfacial “orbital switch” at atomic level is constructed at the interface of Ba Ti_O3/La_1-xSr_xMnO₃,which could modulated the performance of the whole bulk of thin film.In this dissertation,the magnetic and orbital behaviors of manganites are also tuned by strain.A quantitative dependence of charge transfer on orbital reconstruction is established at the surface of LSMO films with different strain states.The LSMO film under large strain behaves interfacial ferromagnetic/antiferromagnetic phase self-assembly,which produces intrinsic exchange bias effect,shedding promising light on fabricating the exchange bias part of spintronic devices in a single step.Utilizing the symmetry-breaking of transport measurements,the magnetization switching in such a system with intrinsic exchange bias is demonstrated to be controlled by the intrinsic exchange bias field in two-dimension.