Electric Field Modulated Physical Properties Of Metastable La_0.67Sr_0.33MnO₃/BaTiO₃ Multiferroic Heterostructures

La_0.67Sr_0.33MnO₃/BaTiO₃multiferroic heterostructures have been widely studied due to their simple structure and rich magnetoelectric coupling properties,which show great potential in practical applications for information storage technology.In this dissertation,we deposited a series of metastable La_0.67Sr_0.33MnO₃thin films on the[011]and[001]orientated BaTiO₃substrates by magnetron sputtering,and the effects of temperature,electric field strength and oxygen concentration on the physical properties of La_0.67Sr_0.33MnO₃films were investigated systematically.Metastable La_0.67Sr_0.33MnO₃films were fabricated on the tensile strained BaTiO₃(011)substrates.The positive and negative pulse electric fields with different strength were applied to the 5 nm thick La_0.67Sr_0.33MnO₃/BaTiO₃(011)heterostructure at 240K.It was found that the in-plane resistance of the heterostructure presents a volatile state when the electric field is removed,and it reaches a non-volatile stable state after a period of time.Moreover,both the duration time of the volatile state and the resistance modulated by the positive and negative electric fields strongly depend on the strength of the electric field.When the pulsed electric field of±4.2 k V/cm is applied,the regulation of non-volatile resistance and volatile resistance reaches?23%and?230%,respectively.We suggest that the volatile and non-volatile electroresistance originate from the redistribution of charge and oxygen vacancies caused by the ferroelectric field effect in the LSMO layer.And the volatile state is mainly induced by the accumulation/depletion of oxygen vacancies at the interface.The volatile resistance state is induced by the relaxation of oxygen vacancies to their original sites when the electric field is removed.We then measured the duration of the volatile state at different temperatures with pulsed electric field of±4.2 k V/cm.The duration was found to be the largest when the BaTiO₃substrate was in the orthorhombic phase.This is due to switching of ferroelectric polarization from the other directions to the out of plane direction,which makes the effect of the ferroelectric field stronger.In contrast to the metal-insulator transition in the bulk La_0.67Sr_0.33MnO₃,the ultra-thin La_0.67Sr_0.33MnO₃film shows a semiconducting transport behavior when it was grown on the BaTiO₃substrate with tensile strain.The electric double layer transistor on the 15 nm thick La_0.67Sr_0.33MnO₃/BaTiO₃(001)heterostructure was formed by using the ionic liquid as dielectric material.It can dynamically change the oxygen content of the film by electrochemical reaction.When the BaTiO₃substrate undergoes a phase transition with the temperature,the change of the biaxial stress on the film causes the spatial reconstruction of the oxygen vacancies.The introduction of oxygen vacancies and spatial reconstruction changes the conductivity mechanism of La_0.67Sr_0.33MnO₃thin film when the BaTiO₃substrate is in the rhombohedral phase,leading to the transition of semiconducting to metallic behaviors.