Oxygen Activity And Its Effect On The Transport Behavior Of La_0.8Ba_0.2MnO₃ Ultrathin Films

In recent decades,perovskite manganites have been extensively studied because of their novel physical properties,such as colossal magnetoresistance,electroresistance(ER)effect,multiferroicity,photoelectric effect etc.In this work,we focus on the magnetoresistance and electroresistance effect due to their potential applications in memory devices,sensors and neuomorphic computing.We alsopay close attention to the oxygen vacancies in our samples.Oxygen vacanciesin transition metal oxides always plays an important role in the mechanism of ER effect and they also have a significant effect on other physical properties,such as magnetism,magnetoresistance,superconductivity,etc.The object of this thesis is La0.8Ba0.2MnO3 ultrathin films prepared by the sol-gel method on LaAlO3 single crystal substrates.In previous research,ER was mainly observed in low-and intermediate-bandwith manganites possessing electronic phase separation characteristics.As for large-bandwith manganites dominated by the double-exchange mechanism few results about ER have been reported.So we chose the large-bandwith manganite La0.8Ba0.2MnO3 thin film in the hope of new findings.Considering the big difference between lattice parameter of the LBMO bulk(3.894 A)and the LAO substrate(3.790 A),we chose the latter as substrate to study the strong strain effect or new growth modes.In addition,oxide films prepared by chemical methods often have a porous morphology,which may leads to novel physical properties,such as the increase in the surface area,which enhances the exchange of matter and energy between the film and environment.Chapter 1.The crystal structure and some basic theories of the perovskite manganites are firstly introduced,including exchange interaction,magnetostriction,and Jahn-Teller distortion.Then the negative magnetoresistance as well as positive magnetoresistance effects in previous research are reviewed.The negative magnetoresistance mainly includes colossal magnetoresistance,giant magnetoresistance and magnetoresistance due to spin-dependent tunneling at low temperatures.The mechanisms of positive magnetoresistance include the quantum interference effect and spin-dependent tunneling in p-n junctions.We then describe the ER effects in transition metal oxides and their mechanisms including the establishment of conductive filaments under current/electric field,the transformation of a large number of insulating phase to ferromagnetic metal phase,the injection of metal ions of the electrode into the sample as well as the dielectrophoresis model,etc.Finally,the method of producing oxygen vacancies,measurement of oxygen vacancy,the effect of oxygen vacancies on the physical properties and their applications are presented.Chapter 2.Firstly,we describe the sol-gel method and the specific preparation process of the samples.Then we introduce the fundamental principle of X-ray diffraction.Next,we introduce the basic principle of scanning electronic microscope(SEM)and analyze the SEM images of our samples.We find that surface morphology of the prepared films is strongly dependent on the pulling speed during the dip coating process.Next,we observed the surface morphology of our sample by atomic force microscopy(AFM)and saw more clearly the relationship between surface morphology and the pulling speed,which is consistant with the results observed in the SEM images.We then discuss the fundamental principle of X-ray photoelectron spectroscopy(XPS)and its advantages and disadvantages.Finally,we introduce the fundamental principle of thermocouple,its connection methods,and selection of the thermocouple materials.Chapter 3.Electric current induced Joule heating effects have been investigated in La0.8Ba0.2MnO3 ultrathin films deposited on LaAlO3(001)single crystal substrate with smaller lattice constant by using the sol-gel method.With applying moderate bias currents(～10 mA),it is found that the Joule self-heating simply gives rise to a temperature deviation between the thermostat and the test sample,but the intrinsic p(T)relationship measured at a low current(0.1 mA)changes little.However,it is noteworthy that the low-temperature transport behavior degrades from metallic to insulating state after applying higher bias currents(>31 mA)in vacuum.Furthermore,the metallic transport can be restored by placing the degraded film in air.The results clearly suggest that the oxygen vacancy in the La0.8Ba0.2MnO3 films is controllable in different atmospheres,particularly with the aid of the Joule self-heating.According tothe SEM images,we attribute the controlled oxygen vacancy to the nano-sized labyrinth pattern of the films,where the large surface-to-volume ratio plays a curial role.Chapter 4.We present the ionic liquid gating effect of LBMO ultrathin films.It is found that the vacuum degree has a strong influence on the resistivity of the sample after applying the gate voltage Vg.We attribute the ionic liquid gating effect to the depletion of oxygen in the sample caused by the positive gate voltage.In addition,the resistivity of the sample is significantly affected by both the duration and magnitude of the Vg,and the resistivity of the film presents a tenfold increase after increasing the Vg from 2.5 to 3.5 V.The resistivity of the filmdecreases with time after the gate voltage is removed,and we attribute it to the transfer of oxygen ion from grain interior to grain boundary.The transfer rate of oxygen ion is very sensitive to temperature.At temperature around 300 K,one can see 17%decrease in resistivity due to oxygen transfer within 5 hours,while itremains little changewith decreasing temperature below 250 K.Then werestore oxygen level by external heating or Joule heating in air.Finally,we describe a transient effect during the measurement and the interference of the additional voltage signal from the gate voltage.Chapter 5.Electroresistance(ER)has been intensively studied in low-and intermediate-bandwith manganites,which possess the electronic phase separation characteristics.As for the Sr-and Ba-doped large-bandwith manganites,however,few results about ER have been reported so far.Here we report ER effect in oxygen-deficient La0.8Ba0.2MnO3-δ thin films,which were obtained by applying a large electric current(33 mA)to the pristine films in vacuum.While the pristine film displays a negligible change in resistivity with respect to the test current,the oxygen-deficient film shows significant ER effect,i.e.ER ratio of-22%at 260 K under a test current of 0.3 mA.By gradually restoring oxygen content in the films,it is found that the ER effect is closely related to the residual resistivity at low temperatures,demonstrating the key role of grain boundaries.Furthermore,the residual resistivity can be readily tuned by heating the oxygen-deficient films in air,suggesting strong oxygen activity in grain boundaries.The magnetoresistance(MR)data show current dependent feature,also revealing the role of grain boundaries.At 40 K,the MR ratio of the 100℃ restored film under 30 kOe increases from-15%to-25%when decreasing the test current from 1 to 10-3 mA.The large ER effect in the oxygen-deficient films is discussed based upon the conductive filament picture in grain boundaries.Our approach to manipulate the ER effect through oxygen deficiency makes oxide films more promising for potential applications in the memristive devices and neuomorphic computing.Chapter 6.Magnetoresistance(MR)of the La0.8Ba0.2MnO3 ultrathin films with different thicknesses has been investigated.While the 13-nm-thick film shows the commonly reported negative MR effect,the 6-and 4-nm-thick films display an unconventional positive magnetoresistance(PMR)behavior in certain conditions.Besides the film thickness dependence,it is found that the electrical resistivity as well as PMR effect of the thinner films is very dependent on the test current.For example,the MR ratio of the 4-nm-thick film changes from+46%to-37%when increasing the current from 10 to 100 nA under 15 kOe at 40 K.In addition,the two thinner films present opposite changes in electrical resistivity with respect to the test current,i.e.electroresistance(ER)effect,at low temperatures.We discuss the complex MR and ER behaviors by taking the weak contact at grain boundary between the ferromagnetic metallic(FMM)grains into account.The PMR effect can be attributed to the breaking of weak contacts due to anisotropic magnetostriction of the FMM grains under magnetic field.Considering competition effects on the transport properties of conductive filament and local Joule self-heating at the grain boundary,the opposite ER in the two thinner films is also understandable.These experimental findings provide an additional freedom in tuning MR effect in the mangantie films.Chapter 7.We provide a summary of the previous six chapters and make some prospects of the future research on deeply understanding physics behind the oxygen activity phenomenon in the LBMO films.