He-doping Effects In The Manganese Oxide Films

In recent years,perovskite manganites have been extensively studied due to their rich physical properties,such as multiferroicity,colossal magnetoresistance and thermoelectricity.As a typical kind of strongly correlated materials,there are strong couplings between spin,orbital,charge,lattice and other degrees of freedom in manganese oxides.This strong coupling results in the physical properties of manganese oxides being very susceptible to modulation by a variety of external excitatory factors.For manganese oxide films,researchers always focus on choosing different single crystal substrates to modify epitaxial strain in the films,so as to regulate the crystal structure and a variety of physical properties of the films.However,this way of adjusting the epitaxial strain of thin films usually leads to extra multi-dimensional lattice distortion,which greatly increases the uncertainty of studying the exact role of epitaxial strain on film properties.Therefore,we expect to explore a flexible and uniaxial method to change the strain state in epitaxial films.Considering the characteristics of He atoms such as small volume and chemical stability,we intend to insert helium atoms into the interstitial void of film’s lattice structure,so as to change the lattice constant of the films uniaxially in the out-of-plane direction,and achieve the so-called He-doping effect.In this thesis,we take manganese oxide films as the subject of our research.Firstly,He-doping process was preliminarily explored based on the tensile strained La0.7Sr0.3MnO3/SrTiO3 films.Then,through continuously optimizing the He-doping process conditions,the helium doping effect of the La0.7Ca0.3MnO3/SrTiO3 was studied.Finally,we investigated the He-doping effect of La0.8Ca0.2MnO3 films grown on different substrates.Chapter 1:As the introduction of this whole thesis,we provided a detailed description of the lattice structure characteristics of perovskite oxides,the main indirect exchange interactions and orderings in manganese oxides.Immediately after,we discussed the effect of epitaxial strain on the physical properties such as lattice structure and transport characteristics of manganese oxide epitaxial films.Then,we illustrated the dependence of transition temperature on lattice structure and other degrees of freedom of manganese oxide films and introduced the Millis phenomenological model.Finally,we introduced the He-implantation technology and its application in the regulation of physical properties of thin films.Chapter 2:In this chapter,we first briefly introduced several mainstream experimental methods of film preparation.Then,the basic principle of magnetron sputtering technology were explained in detail,as well as the experimental procedures for preparing thin films using this technology.Finally,several experimental methods for characterizing the structure of thin films and measuring the physical properties of films were introduced,which include X-ray diffraction(XRD),reciprocal maps(RSM),atomic force microscope(AFM),and low-temperature magnetic and electrical transport measurement techniques.Chapter 3:We explored the He-doping process for in-plane tensile strained(001)La0.7Sr0.3MnO3/SrTiO3 films,and proposed a direct He-doping approach by post treating as-grown LSMO films in He glow-discharge plasma.The experimental results showed that although neither X-ray diffraction patterns nor magnetic measurements characterized the He-doping effect induced by this direct He-doping approach,by observing the surface morphology of the films by AFM and measuring the electrical transport properties,we determined that this direct He-doping method had some degree of effect on the films.Therefore,we identified the possibility of the existence of He-doping effect in manganite films.Meanwhile,we need to find more suitable research objects and efficient He-doping approachs.Chapter 4:We chosed the in-plane tensile strained(001)La0.7Ca0.3MnO3/SrTiO3 funs as research object and proposed a so-called He-co-sputtering method for the He doping.We used magnetron sputtering technique to prepare a series of films with different He-doping amounts at different Ar/O2/He gas flow ratios.Trough the characterization of X-ray diffraction pattern,we have confirmed that the out-of-plane lattice collapsed by in-plane tensile strain in LCMO films can be stretched by the He-co-sputtering method,with the out-of-plane lattice elongating from 2.823 A to 3.845 A.At the same time,accompanied by the lattice expansion in the out-of-plane direction,the metal-insulator transition temperature of the films also increases from 260 K to 280 K.In addition,when we carried out high-temperature annealing on the He-doped films,a massive helium was released from the films at temperatures around 540℃,which in turn results in the degradation of the He-doping effect in the films.Considering the enhanced Jahn-Teller distortion due to the in-plane biaxial strain in the films,we attributed the elevated transition temperature to the distortion relaxation induced by He doping in the tensile strained films.The effective He doping by magnetron sputtering technique provides a simple strategy for manipulating functionality of oxide films.Chapter 5:In this chapter,we conducted a further study on He-doping effect in in-plane tensile strained La0.7Ca0.3MnO3/SrTiO3 films.Here,we proposed two possible mechanisms for the He doping process and verified them by experiments.We carried out He plasma treatment for the target surface and He atom adsorption on the substrate surface respectively before/between sputtering deposition.The films deposited after the target surface pretreatment showed remarkable changes in the out-of-plane lattice and magnetotransport properties,while the films deposited trough the later way changed little.Accordingly,it was demonstrated that the He ion penetration in the cathode target plays a key role for the He doping in the manganite films.Inspired by this result,we adopted a more direct method to realize the He-doping in thin films,i.e.,placing the as-grown films at the cathode and exposing them directly to the He discharge plasma.After the He plasma post-treatment,the out-of-plane lattice constant increased from 3.823 A to 3.851 A,accompanied by an increase in the metal-insulator transition temperature from 260 K to 290 K.The results demonstrate that the He plasma treatment is a simple strategy for the single-axis control of epitaxial thin films.Chapter 6:We deposited(001)La0.8Ca0.2MnO3 films on SrTiO3,LSAT and LaAlO3 substrates,respectively,and obtained a series of He-doped films by using different He-doping methods.By measuring the lattice structure,electrical transport and magnetic properties,we found that for the La0.8Ca0.2MnO3/SrTiO3 inplane-tensile strain films,He-doping caused c-axis lattice of the films to increase dramatically from 3.833 A to 3.860 A,meanwhile,the transition temperature increased from 148 K to 172 K dramatically.While for the in-plane compressive strain LCMO films grown on LSAT substrates,although the metallicity of the films increased substantially with He-doping,i.e.,the resistivity shows an order of magnitude decline,there was no significant evolution of the lattice structure with He-doping.Finally,we performed He-doping experiment on La0.8Ca0.2MnO3/LaAlO3 films with more severe in-plane compressive strain,the experimental results show that He-doping effect can be observed in terms of lattice structure,but the electrical measurement was unable to characterize the He-doping effect.