Structural And Low-Filed Magnetoresistance Properties Of La_0.7Sr_0.3MnO₃ Polycrystalline Thin Films

Perovskite manganites,La1-xSrxMnO3-δ(LSMO),has been studied intensively over the past years,because they show interesting physical properties due to the coupling of lattice,orbital,spin and electron.Especially,LSMO films are attracting more attention because of their excellent magnetoresistance properties.The magnetic and electric properties of manganites are characterized by two important parameters,TC and TIM.The regime in which ferromagnetism and metallic conduction coexist in the manganites was explained qualitatively by the double exchange(DE)mechanism.Generally,the DE interaction relies strongly on the Mn3+/Mn4+ ratio and affects TC and TIM ultimately.The temperature range of LFMR is restricted because the variation of TIM is always similar to Tc’s transformation.In this work,we have effectively modulated the LFMR properties of LSMO polycrystalline films with the method of regulating Tc and TIM simultaneously by changing deposition temperature.Firstly,we prepared the LSMO films with high quality and investigated the effect of deposition temperature on the structural properties like crystallization quality,surface morphology and microstructure with XRD,SEM and TEM.The results showed that the crystallization quality and grain size increased with increasing the deposition temperature.The surface morphology transformed distinctly for the film deposited above 650 ℃.Dense and columnar LSMO films were achieved with thickness of 350nm approximately and the interface between the film and the substrate is clear and straight.In addition,the effect of deposition temperature on the electron structure of LSMO films was studied with XPS.The results demonstrated that Mn3+/Mn4+ ratio varied with changing the deposition temperature.Secondly,the effect of deposition temperature on the electric and magnetic properties of LSMO films was investigated by PPMS and SQUID.It revealed that the resistivity of LSMO films decreased firstly and then increased again and the MS increased firstly and then decreased again with increasing deposition temperature.TC increased with increasing the deposition temperature up to 700 ℃ and then decreased,but TIM increased firstly with increasing the deposition temperature up to 650℃ and decreases subsequently.The divergence between the TC and TIM(△T = TC-TIM)emerged and the △T increased with increasing deposition temperature from 650 ℃ to 750℃.The simultaneous variation of TC and TIm relies on the variation of Mn3+/Mn4+ratio and the transformation of microstructure.LSMO polycrystalline films exhibited big negative magnetoresistance and the LFMR value increased with increasing the AT.We have regulated TC and TIW of LSMO polycrystalline films simultaneous by changing deposition temperature and broadened the temperature range of LFMR.Therefore,This paper provides an effective way to tune the electric and magnetic properties of LSMO films.