Sm₂O₃ And Eu₂O₃The Second Phase Introduction Pair La_0.67Ca_0.33MnO₃ The Influence Of Electromagnetic Transport Properties

The doped perovskite manganites have been an important branch of the strongly correlated electronics materials due to their plentiful physical properties,such as nearly100%spin polarization,insulator-metal?I-M?transition,charge ordering and the colossal magnetoresistance?CMR?etc,and the potential application in magnetic memories,optoelectronic devices,etc.However,the CMR effect based on the double exchange mechanism only occurs in a narrow temperature range near the Curie temperature?Tc?and requires a large magnetic field,which restrains its practical application.In contrast,the low field magnetoresistance?LFMR?related to the grain boundaries occurs over a wide temperature range and requires only a small magnetic field,which may have greater development potential in practical application.But a relatively high LFMR is always achieved in a significant low temperature range while the LFMR near Tc is really smal.In this thesis,the preferentially dopped La_0.67Ca_0.33MnO₃?LCMO?was synthesized by the sol-gel metnod,and it was used as the matrix material.Sm₂O₃ and Eu₂O₃,which are chemically similar but have different physical properties,were introduced to the interface of the LCMO by the solid state sintering,respectively.It is desirable to achieve a larger LFMR over a higher temperature range close to Tc and gain a better understanding of the electaic transport and magnetoresistance properties of the materials by regulating Sm₂O₃/Eu₂O₃ adding amount x?mol%?,the sintering temperature T and holding time t.The structural characterizations of all samples were carried out by X-ray diffraction?XRD?.The surface morphologies and microstructures of the samples were observed by field emission scanning electron microscope?FE-SEM?.The temperature-dependent electrical transport properties and the transversal MR were measured by a standard dc four-probe method for all materials.The experimental results confirmed that:?1?LCMO and Sm₂O₃/Eu₂O₃ coexist in the materials,and with the increase in the addition amount of Sm₂O₃/Eu₂O₃,the sintering temperature and the holding time,the substitution of Sm³⁺/Eu³⁺ions to the A-site of LCMO rises;?2?The adding of Sm₂O₃ and Eu₂O₃ has a significant impact on the electrical transport properties of LCMO.At T=1000°C,t=3h,the resistivity of LCMO-Sm₂O₃/Eu₂O₃ composites increased rapidly with the increase in the content of Sm₂O₃ and Eu₂O₃,and two I-M transitions were observed in LCMO-Sm₂O₃ composites.The reason for the increase of the resistivity of the composites may be that as the amount of Sm₂O₃ and Eu₂O₃ increases,the volume fraction of the grain boundary and the magnetic disorder increase,which enhances the scattering effect of the grain boundary.With the increase of sintering temperature and holding time,the resistivity of LCMO-Sm₂O₃ composites decreases first and then increases,while the resistivity of LCMO-Eu₂O₃ composites decreases.This may be because that:on the one hand,as the sintering temperature and holding time of the composites increase,the substitution of Sm³⁺/Eu³⁺ions for the A-site increases,leading to the weaken of the double exchange.On the other hand,as the LCMO grains grow,the grain boundary volume fraction is reduced.?3?The adding of Sm₂O₃ and Eu₂O₃ also improves the magnetoresistance properties of LCMO.A vertical magnetic field of 500 mT was applied.At T=1000°C and t=3 h,an enhanced LFMR of below²60 K was observed in LCMO-0.03Sm₂O₃ composite;at T=1100°C,t=3 h,an enhanced LFMR with a variation of no more than 3%was observed in the-0.03Sm₂O₃ composite in the range of 150-250 K;at T=1100°C,t=5h,the LFMR was more than 50%in the LCMO-0.03Sm₂O₃ composite close to 100 K;at T=1000°C,t=7 h,an enhanced LFMR was observed in the LCMO-0.01 Eu₂O₃composite in the range of¹50-250 K.Overall,the magnetoresistance properties of LCMO-Sm₂O₃ composites is superior to that of LCMO-Eu₂O₃ composites.