| In this dissertation, sol-gel method was adopted to prepare Re-doped (La1-xLnx)2/3Ca1/3MnO3powders. Through the replacement of La element with Ho and Dy elements with large magnetic moment the change in properties of samples was investigated, such as the electromagnetic properties, the low field magnetoresistance and magnetocaloric effect; role of Lanthanide contraction on the structure and electrical transport performances of samples were also compared and researched through the replacement of La element with most lanthanide.1.(La1-xDyx)2/3Cai/3MnO3series of samples research shows that, electrical transport properties of samples were not only affected by different Dy-doping content and magnetic field were but also closely related with measurement process. It is found that Tc of samples decreases and magnetoresistance of samples increase with the increase of Dy content in the same field, Tc of sample increases with increasing the field in the same Dy content. Contrasting with the warming and cooling R-T curves of (La0.7Dy0.3)2/3Ca1/3MnO3samples under0-3000Gs field, results indicate that increase of Tc as well as decrease of resistance is more obvious during cooling process than the Warming one, and the magnetoresistance of samples is remarkably larger than that of the Warming process; Besides, the increase speed of magnetoresistance of samples during cooling process is also larger than that of the Warming one.2. It demonstrates that magnetic moment of Ho ion has an effect on the magnetic properties of (La1-xHox)2/3Ca1/3MnO3. Magnetic coupling of Mn ion and Ho ion was produced to decrease the low-temperature saturation magnetization with the increase of Ho doping content (x=0.02,0.05and0.1); While the Ho doping content reached higher (x=0.2,0.3,0.4and0.5), the low-temperature saturation magnetization had remarkable Change with the increase of Ho doping content, and the transformation from approximate ferromagnetic states to ferrimagnetic states can be found in the low temperature region with the increase of Ho content; phase transformation from spin cluster-glass to spin glass state can also be found While the transformation to PM state is unconspicuous owing to the existence of ferromagnetic state at low temperature.3. Magnetocaloric effect of (La1-xHox)2/3Ca1/3MnO3(x=0.2,0.5) was investigated through the measurement of M-H around Tc of samples. It demonstrates that the second order magnetic change can be found with the variation of external magnetic field. Magnetic entropy peak of samples (ΔSM) under the5T magnetic field is up to1.19J/kg·K and2.03J/kg·K at100K and152K, respectively. Since the δTFWHM of ΔSm is large, relative rate of magnetic refrigeration (RCP) of samples in the corresponding temperature region is superior. As a result,(La1-xHox)2/3Ca1/3MnO3materials are promising candidate of excellent magnetocaloric materials.4. Structure and transport properties of (La0.8Ln0.2)2/3Ca1/3MnO3(Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho and Er) were compared. It is shown that the average the lattice constant and tolerance factor varies monotonically with the increase of atomic number of doping elements, just with the lanthanide contraction of samples. Transport properties around Tc for samples with different doping elements are remarkably affected. At zero field, Tc of Ce-team (La,Ce,Pr,Nd,Sm and Eu) doped samples exhibit nonmonotonic changes, while the behavior changes from metallic state to insulated state with the increase of atomic number, and Y-team(Gd,Dy,Ho and Er) is in accordance with the results of lanthanide contraction. At a certain field, magnetic moment and mean radius of A-site ions have a remarkable influence on the LFMR of samples; LFMR of Ce-doped (La,Pr,Nd and Sm)samples increase remarkably with the increase of atomic number. |
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