Studies on the Colossal Magnetoresistance and the Magnetic Properties of (ND,Sr)MnO3 Bulks and Films

• Main Authors: Lin Sheng-Yu, 林盛裕
• Other Authors: Kuo Pao-Chang
• Format: Others
• Language: en_US
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/27177973316293861467

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 87 === The polycrystalline samples of Nd0.7Sr0.3Mn1-xFexO3 and Nd0.7Sr0.3Mn1-xCrxO3 (x=0~0.25) were prepared by the solid-state reaction；while the thin films of Nd0.6Sr0.4MnO3 were grown on LaAlO3 substrates by RF magnetron sputtering at ambient temperature. The structural, magnetic and electric properties have been studied. The results of the doping effect on the structures of bulk samples reveal that there is slight structural change with Fe or Cr doping. However, conductivity and ferromagnetism have been suppressed significantly. The magnetic transition temperature drastically decreases as the Fe content increases, whereas for Cr-doping the transition temperature decreases only a few percent in the doping level of 25%. According to the double exchange theory, the metal-to-insulator transition temperature (Tp) should coincide with the Curie temperature (Tc) for the hole-doped CMR compounds. However, we have found that Tp is much lower than Tc for the samples doped with Fe or Cr. These results may originate from the weakening of the double exchange interaction and the enhancement of the impurity scattering owing to the replacement of Mn3+ by Fe3+ or Cr3+. The x-ray results for the thin films of Nd0.6Sr0.4MnO3 indicate that we have obtained the c-orientation films deposited on LaAlO3 substrate after the oxygen annealing. The films of Nd0.6Sr0.4MnO3 have higher Tc and a comparable MR ratio compared with films of Nd0.7Sr0.3MnO3. The film thickness dependence of MR is related to the variation of the lattice strain associated with the interface mismatch between the substrate and the film. Both the resistivity and MR ratio increase with decreasing film thickness.