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Study of the magnetotransport behavior and electrical properties in the colossal magnetoresistance materials La0.7-xLnxPb0.3Mn1-yMeyO3 (Ln=Pr, Nd and Y, Me=Fe and Co)

by Young, San-Lin

Abstract (Summary)
The hole-doped perovskite manganese oxide such as Ln1-xAxMnO3 (Ln = La, Nd, Pr, and A = Ca, Sr, Ba, Pb) is one of the most studied topics in the recent years due to the observation of colossal magnetoresistance (CMR). Basically, LaMnO3 has an almost insulating behavior and on antiferromagnetic arrangement. By substituting a divalent cation (A2+) in place of La3+, LaMnO3 can be driven into metallic and ferromagnetic state. Mixed valence of Mn 3+ / Mn4+ is needed for both metallic behavior and ferromagnetism in these materials. The CMR characteristic occurs in the ferromagnetic state. A systematic investigation of the structural, magnetic and electrical properties in the perovskite colossal magnetoresistance materials La0.7-xLnxPb0.3Mn1-yMeyO3 (Ln=Pr, Nd and Y, Me=Fe and Co) has presented in this thesis. By subatituting Nd, Pr, Y for the La and Co, Fe for the Mn, the substitution effects on the crystallographic deformation, magnetotransport behavior and electrical properties in these compounds have been studied. According to the results of this research, crystallographic distortion is induced by the substitution of smaller ions, Pr or Nd, onto the La-site. Powder $x$-ray diffraction patterns show a crystallographic transition from rhombohedral symmetry (R-3c) to orthorhombic (Pbnm) crystal structure as the doping content is increased. The increase of deformation from R-3c to Pbnm decreases the bond angle of Mn3+¡ÐO2-¡ÐMn4+ , increases the cant of Mn spin, weakens the double-exchange interaction and results in decrease of ferromagnetism, low ferromagnetic transition temperature Tc, eg electron bandwidth and conductivity. However, the great quantity of decrease in resistivity by an external field leads to the increase in the magnetoresistance ratio. We also find that the increase of saturation magnetization results from the contribution of magnetic ion of Pr or Nd. In addition. in contrast to substitution La by magnetic ion of Pr and Nd, the saturation magnetization is decreased as Y content is increased. The zero-field-cool (ZFC) and field-cool (FC) magnetic measurements indicate that the range of spin ordering for Y one is shorter than Pr one or Nd one with the same doping content. It is because of the small ionic radius of Y, which results in larger distortion, increases the bond angle of Mn3+¡ÐO2-¡ÐMn4+, and corresponds low ferromagnetic transition temperature. The distortion induced by Mn-site substitution is not obvious due to the similar radius of Mn, Co and Fe. Powder x-ray diffraction patterns show a single phase of rhombohedral symmetry (R-3c) for Co doped ststem and a slight crystallographic transition from rhombohedral (R-3c) to orthorhombic (Pbnm) symmetry for Fe doped system. Values of temperature dependence of magnetization indicate that the ferromagnetic double-exchange interaction is gradually substituted by the superexchange interaction. The ZFC-FC curves also indicate that long-range spin ordering is progressively substituted by the short-range spin ordering. The substitution of Mn by Co and Fe supresses the double-exchange interaction, decreases the ferromagnetism and the ferromagnetic transition temperature. Due to the synthesis of the substitution of Nd, Pr, Y for La and Co, Fe for Mn, the mechanism of substitution effects are proved different. The substitution of Nd, Pr and Y for La distorts the crystal, decreases the Mn3+¡ÐO2-¡ÐMn4+ bond angle, and results in the transition of properties, while the substitution of Co and Fe for Mn decrease the percentage of ferromagnetic Mn3+¡ÐO2-¡ÐMn4+. The purpose of this thesis is to clear up the role functions of all elements in these compounds and properties of these compounds. Based on the knowledge of these compounds, it would be helpful to control the physical mechanism and improve the characteristics on preparing their thin film devices.
Bibliographical Information:

Advisor:Ying-Chung Chen; Hore-Zen Chen; Tseung-Yuen Tseng; Hung-Duen Yang; Jen-Bin Shi; Lance Horng; Hsiung Chou

School:National Sun Yat-Sen University

School Location:China - Taiwan

Source Type:Master's Thesis

Keywords:field cooling zero long range spin ordering short resistivity ferromagnetic transition temperature antiferromagnetic spi nordering superexchange interaction magnetoresistance ratio saturation magnetization perovskite rhombohedral orthorhombic colossal double exchange

ISBN:

Date of Publication:07/08/2002

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