La_（0.67）Ca_（0.33）MnO_3的电子能态密度以及Nd_（1-x）Zr_xMnO_3的磁化逆转现象研究
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摘要
类钙钛矿结构的稀土锰氧化物因为其显著的CMR效应而引起了人们的兴趣。这类材料不仅在自旋电子器件中具有潜在的应用价值,其本身也为凝聚态和材料物理的研究提供了良好的场所。本论文中,我们利用溶胶—凝胶法分别制备了空穴和电子掺杂的稀土锰氧化物,并对其物理性质进行了较为系统的研究。
在晶粒连通性比较好且经过抛光处理的La_(0.67)Ca_(0.33)MnO_3块体上用磁控溅射的方法制备了La_(0.67)Ca_(0.33)MnO_3/Al2O3/Al隧道结,通过分析不同温度下dI/dV-V曲线得到了La_(0.67)Ca_(0.33)MnO_3在费米面附近电子能态密随温度的变化关系。我们发现,居里温度之下电子能态密度对能量的依赖程度要大于居里温度附近以及之上的依赖程度;用公式G (V ) = G0 [1 +(| V |/V *) n]对费米能级附近(|E - E_F|≤0.3eV)不同温度下的隧穿电流与电压的关系曲线进行了拟合,拟合结果显示,体系在低温下表现出很好的金属性质,并且属于强无序体系,电子—电子相互作用比较明显。
成功地合成了Nd_(1-x)Zr_xMnO_3 (x=0,0.05,0.1)以及Pr1-xZrxMnO3 (x=0,0.05,0.1)化合物。XPS分析结果表明,Zr掺杂后的Nd_(1-x)Zr_xMnO_3 (x=0.05,0.1)和Pr1-xZrxMnO3 (x=0.05,0.1)样品中Mn以Mn2+和Mn~(3+)离子的形式存在,说明样品是电子掺杂型的。Nd_(1-x)Zr_xMnO_3的FC以及ZFC的M-T曲线中,磁化强度均出现了负值,即出现了磁化逆转现象。而同属于轻稀土元素且在元素周期表中与Nd相邻、离子半径相近的元素Pr掺杂的系列样品Pr1-xZrxMnO3 (x=0.05,0.1)在整个温度范围内都没有表现出磁化逆转现象,结合前人的研究结果我们提出,Nd_(1-x)Zr_xMnO_3 (x=0.05,0.1)中磁化强度逆转现象是由两套亚铁磁排列的子晶格Mn2+/Mn~(3+) (M23)和Mn~(3+)/Mn~(3+) (M33)以及它们与Nd~(3+)离子之间负的f-d交换作用共同引起的, Nd~(3+)在13 K以下完全有序,净磁矩与磁场方向相反,导致负的磁化强度。Nd_(1-x)Zr_xMnO_3 (x=0,0.05)的比热与温度的关系进一步证明了上述观点。
Manganites with porverskite structures have attracted researchers’attention not only because their wide prospect of application in electron spin devices, but also they provide platforms for basic research. In this thesis, we prepare hole-doped and electrons-doped manganites by pechini process, and investigate their physical properties systematically.
LCMO/AlO/Al tunnel junction was fabricated with magnetic sputtering technology on the LCMO bulk with good connectivity between grain boundaries which was sufficiently polished. Temperature dependence of density of states (DOS) was investigated via the dI/dV-V curve. It was found that far below the courier temperature the DOS had stronger dependence on energy than that for temperature near and above courier temperature. We fitted the tunneling conductance near the Fermi-level with the equation , the results shows that the system reveals good metallic property and the effect of electron-electron interaction can be quite prominent.
Nd_(1-x)Zr_xMnO_3 and Pr1-xZrxMnO3 compounds was successfully synthesized by sol-gel process, the x-ray photoemission spectroscopy investigation shows that, the Mn cations of the sample are in the divalent and trivalent states after Zr was introduced means that the systems were electron doped. In addition, for Nd_(1-x)Zr_xMnO_3, the M-T curve during both ZFC and FC process shows negative magnetization, which was also called magnetic reversal .In comparison, Pr, being similar with Nd, which belongs to the light thulium material, has no sign of magnetic reversal in higher temperature region. Combining the previous study, we proposed that the appearance of magnetic reversal during the higher temperature region was result from the competition between two antiferromagnetic coupled lattices Mn2+/Mn~(3+) and Mn~(3+)/Mn~(3+), while in low temperature region It may related with the ordering of Nd~(3+) ions .and the peak of the specific heat in low temperature provides an effective evidence for the ordering of Nd~(3+) ions in low temperature.
在晶粒连通性比较好且经过抛光处理的La_(0.67)Ca_(0.33)MnO_3块体上用磁控溅射的方法制备了La_(0.67)Ca_(0.33)MnO_3/Al2O3/Al隧道结,通过分析不同温度下dI/dV-V曲线得到了La_(0.67)Ca_(0.33)MnO_3在费米面附近电子能态密随温度的变化关系。我们发现,居里温度之下电子能态密度对能量的依赖程度要大于居里温度附近以及之上的依赖程度;用公式G (V ) = G0 [1 +(| V |/V *) n]对费米能级附近(|E - E_F|≤0.3eV)不同温度下的隧穿电流与电压的关系曲线进行了拟合,拟合结果显示,体系在低温下表现出很好的金属性质,并且属于强无序体系,电子—电子相互作用比较明显。
成功地合成了Nd_(1-x)Zr_xMnO_3 (x=0,0.05,0.1)以及Pr1-xZrxMnO3 (x=0,0.05,0.1)化合物。XPS分析结果表明,Zr掺杂后的Nd_(1-x)Zr_xMnO_3 (x=0.05,0.1)和Pr1-xZrxMnO3 (x=0.05,0.1)样品中Mn以Mn2+和Mn~(3+)离子的形式存在,说明样品是电子掺杂型的。Nd_(1-x)Zr_xMnO_3的FC以及ZFC的M-T曲线中,磁化强度均出现了负值,即出现了磁化逆转现象。而同属于轻稀土元素且在元素周期表中与Nd相邻、离子半径相近的元素Pr掺杂的系列样品Pr1-xZrxMnO3 (x=0.05,0.1)在整个温度范围内都没有表现出磁化逆转现象,结合前人的研究结果我们提出,Nd_(1-x)Zr_xMnO_3 (x=0.05,0.1)中磁化强度逆转现象是由两套亚铁磁排列的子晶格Mn2+/Mn~(3+) (M23)和Mn~(3+)/Mn~(3+) (M33)以及它们与Nd~(3+)离子之间负的f-d交换作用共同引起的, Nd~(3+)在13 K以下完全有序,净磁矩与磁场方向相反,导致负的磁化强度。Nd_(1-x)Zr_xMnO_3 (x=0,0.05)的比热与温度的关系进一步证明了上述观点。
Manganites with porverskite structures have attracted researchers’attention not only because their wide prospect of application in electron spin devices, but also they provide platforms for basic research. In this thesis, we prepare hole-doped and electrons-doped manganites by pechini process, and investigate their physical properties systematically.
LCMO/AlO/Al tunnel junction was fabricated with magnetic sputtering technology on the LCMO bulk with good connectivity between grain boundaries which was sufficiently polished. Temperature dependence of density of states (DOS) was investigated via the dI/dV-V curve. It was found that far below the courier temperature the DOS had stronger dependence on energy than that for temperature near and above courier temperature. We fitted the tunneling conductance near the Fermi-level with the equation , the results shows that the system reveals good metallic property and the effect of electron-electron interaction can be quite prominent.
Nd_(1-x)Zr_xMnO_3 and Pr1-xZrxMnO3 compounds was successfully synthesized by sol-gel process, the x-ray photoemission spectroscopy investigation shows that, the Mn cations of the sample are in the divalent and trivalent states after Zr was introduced means that the systems were electron doped. In addition, for Nd_(1-x)Zr_xMnO_3, the M-T curve during both ZFC and FC process shows negative magnetization, which was also called magnetic reversal .In comparison, Pr, being similar with Nd, which belongs to the light thulium material, has no sign of magnetic reversal in higher temperature region. Combining the previous study, we proposed that the appearance of magnetic reversal during the higher temperature region was result from the competition between two antiferromagnetic coupled lattices Mn2+/Mn~(3+) and Mn~(3+)/Mn~(3+), while in low temperature region It may related with the ordering of Nd~(3+) ions .and the peak of the specific heat in low temperature provides an effective evidence for the ordering of Nd~(3+) ions in low temperature.
引文
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