多自旋态离子Co替代诱导La_(2/3)Ca_(1/3)MnO_3体系的输运反常

Abnormal transport behavior in the mult-spin Co-doped La_(2/3) Ca_(1/3) MnO_3 manganites

利用具有多自旋态的Co离子进行Mn位替代,制备了La2/3Ca1/3Mn1-xCoxO3(0≤x≤0.15)系列样品并研究了体系的结构和输运特性.结果表明,在替代范围内,样品呈现很好的单相结构,各晶格参数随替代量的增大而减小;Co替代导致体系出现电输运反常,具体表现为在居里温度TC以下电阻-温度曲线的二次金属-绝缘转变(M-I)行为(双峰效应),且随Co替代量的增大,无论是高温峰还是低温峰,其峰值温度均向低温区移动;所不同的是,对Co替代样品而言,随外加磁场增加,高温峰值温度TPH向高温区移动,而低温峰值温度TPL则保持不变,表现出磁场无关的特征;相应的峰值电阻率对Co替代和外加磁场表现出很强的依赖关系,随Co替代含量的增加,各峰值电阻率增加,而低温峰值电阻对Co替代更为敏感;对照样品磁特性测量结果,证明高温峰对应于未替代体系的M-I转变,低温峰对应的反常变化则与Co3+离子替代Mn4+后在体系中引入氧缺位和高的自旋态相关联.

The spin structure and polarization characters are the dominant aspects in the physical properties of manganite systems with colossal magnetoresistance effect. In this paper, a series of Mn-site doping samples La2/3Ca1/3Mn1-xCoxO3（0≤x≤0.15） have been prepared with Co ion which has muh-spin states. The structure and transport properties of this system have been systematically investigated. The results showed that all the samples show good single phase and the lattice parameters decrease with the increasing of doping concentration x. The abnormal transport properties are induced by Co doping. To be specific, the resistivity exhibits two metal-insulator （M-I） transitions （double peak effect）. Both high temperature peak （TMIH） and low temperature peaks （ TMIL） shift to low temperature end with increasing x. While the TMIH shifts to high temperature, the TMIL is almost unchanged under the external field. The corresponding peak values of resistivity show strong dependence on the external field and Co doping concentration x. All the peak values of resistivity increase with the increase of x, and the low temperature peak value seems more sensitive than the high temperature peak. Compared with the magnetization properties, it could be concluded that, the high temperature peak is related with the undoped M-I transition, and the abnormal behavior of low temperature peak has certain relationship with the oxygen vacancy and high spin state induced by Co^3＋ substituting Mn^4＋ in the present system.