Electronic phase separation in the rare-earth manganates (La1−xLnx)0.7Ca0.3MnO3 (Ln = Nd, Gd and Y)
Electronic phase separation in the rare-earth manganates (La1−xLnx)0.7Ca0.3MnO3 (Ln = Nd, Gd and Y)
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DOI：
10.1088/0953-8984/15/19/306
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发表时间：
2003-02
期刊：
影响因子：
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通讯作者：
L. Sudheendra;C. Rao
中科院分区：
文献类型：
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作者：
L. Sudheendra;C. Rao
为了理解相分离的本质，研究了三个系列的锰酸盐的电子输运和磁性，这些锰酸盐的分子式为（La 1-xLnx）0.7Ca0.3MnO3，其中Ln = Nd、Gd和Y，其中只有平均A位阳离子半径rA和相关的无序度发生变化，而不影响Mn 4 +/Mn 3+的比例。所有三个系列的锰氧化物都显示出铁磁性的饱和磁化强度特性，铁磁性Tc随着x的增加而减小，直到临界值x，xc（对于Nd，Gd，Y，xc分别为0.6，0.3，0.2）。当x_{c}$>x > xc时，磁矩明显变小，在TM附近有一个小的增加，TM值随x的增大或rA的减小而略有减小。铁磁组分（x ≤ xc）表现为绝缘体-金属转变，而x_{c}$>x > xc的组分则表现为绝缘体。这些锰酸盐的磁电阻率和电阻率行为与xc周围的组合物中相分离的发生一致，对应于A位阳离子的临界平均半径rAc为1.18 A。当r_{A}^{c }$>rA > rAc或x ≤ xc时，Tc和TIM都线性增加，正如均匀铁磁相所预期的那样。Tc和TM都随着A位阳离子尺寸无序线性降低，如通过方差σ2测量的。因此，σ2的增加有利于绝缘AFM状态。在rA ^c>rA > rAc的组合物中观察到渗透传导。当x_{c}$>x > xc时，绝缘区的电子输运性质符合变程跳跃机制。更有趣的是，当x_{c}$>x > xc时，介电常数的真实的部分（e '）在常温下达到一个很高的值（104 - 106），在低于某一温度时降到一个很小的值（~ 500），其值随频率的降低而减小。
Electron transport and magnetic properties of three series of manganates of the formula (La1−xLnx)0.7Ca0.3MnO3 with Ln = Nd, Gd and Y, wherein only the average A-site cation radius rA and associated disorder vary, without affecting the Mn4+/Mn3+ ratio, have been investigated in an effort to understand the nature of phase separation. All three series of manganates show saturation magnetization characteristic of ferromagnetism, with the ferromagnetic Tc decreasing with increasing x up to a critical value of x, xc (xc = 0.6, 0.3, 0.2 respectively for Nd, Gd, Y). For x_{c}$>x > xc, the magnetic moments are considerably smaller, showing a small increase around TM, the value of TM decreasing slightly with increase in x or decrease in rA . The ferromagnetic compositions (x ≤ xc) show insulator–metal transitions, while the compositions with x_{c}$>x > xc are insulating. The magnetic and electrical resistivity behaviour of these manganates is consistent with the occurrence of phase separation in the compositions around xc, corresponding to a critical average radius of the A-site cation, rAc , of 1.18 A. Both Tc and TIM increase linearly when \langle r_{A}^{c }\rangle $>rA > rAc or x ≤ xc, as expected of a homogeneous ferromagnetic phase. Both Tc and TM decrease linearly with the A-site cation size disorder as measured by the variance σ2. Thus, an increase in σ2 favours the insulating AFM state. Percolative conduction is observed in the compositions with \langle r_{A}^{c }\rangle $>rA > rAc . Electron transport properties in the insulating regime for x_{c}$>x > xc conform to the variable-range hopping mechanism. More interestingly, when x_{c}$>x > xc, the real part of dielectric constant (e') reaches a high value (104 –106) at ordinary temperatures dropping to a very small (~ 500) value below a certain temperature, the value of which decreases with decreasing frequency.