钙钛矿锰氧化物La_(1.2-x)Tb_xSr_(1.8)Mn_2O_7(x=0,0.05)的磁熵变和电输运性质研究

Magnetic Entropy Change and Electrical Transport Properties of Bilayered Perovskite Manganite La_(1.2-x)Tb_xSr_(1.8)Mn_2O_7(x =0,0. 05)

采用传统固相反应法制备钙钛矿锰氧化物La_(1.2-x)Tb_xSr1.8Mn2O7(x=0,0.05)多晶样品,通过测量样品的磁化强度与温度关系曲线(MT)和磁化强度与外加磁场关系曲线(M-H)对两样品的磁熵变研究发现:在整个温度测量区间内,x=0和x=0.05两样品在高温部分均表现出顺磁性;随着温度的降低,两样品分别在245和225 K处发生了二维短程铁磁有序转变(T_C^(2D));在120和70 K处发生了三维长程铁磁有序转变(T_C^(3D));在低温部分,两样品均表现出团簇自旋玻璃行为。x=0样品在T_C^(3D)附近的磁相变为一级相变,x=0.05样品为二级相变,两样品的磁熵变曲线的对称性良好,且在外场2 T下两样品在T_C^(3D)附近出现的最大磁熵变值分别为:2.17和1.60 J·(kg·K)^(-1),因此两样品有利于应用在埃里克森磁制冷循环中。此外,通过测量零场下电阻率与温度关系曲线(ρ-T)对两样品的电输运性质研究发现:x=0样品和x=0.05样品分别在96和93 K处发生了绝缘-金属转变(T_P),对T_P以上的ρ-T曲线拟合表明,两样品在高温部分均遵循三维变程跳跃的导电方式,Tb3+离子的掺杂没有改变高温区的导电方式。

The polycrystalline samples of double-layered perovskite manganite La1.2-xTbxSr1. 8Mn2O7（x = 0,0. 05） were prepared by traditional solid state reaction method. The nature of magnetic phase transitions,the magnetic entropy change and possible applications in magnetic refrigeration were explored by measuring and analyzing magnetization curves M（H,T）. In the temperature range measured,the samples with x = 0 and 0. 05 showed paramagnetic behavior at high temperature. At lower temperature,the samples with x = 0 and 0. 05 formed a two-dimensional short-range order at TC2D= 245 and 225 K,and then formed an three-dimensional long-range ferromagnetic order at TC3D= 120 and 70 K,respectively; when at further lower temperature below TC3D,both samples showed cluster spin glass behavior. Near the TC3D,the sample with x = 0 exhibited the characteristic of a first-order transition and gave a maximum magnetic entropy change of 2. 17 J·（kg·K）-1under a 2 T magnetic field,while the sample with x = 0. 05 only displayed a continuous second-order transition and gave maximum magnetic entropy change of 1. 6 J·（kg·K）-1. Therefore,the two samples were beneficial to the application of Erickson magnetic refrigeration cycle. In addition,the electrical transport properties of the samples were investigated by resistivity-temperature（ρ-T） curve. The results showed that the x = 0 sample and the x = 0. 05 sample appeared an insulator-metal transition（TP） at 96 and 93 K,respectively. The fitting to ρ-T curves at temperatures above TPshowed that,the electron conducting mechanism of both samples could be attributed to three-dimensional variable-range hopping in the high temperature range.