La_(0.7)Ca_(0.3-x)Sr_xMnO_3薄膜的电输运特性研究

Electrical Transport Properties in La_(0.7)Ca_(0.3-x)Sr_xMnO_3 Thin Films

采用溶胶–凝胶法制备La_(0.7)Ca_(0.3-x)Sr_xMn O_3(LCSMO)薄膜,探讨掺杂对结构、磁性能与电输运特性的影响机制。从X射线衍射(XRD)结果来看,所有薄膜均具有典型钙钛矿结构。LCSMO薄膜的居里温度(T_C)和金属绝缘体转变温度(T_(MI))均随Sr掺杂浓度增加而单调增加。总体看来,当x≤0.05时,LCSMO薄膜磁阻率类似于窄带系LCMO系材料,在T_(MI)周围较宽的温度区间内存在相分离,而相分离过程中多相共存的无序状态是该类材料庞磁阻效应的主要来源。对特定温度下的磁阻率随磁场的变化进行分析,当温度低于T_(MI)时,磁阻率随磁场变化出现双梯度,低磁场时晶界隧穿效应起主导,该部分效应对磁场特别敏感,高磁场时磁阻率主要来源于磁场对自旋波动的压制;当温度接近或高于T_(MI)时,晶界隧穿效应逐渐消失,磁阻率随磁场线性变化,磁场对自旋波动的压制起主导作用。

The influence of the chemical composition on the structure, magnetic and electrical transport properties of La（0.7）Ca（0.3-x）SrxMn O3（LCSMO, x≤0.05） thin films was systematically studied. LCSMO thin films were prepared by Sol-Gel method. XRD results show that all the films have typical perovskite structure. With increase of Sr doping concentration, both Curie temperature TC and metal-insulator transition temperature T（MI） of LCSMO thin films increase monotonously. When Sr doping concentration x≤0.05, with increase of temperature, absolute value of MR decreases first, then increases to a maximum value around T（MI）, and finally decreases, which indicates that the disorder coming from phase separation around T（MI） contributes greatly to MR. The field dependence of MR values at certain temperature can be classified into two kinds. When the temperature is much lower than T（MI）, dual gradient of MR with the magnetic field is shown. The tunneling effect on the grain boundaries plays a dominant part in the low magnetic field, and this effect is particularly sensitive to the magnetic field; the suppression of spin fluctuations by magnetic field is the main reason in the high magnetic field range. When the temperature is higher than T（MI）, MR of LCSMO thin films changes almost linearly with the magnetic field which is mainly due to suppression of spin fluctuations by magnetic field.