厚度与应变效应对La0.67Ca0.33MnO3薄膜电输运与居里温度的影响

在厚度为25—400nm范围内，系统地研究了 (001)SrTiO_3(STO), (001)LaAlO_3(LAO)衬底上La_0.67Ca_0.33Mn_O.3 (LCMO)薄膜的电输运与居里温度T_C随薄膜厚度及衬底的变化. 结果表明，随薄膜变薄，电阻率ρ增加，T_C降低. 对于同一薄膜厚度，LCMO/STO薄膜的ρ大于LCMO/LAO基上的薄膜的ρ. T_C衬底的依赖关系则与ρ相反. 分析表明，LCMO薄膜的低温区电阻温度(ρ-T) 符合关系式ρ=ρ_0+Bω_s/sin h^2(ω_s/2/k_BT)+CT^n， 其中ρ_0为剩余电阻；等号右端第二项反映软光学模声子对电子散射的贡献；第三项包括其余可能散射机 理在电输运过程中所起的作用；B，ωs（软光学模声子的平均频率）与C都为拟合系数. 高温区的电输运则由小极化子跃迁模型ρ=DT×exp（E_a/k_BT）描述（E_a为极化子激 发能）. 根据ρ_0，ωs，E_a以及T_Ｃ变化，初步讨论了薄膜中的厚度与应变效应. 进一步 研究发现ωs，E_a的变化与T_C相关，从而说明极化子效应为影响T_C变化的主要因素. 关键词： 锰氧化物薄膜 电输运 居里温度 极化子

Thickness and strain effects on electronic transport and Curie temperature in La_0.67Ca0.33MnO_3 films

La_0.67Ca_0.33Mn_O.3 films with thickness between 25 and 400nm were fabricated on (001)SrTiO_3 and (001)LaAlO_3 substrates, using pulsed laser deposition. The magnetization and resistivity ρ of the fil ms were studied as functions of thickness. The reduction of t causes the decrease of the Curie temperature T_C in addition to the increase of ρ of the films. Com pared with the compressive strain in the La_0.67Ca_0.33Mn_O.3/La AlO_3 films, the tensile strain in the La_0.67Ca_0.33Mn_O.3/SrTiO_3 films causes a faster reduction of T_C with t, and a larger value of ρ_０. The low_temperature ρ is found to follow well ρ=ρ_0+Bω_s/sin h^2(ω_s/2/k_BT)+CT^n， with ρ_０ being the residual resistivity. The second term arises fro m the scattering by a soft optical phonon mode, and the third term denotes the other possible transp ort mechanisms. B, ω_s(the frequency of a soft optical mode) and C are fitting parameters. At temperatures higher than T_C, ρ of all the films show activated conduction with ρ=DT×exp（E_a/k_BT）. Here, E_a is the activation energy. Based on the investigation of ρ_０, ω_s and E_a, the thickness and strain effect s in the films are discussed, separately. Further study shows that the variation of T_C is mai nly associated with E_a and ω_s, confirming the crucial role of the electron_phonon coupling in the strain effects on T_C.