Transport mechanism and magnetothermoelectric power of electron-doped manganites La_(0.85)Te_(0.15)Mn_(1-x)Cu_xO_3 (0 ≤ x ≤ 0.20)

摘要

We present the results of a systematic study of the transport mechanism and magnetothermoelectric power (MTEP) of electron-doped manganites La_(0.85)Te_(0.15)Mn_(1-x)Cu_xO_3 (0 ≤ x ≤ 0.20). Two peaks are observed in thermoelectric power S(T) curves for x < 0.10 samples. For x > 0.10 samples, the very large S value with over 100 μV/K at low temperatures appears, which is attributed to the destruction of ferromagnetic (FM) order and the strong carrier localization at low temperatures due to Cu doping. In addition, a sign variation of S(T) for Cu-doped samples is also observed, which may originate from the narrowing of the concomitant σ(e_g~↑ - 2p) band. Particularly, an anomalous behavior of S(T) is observed in x = 0.10 sample, which is suggested to be related to the contribution of spin polarization and phonon drag. Based on the results of resistivity ρ(T) and S(T), the transport mechanism in the high-temperature paramagnetic region for all the samples and low-temperature FM insulating region below T_C for the samples with x ≤ 0.10 can be described by the variable-range-hopping model. However, in the intermediate-temperature FM metallic region below T_C, ρ(T) and S(T) of the samples with x ≤ 0.10 are well fitted by the formula ρ = ρ_0 + ρ_(2.5)T~(2.5) and S=S_0 + S_(3/2)T~(3/2) + S_4T~4, respectively, implying the importance of electron-magnon scattering. As to the MTEP, only a negative MTEP peak close to T_C is observed in the whole measured temperature range for the samples with x > 0. 10, which is suggested to originate from the spin alignment induced by applied magnetic fields. However, for x ≤ 0.10 samples, an additional positive MTEP peak is induced by Cu doping below T_C besides a large negative MTEP peak in the vicinity of T_C, which is ascribed to the enhancement of electron-magnon interaction caused by the Cu doping and the decrease of magnetic entropy around T_C, respectively.