Effect of Ga doping on charge transport mechanism of La0.85Zr0.15Mn1-xGaxO3

Structural properties, temperature-dependent resistivity rho(T) and thermoelectric power S(T) of La0.85Zr0.15Mn1-xGaxO3 (LZMGO) manganites with 0.0 <= x <= 0.06 have been extensively investigated. It is found that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with increasing Ga content, indicative of a significant distortion of MnO6 octahedra in LZMGO. The local lattice distortion and suppression of double exchange ferromagnetism induced by the Ga doping shift both Curie temperature (T-C) and metal-insulator transition temperature (T-MI) toward lower temperatures. Followed by the metal-insulator transition, rho(T) exhibits an insulatinglike behavior accompanied by a relatively small value of S(T) below T* at low temperatures. An anomalous peak observed in S(T) is attributed to an enhancement of electron-magnon interaction caused by the Ga doping and a decrease in magnetic entropy near T-MI. Three different charge transport mechanisms were identified in three temperature regions based on detailed analyses of rho(T) and S(T) data. The small-polaron hopping mechanism governs the charge transport in the high-temperature paramagnetic region (T>T-MI). The polaron binding energy determined from the rho(T) and S(T) data increases with increasing Ga content, suggesting that polaron in La0.85Zr0.15MnO3 has a magnetic nature. In addition, the electron-magnon scattering dominates the charge transport in the intermediate-temperature metallic region (T*<T<T-MI), whereas the transport behavior in the low-temperature insulating region (T<T*) can be described by the three-dimensional variable-range-hopping model. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3073950]