| Summary: | 碩士 === 淡江大學 === 物理學系碩士班 === 96 === Structural properties, temperature-dependent resistivity (T), temperature-dependent magnetization M(T), and thermoelectric power S(T) of La0.85Zr0.15Mn1-xGaxO3 (LZMGO) manganites with 0.0 x 0.06 were extensively investigated. It was found that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with increasing the Ga content, indicative of a significant distortion of MnO6 octahedra in LZMGO. Both Curie temperature (TC) and metal-insulator transition temperature (TMI) shift toward lower temperatures accompanied by an increase of maximum magnetoresistance ratio in H = 5 T from 45% to 55% as the Ga content increases. A anomalous peak observed in S(T) attributed to an enhancement of electron-magnon interaction caused by the Ga doping and a decrease of magnetic entropy near TMI. (T)of the system exhibits an insulating-like behavior with a relatively small value of S(T) below T* at low temperatures. Three different charge transport mechanisms were identified for LZMGO in three temperature regions based upon detailed analyses of (T) and S(T) data.The small-polaron hopping mechanism governs the charge transport in the high-temperature paramagnetic region (T > TMI).It should be noted that the polaron binding energy determined from the (T) and S(T) data increases with an increase in the Ga content, suggesting that the Ga-doping-induced local lattice distortion of the MnO6 octahedra predominately involves with the small-polaron formation in LZMGO.In addition, the electron-magnon scattering predicted for an itinerant ferromagnet dominates the charge transport in the intermediate-temperature metallic region (T* < T < TMI), whereas the transport behavior in the low-temperature insulating region (T < T*) can be described by the three-dimensional variable-range-hopping model as a result of random potential at Mn-site arising from Ga doping.
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