鎵於錳位置的摻雜對La0.85Zr0.15MnO3載子傳輸機制與磁電阻的影響

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Title:	鎵於錳位置的摻雜對La0.85Zr0.15MnO3載子傳輸機制與磁電阻的影響
Other Titles:	Effect of Ga substitution at the Mn site on transport mechanism and magnetoresistance of La0.85Zr0.15MnO3
Authors:	鄭景鴻;Cheng, Jing-hung
Contributors:	淡江大學物理學系碩士班林大欽;Ling, Dah-chin
Keywords:	小偏極子躍遷;電子-磁振子散射;變程躍遷;small-polaron hopping;electron-magnon scattering;variable-range-hopping
Date:	2008
Issue Date:	2010-01-11 02:22:59 (UTC+8)
Abstract:	我們有系統地對於La0.85Zr0.15Mn1-xGaxO3的多晶塊材樣品(LZMGO)進行結構、電性ρ(T)、磁性M(T)與熱電性質S(T)的研究。結構分析顯示隨著Ga摻雜量的增加，系統的 Mn-O-Mn 鍵角遞減，而Mn-O鍵長遞增，意味著Ga的摻雜使得LZMGO中的MnO6錳氧八面體被扭曲，晶格尺寸因而縮小，減弱了Mn2+-O-Mn3+間的雙交換作用。因此，系統的居禮溫度(TC)，金屬-絕緣轉變溫度(TMI)皆隨著 Ga摻雜量的增加往低溫移動，伴隨著在H=5T時最大磁阻比值，隨Ga摻雜量的增加從45%增加至55%。在TMI附近，系統的 S(T)出現一個隨著Ga摻雜量增加而遞增的峰值，主要成因來自於系統中載子與磁振子的交互作用隨著Ga摻雜量增加而漸增，以及系統自旋組態亂度在溫度稍低於TMI時的降低。在更低溫區域，當T<T時，LZMGO的ρ(T)呈現類絕緣體的行為且 S(T)的值變得很小。仔細地分析ρ(T)與S(T)的數據發現在高溫(T>TMI)、中溫(T<T<TMI)、低溫(T<T)區域，分別為小偏極子躍遷、電子-磁振子散射與三維的變程躍遷來主導載子的傳輸性質。在高溫順磁區域，小偏極子的躍遷距離隨著Ga摻雜量增加而遞減；其活化能隨著Ga摻雜量增加而遞增，顯示小偏極子的形成與 MnO6八面體因Ga摻雜所產生的扭曲有密切的關係。在中溫鐵磁區域，由於Ga的摻雜降低了系統中自旋波的剛性，提昇了載子與磁振子散射的機率，因而主宰了此溫區的載子傳輸機制。在低溫絕緣區域，Ga3+離子無序地分佈所建立的無規式位能井，造成LZMGO的侷域態呈現無規的分布。因此，載子的傳輸是以三維的變程躍遷方式來進行的。 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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