尖晶石結構LiTi2-xMxO4 (M = V and Cr)之電性、磁性與熱性質之研究

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研究生：	徐豐麒 Hsu, Fong-Chi
論文名稱：	尖晶石結構LiTi2-xMxO4 (M = V and Cr)之電性、磁性與熱性質之研究 Investigation of the electrical, magnetic and thermal properties of spinel LiTi2-xMxO4 (M = V and Cr)
指導教授：	彭宗平 Perng, Tsong-Pyng 吳茂昆 Wu, Maw-Kuen
口試委員:
學位類別：	博士 Doctor
系所名稱：	工學院 - 材料科學工程學系 Materials Science and Engineering
論文出版年：	2009
畢業學年度：	97
語文別：	英文
論文頁數：	130
中文關鍵詞：	超導、自旋玻璃、近藤效應、尖晶石、磁性散射、電子對拆散
外文關鍵詞：	superconductivity, spin-glass, Kondo effect, spinel, magnetic scattering, pair breaking
相關次數：	點閱：4 下載：0
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在這個研究中，我們利用固態化學反應的方法，製備具有正尖晶石結構之摻釩 (LiTi2-xVxO4) 與摻鉻 (LiTi2-yCryO4) 的樣品，根據釩與鉻的 L-edge X光吸收光譜，結果顯示釩與鉻離子均為正三價，且占據於八面體晶格位置中。與非磁性鋁離子取代八面體的鈦離子之樣品 (LiTi2-zAlzO4), 及磁性錳離子取代四面體的鋰離子之樣品 (Li1-xMnxTi2O4) 相比較時，在摻釩與摻鉻的樣品中，利用不同離子半徑大小，且具有的磁性釩或鉻取代八面體之鈦離子時，超導轉變溫度快速下降，顯示磁性摻雜與軌域雜化程度對壓抑超導現象，扮演著重要的角色。
在稀薄摻雜濃度範圍內，超導轉變溫度與摻雜量呈現線性下降。相對於軌域雜化比較弱的錳-氧 3d-2p 鍵結強度，這個結果可用相對比較強的釩-氧與鉻-氧之 3d-2p 軌域雜化，來加以解釋超導電子配對破壞 (superconducting pair-breaking) 的效應. 在相同價態與化學環境之下，即使釩離子自旋量子數(s = 1/2) 比鉻離子 (s = 3/2) 來的小，但由於釩離子半徑比鉻離子半徑大，表示釩離子之外層 3d 軌域比鉻離子往外延伸更遠，並使得侷域磁矩與傳導電子之交互作用比較大，對壓抑超導的現象，導致較為明顯的效應。改變外加磁場大小，利用量測比熱性質，可以估計出，摻釩與摻鉻樣品之超導相關的物理參數。
經由扣除傳導電子與聲子之比熱貢獻，由磁性比熱的分佈，隨者外加磁場大小演變的特性，可以推論在低摻雜量之壓抑超導的作用力，主要是類近藤效應 (Kondo-like effect) 所造成的。並利用磁場增加時，將低於 0.4 K 以下的磁性比熱，位移至高溫的特性，得以估計出未摻雜LiTi2O4 的樣品中，磁性缺陷濃度之下限為 651 ppm。
隨者釩與鉻取代量的增加 (x ≤ 0.4, y ≤ 1)，類自旋玻璃 (spin-glass-like) 的特性，開始出現於直流或交流磁化率的量測結果中，這些樣品的電阻率增加，且電性也傾向變成絕緣體行為，在低溫的定溫磁阻會由正磁阻效應，轉變成負磁阻效應，這種現象與自旋玻璃之自旋相關的電性傳輸行為一致。於接近或略大於自旋凍結溫度 (spin freezing temperature) 時，磁性比熱也會出現一個峯值；經由數據擬合，並對組成做外插處理，摻釩樣品之自旋凍結溫度，與摻雜量的 1.74 次方成正比關係，出現類自旋玻璃的釩臨界濃度是 0.037，顯示與先前敘述之磁性比熱，隨不同磁場大小的行為演變，所推論之類近藤效應的結果，互相吻合。

The normal spinel LiTi2-xVxO4 and LiTi2-yCryO4 samples were synthesized by solid state reaction. According to X-ray absorption spectra of V L-edge and Cr L-edge, both cations show trivalent state and occupy octahedral sites. Comparing to the non-magnetic Al substitution for Ti at octahedral site and magnetic Mn substitution for Li at tetrahedral site, the substitution of the magnetic V or Cr for Ti at octahedral site plays an important effect on abruptly superconductivity suppression.
Because the transition temperature shows a linear suppression of superconductivity with diluted magnetic impurities, the strong 3d(V) or 3d(Cr) hybridization with 2p(O) is proposed to explain the pair breaking effect rather than weak hybridization of 3d(Mn)-2p(O). From the ionic radii point of view, one can expect that the more extended 3d(V)-2p(O) hybridized orbital should result in a larger superconductivity suppression in the LiTi2-xVxO4 samples than the LiTi2-yCryO4 samples, even though V3+ (s = 1/2) has smaller spin number than Cr3+ (s = 3/2). The superconductivity properties of the LiTi2-xVxO4 and LiTi2-yCryO4 samples have been also evaluated from their magnetic field dependence of specific heat properties.
By subtracting the specific heat from the conducting electrons and phonons, the field dependences of residual specific heat, from the magnetic contribution, of low substituted samples (x, y ≤ 0.025) are Kondo-like. For the non-substituted LiTi2O4 sample, the minimum content of magnetic defect is estimated to be 651ppm from the magnetic field shifting the magnetic entropy below 0.4 K to higher temperature.
With further increasing doping level (x ≤ 0.4, y ≤ 1), the spin-glass-like behavior is observed in dc and ac susceptibility measurement for both LiTi2-xVxO4 and LiTi2-yCryO4 samples. These samples tend to become insulating and their magnetoresistance changes from positive to negative with increasing doping level at low temperature. This resistive property is consistent with spin-dependent transport property of spin-glass. The magnetic specific heat also shows a corresponding peak slightly larger than the freezing temperature determined from the ac magnetic susceptibility measurement. With increasing the concentration of V, the freezing temperature increases and shows an x1.74 dependence on the substitution level. The minimum doping level of V in the emergence of spin-glass-like, by extrapolation, is 0.037 and the dominant Kondo-like effect in LiTi1.975V0.025O4 is thereby confirmed consistent with the feature of magnetic field dependence of magnetic specific heat.

中文摘要    1
Abstract    3
致謝    5
Contents    I
List of Figures    III
List of Tables    XI

Chapter 1. Introduction    1-1

Chapter 2. Sample Preparation and Characterization    2-1
2-1    Sample Preparation    2-1
2-2    Sample Characterization    2-2
2-2-1    Phase Characterization    2-2
2-2-2    Chemical Composition Determination    2-2
2-2-3    X-ray Absorption Spectroscopy    2-3
2-2-4    Electric Transport Measurement    2-3
2-2-5    DC Susceptibility Measurement    2-4
2-2-6    AC Susceptibility Measurement    2-5
2-2-7    Specific Heat Measurement    2-7

Chapter 3. Theorem of the Magnetism, Specific Heat and Superconductivity    3-1
3-1    Magnetism    3-1
3-1-1    Paramagnetism    3-1
3-1-2    Diamagnetism    3-4
3-1-3    Ferromagnetism, Ferrimagnetism and Antiferromagnetism    3-4
3-1-4    Spin-Glass    3-5
3-2    Superconductivity    3-11
3-3    Specific Heat    3-19
3-3-1    Phonon Specific Heat    3-19
3-3-2    Electronic Specific Heat    3-20
3-3-3    Schottky Anomaly    3-22
3-3-4    Specific Heat of Superconductivity    3-24
3-3-4-1    Electronic Specific Heat in a Magnetic Field    3-25
3-3-4-2    Superconductivity of a Type-II with s-wave Symmetry in Dirty Limit    3-26

Chapter 4. Results and Discussion    4-1
4-1    Characterization of Samples    4-1
4-1-1    X-ray Diffraction − Structure Identification    4-1
4-1-2    Chemical Composition Determination    4-9
4-1-3    X-ray Absorption Spectroscopy    4-10
4-2    Superconductivity Suppression in Low Doping Level            (x ≤ 0.05, y ≤ 0.05)    4-16
4-2-1    Resistive Properties    4-16
4-2-2    Magnetic Properties    4-26
4-2-2-1    Magnetism in Normal State    4-26
4-2-2-2    Magnetism in Superconducting State    4-30
4-2-3    Specific heat properties    4-34
4-2-4    Magnetic Entropy    4-49
4-3    Spin-Glass Transition in High Doping Level (x, y ≥ 0.1)    4-50
4-3-1    Magnetic Properties    4-50
4-3-2    Resistive Properties    4-67
4-3-3    Specific Heat Properties    4-76

Chapter 5. Conclusions    5-1

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