鈷氧化物的比熱與磁性研究

Abstract

近年來對於NaxCoO2有許多的研究，但對於高Na含量(x > 0.75)的NaxCoO2相圖，礙於對Na成分比例控制的不成熟，仍然有許多混沌不明的地方。本論文藉由對Na0.82CoO2及Na0.833CoO2兩種鈉成份差異很小的單晶樣品進行低溫比熱量測實驗，並根據外加磁場下比熱數據的變化，以及磁化率量測實驗的數據，我們觀察到Na0.833CoO2單晶樣品具有T = 15 K及 T = 8 K的反鐵磁相變。Na0.82CoO2單晶樣品具有T = 22 K的反鐵磁相變及 T = 6 K的anomaly。本研究使NaxCoO2的相圖更趨完整。 我們還測量Na0.833CoO2單晶磁化率變化，推測T ~ 50 K有個未知相變，並討論Na0.82CoO2、Na0.832CoO2及Na0.833CoO2三個樣品easy axis的異同，試圖更了解NaxCoO2在這1 %鈉含量變化所造成的影響。 這十年來，Ca3Co4O9同樣有許多的研究探討其熱電性質，但多集中在室溫以上的溫區，對於低溫的磁性結構討論較少。我們藉由Ca3Co4O9塊材低溫比熱量測實驗及磁化率實驗，對Ca3Co4O9塊材低溫磁性結構做一檢視，在磁化率量測實驗上看到亞鐵磁態的磁性行為，但比熱數據卻呈現短程有序anomaly，並非長程有序相變。本論文也探討摻雜的Ca3Co4O9，觀察到Ca3Co4O9塊材T = 16 K的anomaly在摻了Bi原子後即消失。

Recently, there have been intensive research activities on NaxCoO2. However, for the phase diagram of NaxCoO2, especially when x > 0.75, there are still puzzles due to the inadequate sodium content control. This thesis compared the differences of single-crystalline Na0.82CoO2 and Na0.833CoO2 by low temperature heat capacity experiments and magnetic measurments. Na0.833CoO2 have antiferromagnetic transitions at T = 15 K and T = 8 K, respecting Na0.82CoO2 has an antiferromagnetic transition at T = 22 K, and an anomaly around T = 6 K. This study intends to further complete the phase diagram of NaxCoO2. We have also measured the magnetic susceptibility of single crystal Na0.833CoO2, and point out that there was an unknown transition aroud T = 50 K. We discusse the differences of the easy axis of the three samples: Na0.82CoO2, Na0.832CoO2 and Na0.833CoO2, and tried to understand the effect made by the 1% Sodium content difference in NaxCoO2. Ca3Co4O9 has also been an interesting compound duo to its thermoelectric properties. Most previous studies focused on the room temperature properties, and rarely discussed those at low temperature. We measure bulk Ca3Co4O9 by heat capacity and magnetic susceptibility experiments at low temperatures. The magnetic susceptibility behaves like a ferrimagnetice, but the specific heat data do not indicate a short range ordering. The 16 K anomaly of Ca3Co4O9 in specific heat disappears when we for Bi-doped Ca3Co4O9.