Physical Property and Skyrmion Phase Stability Studies of Transition-metal Silicides

本論文探討一系列過渡元素矽化物單晶材料在熱電，熱導，比熱，磁性，以及skyrmion在磁場－溫度相空間中穩定度的研究。針對錳缺陷濃度與晶粒尺寸對矽化錳磁性相圖的影響，研究結果顯示: 錳空缺增加與晶粒徑縮小會抑制矽化錳三維螺旋磁結構形成並降低磁序相轉變溫度。首先利用化學計量比例控制的單晶成長方式，製造出不同晶粒徑大小與不同程度的錳缺陷量。藉由量測不同錳缺陷與晶粒徑大小的矽化錳的電性傳輸特性，比熱，與交流磁化率變化，得以深入探討其間物理特性差異，我們證實矽化錳的螺旋磁序相變溫度隨著晶粒徑增加而上升。透過精密交流磁化率的數據分析，成功發現在磁場－溫度相空間中，螺旋三維磁結構如何在特定磁場與溫度下成核成長，及其相變過程中的系統自由能能量差異的變化。本論文同時探究了矽化鈷鐵與矽化錳單晶樣品間熱力學上的物性差異。在矽化鐵系統中，當三分之一的鐵原子被鈷原子所置換取代時，磁場－溫度相空間中的螺旋三維磁結構區域會增大，而相變溫度也大為提升。Fe0.7Co0.3Si 的螺旋三維磁結構相較於矽化錳樣品展現在更高的相轉變溫度與較低的外加磁場，約 280 ± 50 奧斯特，展現了更穩定的熱力學特性。本論文完整比較了Fe0.7Co0.3Si 與矽化錳樣品於磁場-溫度相空間中的螺旋三維磁結構區域差異和自由能在外加磁場下的變化量。We present a comprehensive study of physical properties for MnxSi, FeSi, CoSi, and Fe1-yCoySi (0.1 ≤ y ≤ 0.7) single crystals, including thermoelectric performance, electrical transport property, heat capacity, and spin susceptibility. Mn vacancy defect level and grain size are shown to modify the magnetic phase diagram of MnSi significantly, especially near the critical regime of skyrmion formation and the helimagnetic phase transition. Crystals grown using controlled nonstoichiometric initial precursors have both grains and intrinsic Mn vacancy defects form in MnSi. The measurement results of transport, specific heat, and AC spin susceptibility are compared for MnSi single crystal samples of various manganese deficiency levels and grain sizes. A finite-size effect of helical phase transition temperature Tc has been identified and verified. The stability of skyrmion state (i.e., the A phase) in magnetic field-temperature (H-T) phase space has been examined using AC spin susceptibility data and compared with the theoretical prediction. Similar studies on Fe1-yCoySi (0.1 ≤ y ≤ 0.7) single crystals are compared with that of MnSi. The range of skyrmion phase in the H-T phase space has been shown enlarged and moved to higher temperatures for Fe1-yCoySi with y = 1/3. Comparing with MnSi, the skyrmion phase of Fe0.7Co0.3Si was found thermodynamically more stable under a much lower and narrower critical field near 280 ± 50 Oe. The stability range in the H-T phase space and the free energy reduction for both the Fe0.7Co0.3Si and MnSi are compared