實驗及理論探討單根鎳奈米線之熱與電傳遞

Abstract

磁性材料被廣泛應用於宏觀和奈米結構中，包括生物技術、量子計算、記憶技術等，其中一個應用為磁阻隨機存取記憶體（Magnetoresistive random access memory，MRAM）元件中的磁隧道結（Magnetic tunnel junctions, MTJ）。MRAM具有取代現在市面上常見的靜態隨機存取記憶體（Static random access memory，SRAM）和動態隨機存取記憶體（Dynamic random access memory，DRAM）的潛力，故在近十年逐漸開始受到廣泛的關注及開發。在MRAM的結構中，MTJ隨著元件尺寸縮小，而隨之上升的功率密度使得元件的散熱能力成為影響其效能的重要指標。因此，對於MRAM和其他含有磁性材料的微米及奈米元件的電與熱傳遞機制之研究變得至關重要。在固體材料中，主要是透過量子化的晶格震動，也就是聲子，作為非導體中主要的熱量傳遞粒子。在導體中，電子亦會傳遞材料中的熱量。而除了上述的兩種粒子外，在磁性材料中，原子間的磁矩會相互影響而形成自旋波，此自旋波量子化後即可得到磁子。故在鐵磁性材料中，熱的傳遞便是藉由上述三種粒子來傳遞，相較於對電子和聲子的傳遞機制已被廣泛地研究，磁子的傳遞機制尚未完全闡明。許多研究發現在磁性材料中，磁子對於熱傳遞有顯著的貢獻，然而這些研究大多僅探討低溫環境中磁子的熱傳遞機制。而由於一班電子元件的使用皆在室溫下的環境，故探討室溫下的磁子熱傳遞機制是有其必要性的。 鑒於鎳為一常見的鐵磁性金屬材料，因具有良好的物理及機械性質，故被廣泛的使用在許多地方，亦是奈米製程中十分常見的鐵磁性材料，本研究探索了單一鎳奈米線（NiNWs）在外加磁場下的店與熱傳遞機制。本研究以半導體製程製作一微型量測元件，並輔以真空腔體系統量測20K至300K的溫度範圍下不同外加磁場下鎳奈米線的電阻率及熱傳導率。研究中的鎳奈米線是以電鍍法將鎳電鍍於奈米多孔性陽極氧化鋁薄膜中製備而成，並透過掃描式電子顯微鏡、能量散佈光譜儀和X 光繞射。由分析結果可得知本研究製備之鎳奈米線線徑約為 350 nm，為多晶之面心立方晶體結構，晶粒大小約為 20 nm。本研究亦透過超導量子干涉儀得到鎳奈米線之磁滯曲線及使用磁力式顯微鏡觀察不同外加磁場下鎳奈米線中磁域的大小及分布狀況。 根據研究結果，可得知鎳奈米線之電阻率會隨著溫度上升而增加，原因在於溫度上升時，被激發之聲子數量也會隨之增加，導致電子和聲子間的散射越來越劇烈，使得電阻率增加。另一方面，外加磁場對鎳奈米線的電阻率影響較小，在室溫下的飽和磁化率下僅觀察到1.4％至2.4％的電阻率變化。根據研究結果亦可得知鎳奈米線之熱傳導率會隨著溫度降低而變小，主因為鎳奈米線的熱傳導率貢獻主要來自於電子，而奈米線之晶界、線徑、線長和表面粗糙度等會限制電子之平均自由徑。同時，當溫度下降時，被激發之電子數量會隨之減少，使得電子之比熱容亦會隨著溫度下降而減少，進而導致電子的熱傳導率下降。在外加磁場下鎳奈米線之熱傳導率會隨著磁場強度增加而減小，原因在於磁場強度增加時，被激發的磁子數量會減少且磁子平均自由徑會變小，使得磁子對於熱傳導率之貢獻降低。根據實驗結果，可得知在室溫且無外加磁場的環境下，磁子熱傳貢獻率佔鎳奈米線總熱傳導率約22%至33%。本研究亦建立一理論模型來探討磁子之熱傳機制，模型中考慮了四種磁子散射機制，分別為邊界散射、缺陷散射、磁子與聲子間的散射、及磁子與磁子間的散射。根據理論模型和實驗數據的擬合結果可知，磁子的邊界散射、缺陷散射在磁子傳遞的過程中有較為顯著的影響。此外，磁子的平均自由徑會隨著外加磁場強度的增加而變小，且通過比較外加磁場強度對磁子的比熱容、速度和平均自由徑的影響程度，可得知當外加磁場強度增加時，磁子平均自由徑的變好為造成磁子熱傳導率降低的主因。 本研究探討了單根鎳奈米線的電與熱傳遞機制，並藉由推討理論模型來深入了解磁子的熱傳遞機制，得知不同的散射機制對於磁子熱傳遞的影響。本研究探探討之電與熱傳遞機制將對於MRAM和其他含有磁性材料的微米及奈米元件發展有所貢獻。

Magnetic materials employed in macro/nanostructures have widespread applications, such as biotechnology, quantum computing, memory technology, and others. One of the most important implementations is the component in the magnetoresistive random-access memory (MRAM), which is the magnetic tunnel junction (MTJ). However, with the reduction of component size, the increased power density makes the heat dissipation capacity of the component become an important indicator, directly affecting its efficiency. Therefore, a comprehensive investigation of heat and electrical transport properties within magnetic materials in micro/nanostructures becomes essential for developing MRAM and similar devices incorporating magnetic materials. Electrons, phonons, and magnons carry heat transfer in a magnetic material. Compared with the extensive studies on electrons and phonons, the transport mechanism of magnons has not yet been well discussed. Most of the studies only focus on magnon thermal behavior in low-temperature environments. Due to the applicable temperature of the general electronic devices, investigating the magnon heat transfer mechanism near room temperature is required. Since nickel is among the most common ferromagnetic metals in various applications, this study explores the magnetic field effect on the thermal and electrical properties of single nickel nanowires (NiNWs). This study measures the electrical resistivity and thermal conductivity of NiNWs by a measurement microdevice. The NiNWs were fabricated by electroplating using an anodic aluminum oxide (AAO) nanoporous membrane. The synthesized NiNWs were thoroughly characterized. The results show that the NiNWs were polycrystalline with a face-center cubic (FCC) crystal structure and a grain size of approximately 20 nm. In addition, the magnetization of the NiNWs array increased with an elevating magnetic field and reached saturation at around three kOe. Moreover, the NiNW's magnetic domain shifted from a multi-domain distribution to a single domain as the external magnetic field intensity's strength increased. The electrical resistivity of NiNWs increases as the temperature increases, attributed to the more intense scattering between electrons and phonons. On the other hand, the magnetic fields had little effect on the electrical resistivity of NiNWs, only accounting for approximately 1.4% to 2.4% difference at room temperature at the saturated magnetization. The thermal conductivity of NiNWs decreases with decreasing temperature due to the constrained mean free path and the decline in specific heat capacity as temperature decreases. Furthermore, the thermal conductivity of NiNWs decreases as the magnetic field's strength increases due to suppression of magnons, leading to the decrease of the magnon’s mean free path and in turn cause a reduction in magnon thermal conductivity. The contribution of magnon thermal conductivity to the total thermal conductivity of NiNWs is estimated to be around 22% to 33% at room temperature. A theoretical model based on the Boltzmann transport equation, using the full dispersion relation, was established to apprehend the behavior of magnon transport. This model considered four scattering mechanisms: magnon-boundary, magnon-defect, magnon-phonon, and magnon-magnon scattering. The theoretical modeling indicated that the magnon-boundary and magnon-defect scatterings played crucial roles in magnon transport. Furthermore, the total mean free path of the magnon decreases as the strength of the external magnetic field increases. By comparing the effects of the external magnetic field's strength on magnon heat capacity, velocity, and mean free path, it can be confirmed that the predominant factor responsible for the reduction in magnon thermal conductivity under an external magnetic field is the decrease in mean free path. This study investigates the electrical resistivity and thermal conductivity of single NiNWs. The mechanism of the electrical resistivity and thermal conductivity are discussed. The insights gained from this research are anticipated to play a valuable role in the advancement of technologies such as MRAM and other micro/nanodevices that incorporate magnetic materials.