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標題: | 以第一原理方法計算過渡金屬奈米線的結構, 電子與磁學性質 Ab initio studies of the structure, electronic and magnetic properties of the transition metal linear and zigzag nanowires |
作者: | Jen-Chuan Tung 董人銓 |
指導教授: | 郭光宇 |
關鍵字: | 第一原理,奈米線,磁異向能,自旋極化,自旋軌道交互作用, first principle,magnetic anisotropy energy,transition metal,spin orbital coupling,spin polarization, |
出版年 : | 2008 |
學位: | 博士 |
摘要: | 這幾十年來, 由於實驗技術的進步, 許多奈米尺度的新奇材料不斷的被製造出來, 這些具有低維度結構的物質, 在在的顯示出了許多新奇的物理性質, 像是自然狀態下不具有磁性的物質, 在低維度結構中存在著穩定的磁性的狀態. 又或者像是在低維度的結構中, 有些物質具有相當大的磁各向異性能. 這些低維度的新穎材料, 在可預見的將來, 有相當大的潛能可以被人們加以開發應用. 另一方面這些低維度材料也可以作為驗證基本量子理論的舞台.
在本研究中,我們利用密度泛函理論的方式找出各個過渡金屬元素奈米線的穩定結構,我們發現到幾乎每一個過渡金屬元素奈米線直線(Linear)結構或是鋸齒型(Zigzag)結構都有穩定的鐵磁(FM)或是反鐵磁(AF)的磁性結構,以3d過度金屬為例,像是在鈧(Sc)、鈦(Ti)、鐵(Fe)、鈷(Co)、鎳(Ni)的直線型奈米線中,鐵磁態是穩定的結構而釩(V)、鉻(Cr)、錳(Mn)則有著穩定的反鐵磁結構。而在4d與5d的過渡金屬中我們發現穩定態是反鐵磁的元素有著相同的電子組態, Mo(W), Tc(Re)。而其他的4d與5d過渡金屬元素則是鐵磁態比較穩定。 利用所計算出的態密度(Density of States)曲線我們也討論了這些結構下的自旋極化率(Spin-polarization),我們發現在這樣的結構中,3d的過渡金屬元素像是鈧、釩、錳、鐵、鈷、鎳有著大約90%的自旋極化率,這麼高的自旋極化率顯示這樣的過渡金屬奈米結構將來有機會可以應用在自旋傳輸的裝置上, 然而在4d與5d的過渡金屬中, 我們就沒有發現有任一個元素具有這麼大的自旋極化率。 更有甚者在釩、鉻、錳、鐵直線型奈米線中,鉻的磁晶格膨脹 (magneto lattice expansion)更是達到了54%之譜, 然而在4d與5d的過渡金屬中, 因為平衡態的磁性普遍的比3d過渡金屬來的小, 所以磁晶格膨脹效應也相對的比較小。我們也同時討論了在這些結構中,各個3d過渡金屬元素奈米線的軌道磁矩隨著鍵長的變化關係,非常有趣的是,當我們考慮了自旋與軌道偶合的效應時,4d與5d的過渡金屬元素顯示了許多新奇的物理,好比說若不考慮自旋軌道偶合效應,Ir奈米線的平衡態將具有磁性,然而一但考慮自旋軌道偶合效應,理論計算所給出的穩定態將是不具有磁性的,此外,若自旋磁矩的方向與原子鍊的方向不同,在某些元素中也給出不同的結果。 在4d與5d的過渡金屬奈米線中,被發現了具有巨大的磁異向性(Colossal magnetic anisotropy)這個特殊的物理現象。磁異向性指的是當考慮自旋與軌道交互作用時,系統沿著某個方像具有磁性,而沿著另一個方向(通常與前一個方向垂直)則不具有磁性。這樣的性質非常有趣,首先,對電子來說在這樣的系統下可以算是多了一種選擇定則,因為它限制了電子軌道磁矩的方向, 此外,因為磁矩的方向被限制住了,這使得這樣的系統也某種程度上,可以算是類Ising系統。在我的研究中,鉑(Pt) 、銠(Rh)和 鉿(Hf) 都具有這樣的性質。 本文的最後則同時考慮了兩種不同的磁異向能,分別是古典的磁偶極異向能(shap anisotropy)與量子的磁各向異性能 (magnetocrystalline anisotropy)。 現在因為硬碟容量在相同的體積下越做越大,這表示磁性媒體的體積越做越小,當體積變小時,磁粒子所受的熱擾動就不能忽略了。一般來說,磁粒子所具有的磁能是磁各向異性能與粒子體積的乘積。而熱能為玻茲曼常數(Boltzmann Constant)與溫度的乘積。如果磁能大於熱能,則該磁微粒就不太會受熱能的影響,如果低於熱能,磁能就會被熱擾動所消除。 所以,尋找高磁異向能的材料也是科學家們近年來努力的方向。此外,通過分解的軌道態密度曲線(decomposed density of state)我們也對所計算出的磁各向異性能的方向變化給出了定性上的解釋。 Physical properties in low dimensional systems, such as magnetism at the nanoscale have been a very active research area in the last decades, because of its novel fundamental physics and exciting potential applications. Modern methods of preparing nanostructured systems not only made us possible to investigate the influence of dimensionality on the magnetic properties but also a test ground for quantum mechanics. The magnetic and electronic properties of both linear and zigzag atomic chains of all transition metals (TM) have been calculated within density functional theory with the generalized gradient approximation (GGA). The underlying atomic structures were determined theoretically. It is found that all the zigzag chains, except the nonmagnetic (NM) Ni, Re, and antiferromagnetic (AF) Fe chains which form a twisted two-legged ladder, look like a corner-sharing triangle ribbon, and have a lower total energy than the corresponding linear chains. All the 3d transition metals in both linear and zigzag structures have a stable or metastable ferromagnetic (FM) state whilst some elements such as Y, Tc, Pd, La, Ta, Re, Os, Ir and Pt do not have a stable or metastable FM state. Furthermore, in the V, Cr, Mn, Fe, Co, Mo, Tc, W, and Re linear chains and Cr, Mn, Fe, Co, Ni, Ru, Pd, and Os zigzag chains, a stable or metastable AF state also exists. In the Sc, Ti, Fe, Co, Ni, Zr, Ru, Rh, Hf, and Ir linear structures, the FM state is the ground state whilst in the V, Cr, Mn, Mo, Tc, W and Re linear chains, the AF state is the ground state. It is well known that the spin-orbital coupling (SOC) effect is weak in 3d transition metals but stronger in 4d and 5d transition metals. Dramatically, when we introduce the SOC in our studies, we find that the spin magnetic moments for those 3d TM are almost identical to the spin-polarized results whilst for the 4d and 5d TM, the spin magnetic moments are quite different. Very interestingly, for the Os TM linear chain, it shows a spin magnetic moment if the magnetic moment lies perpendicular to the chain direction whilst if the magnetic moment lies parallel to the chain direction, the spin magnetic moments are almost zero. In general, the parallel orbital moments are larger than perpendicular orbital moments in all TM linear and zigzag chain. The electronic spin-polarization at the Fermi level in the 3d FM Sc, V, Mn, Fe, Co and Ni linear chains is close to 90% or above, suggesting that these nanostructures may have applications in spin-transport devices. The electronic spin-polarization at the Fermi level in the 4d and 5d linear chains is much lower, all smaller than 65%. Interestingly, in the 3d transition metal, V, Cr, Mn, and Fe linear chains show a giant magneto-lattice expansion of up to 54 % whilst this effect is very small in the 4d and 5d transition metals, this is because the spin magnetic moment is small in 4d and 5d transition metals. In the zigzag structure, for all transition metals, the AF state is more stable than the FM state merely in the Cr, and Pd chain. Both the electronic magnetocrystalline anisotropy and magnetic dipolar (shape) anisotropy energies are calculated. It is found that the shape anisotropy energy in the 3d TM may be comparable to the electronic one and always prefers the axial magnetization in both the linear and zigzag structures, whist in the 4d and 5d TM this effect is very small. In the zigzag chains, there is also a pronounced shape anisotropy in the plane perpendicular to the chain axis. Nonetheless, in the FM Ti, Mn, Co, Zr, Nb, Ru, Ir and AF Cr, Mn, Fe, Mo, W, Re linear chains, the electronic anisotropy is perpendicular, and it is so large in the FM Ti, Co, Ru and Ir as well as AF Cr and Tc. Mn, Fe and Tc linear chains that the easy magnetization axis is perpendicular. In the AF Cr and FM Ni, Ru, Pd, W, Os zigzag structures, the easy magnetization direction is also perpendicular to the chain axis but in the ribbon plane. Remarkably, the axial magnetic anisotropy in the FM Ni, Rh, and Ir linear chain is gigantic, being ~12, 11, 18 meV/atom, suggesting that Ni, Rh and Ir nanowires may have applications in ultrahigh density magnetic memories and hard disks. Interestingly, there is a spin-reorientation transition in the FM Fe, Co, Ru, Ta, and Ir linear chains when the chains are compressed or elongated. Large orbital magnetic moment is found in the FM Fe, Co, Ni, and Ir linear chains. Finally, the band structure and density of states of the nanowires have also been calculated to identify the electronic origin of the magnetocrystalline anisotropy and orbital magnetic moment. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/37244 |
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