"以第一原理計算Fe/(Ga,Mn)As介面與Na2Ni2TeO6的電子與磁性結構"

Abstract

本文以密度泛函理論及投影擴充波方法研究兩個低維度系統鐵/砷化鎵錳雙層膜及碲酸鎳鈉的電子與磁性結構。為了更準確地處理系統中的鐵, 錳, 鎳原子，我們運用哈柏模型配合密度泛函理論描述局域d電子間的交互作用。鐵, 錳, 鎳的d電子是這兩類材料磁性的來源，我們將以海森堡模型分析局域磁矩之間的交互作用。 砷化鎵錳是被廣泛研究的稀磁半導體，即在半導體中參雜少量錳使得材料帶有磁性，配合上已發展的半導體技術可望成為自旋電子學可應用的材料。但由於量測到的居禮溫度介於200 K以內，無法成就室溫的應用。為了提升材料磁性存在的溫度，實驗上發現可在砷化鎵錳表面附加鐵磁性金屬的薄膜，藉由磁邊際效應極化鐵磁性金屬(例如鐵)附近數奈米內砷化鎵錳的磁矩。X光磁圓偏振二向性實驗量測到鐵/砷化鎵錳雙層膜中鐵與錳的磁矩反鐵磁耦合，目前尚缺乏理論方面的解釋。透過第一原理計算我們嘗試了解鐵與錳磁矩間的耦合機制，並計算出耦合的強度。另外我們從考慮自旋軌道耦合的計算得到鐵/砷化鎵錳雙層膜的非共線磁性結構。在砷化鎵錳受到張力的情況下，靠近鐵薄膜的錳的磁矩在平行介面方向有較大的分量，而越遠離鐵錳的磁矩方向越接近垂直於介面。我們利用描述磁區的模型來了解錳磁矩方向的變化。 碲酸鎳鈉為近年發現的材料，緣起於實驗學家尋找以鈉離子電池可替代鋰離子電池的可能性。碲酸鎳鈉中鎳原子處在二維的六角晶格上，鎳原子層與層之間有著高離子導電性的鈉原子層。我們計算數種可能的鎳原子磁性結構，並由計算所得的總能量用海森堡模型反推磁矩間的耦合強度。結果顯示鎳原子磁矩層與層間有微弱的反鐵磁耦合，而層內的磁矩形成帶狀的結構，相鄰兩帶之間磁矩反向排列。得到磁矩間的耦合強度後我們以平均場假設估算材料的居禮-外斯溫度約在52 K左右。

In this work we use density functional theory (DFT) and projector augmented-wave method to study the electronic and magnetic properties of two low-dimensional systems: Fe/(Ga,Mn)As bilayer and Na2Ni2TeO6. In order to accurately describe the Fe, Mn, and Ni atoms in these systems, we adopt the DFT+U approach, where the on-site Coulomb interaction (U) between localized d- electrons is treated by Hubbard model. The magnetic moments of these systems originated from the d-electrons. We will use Heisenberg model to describe the interactions between the localized magnetic moments. (Ga,Mn)As is a widely studied dilute magnetic semiconductor; by doping Mn impurities into the semiconductor GaAs, the material becomes ferromagnetic. It is anticipated that the developed semiconductor technology can pave the way for spintronics application. However, the highest value of Curie temperature reported falls below 200 K, and thus restricts practical applications. To raise the Curie temperature, one approach is to apply a thin film of iron on the (Ga,Mn)As surface. Experimentalists found that indeed the Mn atoms within a few nanometers near the interface become spin-polarized at room temperature due to magnetic proximity effect. Element-specific spectra measured by X-ray magnetic circular dichroism show that the magnetic moments of Fe and Mn near the interface align antiparallel to each other. We try to understand from first-principle calculations why the antiferromagnetic coupling is energetically favorable and find out the coupling strength. On the other hand, we perform calculations including spin-orbit coupling to obtain the non-collinear magnetic structure. If the (Ga,Mn)As is under biaxial tensile strain (along directions parallel to the interface), the magnetic moment of a Mn atom near the interface is almost parallel to that of Fe (whose magnetic moments are parallel to the interface), and perpendicular to the interface for Mn away from the interface. We fit the direction-changing local magnetic moments to a simple domain wall model. Na2Ni2TeO6 is a new material recently discovered in the experimental efforts to replace lithium batteries with sodium batteries. The nickel atoms locate on layers of honeycomb lattice; between two Ni layers is one layer of conductive sodium ions. We calculate the total energies of several possible magnetic configurations, and deduce the magnetic coupling constants between Ni atoms assuming classical Heisenberg model holds. Results show that the magnetic couplings between interlayer Ni is weakly antiferromagnetic, and magnetic moments of Ni within one layer form a stripe-like structure; the magnetic moments within adjacent stripes align antiparallel to each other. The Curie-Weiss temperature is calculated to be 52 K according to mean-field approximation.