表面原子尺度建造之自旋的第一原理計算

Abstract

為邁向原子尺度磁系統元件應用的道路，我們與IBM 矽谷研究中心的 掃描穿隧顯微鏡實驗室合作，在特別的表面建造磁原子結構。該表面是在銅 (100)上覆蓋單層氮化銅。此獨一設計使吸附原子的自旋免受傳導電子的遮 蔽，又容許足夠的穿隧電流從掃描探針通到表面來探測自旋激發，並更進一 步讓探針對個別原子進行建構、探測與改造。為了製成具有較強磁性的表面 原子尺度結構，以下有三個重要課題：計算並調控自旋間的耦合、探索單一 原子自旋的異向性、自旋的耦合與異向性之相互影響。第一個課題關係到大 磁化量原子尺度結構的製成，該製成乃藉由將上述表面建造的原子自旋進行 鐵磁耦合。第二者能藉由鎖定原子自旋的方向來幫助建構磁儲存位元。最後 是進一步於原子尺度發展出巨大磁異向性。 研究此一新穎磁系統的方法是相當性先進的密度汎函理論搭配擴增平 面波法。我們首先將尋找具有鐵磁耦合的原子自旋系統並從中學習如何藉由 不同吸附磁原子與排列幾何來改變耦合強度，我們預期會找到數個合適的原 子與排列幾何，並用計算這些不同系統的自旋耦合及其他磁性特質。我們也 將計算不同吸附磁原子的自旋異向性，並以具有f 軌域者為優先。我們也將 研究不同軌道角動量對自旋異向性的影響。把數個具有自旋異向性的原子耦 合成為較大的量子自旋結構，將有助於在原子尺度建構出巨大磁異向性。我 們將會在表面放置同時具有自旋耦合與自旋異向性的原子，並研究這兩個效 應在共存時如何相互影響。

Targeting the device application of atomic-scale magnetic systems (1~10 atomic spins), we will collaborate with the scanning tunneling microscopy (STM) Lab at IBM Almaden Research Center in engineering magnetic atoms on a specially designed surface, an insulating copper-nitride monolayer on top of the Cu(100) surface (to be called CuN surface latter). This unique design provides the opportunity to preserve the spins of magnetic adatoms from being screened by the underlying conduction electrons while at the same time allowing enough tunneling current from an STM tip to probe the spin excitations. Moreover, the magnetic atoms on this surface can be constructed, probed, and manipulated atom-by-atom. In order to fabricate surface-embedded atomic-scale structures with strong magnetism, there are three important topics of study for such a system: How strong is the coupling between two spins on this surface and how can the coupling strength be tuned? What is the detailed structure of the anisotropy of a single atomic spin and how can one possibly control such anisotropy? What is the interplay between the spin coupling and anisotropy? The first topic is related to fabricating of atomic-scale structures with a large magnetization on a surface by coupling those surface-engineered atomic spins ferromagnetically. The second helps construct magnetic storage bits by aligning individual atomic spins. The last can enable further development of giant magnetic anisotropy at the atomic scale. The state-of-the-art Density Functional Theory (DFT) in the Full-potential Linearized Augmented Plane Wave basis will be used to study the magnetic properties of such novel systems. We will first search for magnetic adatoms that exhibit ferromagnetic coupling between their spins and learn how the coupling strength can be changed by comparing different magnetic adatoms and geometries. We expect to find one or more suitable atoms and geometries and plan to calculate their spin coupling and other magnetic properties. The magnetic anisotropy is responsible for the orientation of spins. We will also calculate spin anisotropy of various different magnetic adatoms on the same surface, presumably magnetic atoms with f orbitals. The effect of different orbital angular momenta (e.g. d or f) on the spin anisotropy will be studied. Combining the spin anisotropy with the ability to couple atomic spins into extended quantum spin structures has the potential in completely engineering giant magnetic anisotropy at the atomic scale. We will place on the CuN surface the magnetic atoms that exhibit both spin coupling and anisotropy. The interplay of these two effects upon their coexistence in engineered spin systems will be studied.