Magnetocrystalline anisotropy energy of Co and Fe adatoms on the (111) surfaces of Pd and Rh
Magnetocrystalline anisotropy energy of Co and Fe adatoms on the (111) surfaces of Pd and Rh
复制标题
DOI：
10.1103/physrevb.81.104426
复制
发表时间：
2010-03-01
影响因子：
3.7
通讯作者：
Hafner, Juergen
中科院分区：
文献类型：
--
作者：
Blonski, Piotr;Lehnert, Anne;Hafner, Juergen
我们对孤立的Co和Fe原子在Pd(111)和Rh(111)上的轨道磁性和磁晶各向异性进行了理论和实验相结合的研究。自旋和轨道矩的理论计算是基于从头算的自旋极化密度泛函理论(DFT)，包括自旋-轨道耦合的自洽处理。计算使用平板模型来表示吸附/底物络合物，并允许完全的结构驰豫，导致吸附原子强烈的内移和适度的底物原子的垂直和横向驰豫。与原子保持在体晶格位置直到表面的理想几何结构相比，弛豫导致了更强的吸附原子/配体杂化。这也反映在轨道矩和磁晶各向异性能(MAE)的结果中。增强的杂化导致吸附原子轨道矩的强烈猝灭，但也导致在衬底中形成大的诱导自旋和轨道矩。因此，我们发现衬底对MAE的贡献比以前基于理想化几何的研究估计的要重要得多。我们还发现了一个令人惊讶的结果，即MAE强烈依赖于吸附位置。相对于(111)表面六方密排空位上的吸附原子，面心立方表面的吸附原子的MAE的大小甚至符号都发生了变化。从电子结构的角度分析了MAE与吸附原子和衬底结合的关系，给出了MAE起源的物理图像。然而，一个根本的问题是对吸附原子轨道矩大小的正确预测。我们认为，这个问题只能通过引入交换势的轨道依赖的后DFT校正来解决。将理论结果与现场平均、元素特定的X射线磁性圆二色谱(XMCD)测量结果进行了比较。低温XMCD谱和磁化曲线揭示了Fe原子在两种衬底上的弱的面外各向异性。有趣的是，Co原子在Rh(111)面上存在面内各向异性，MAE约为-0.6 meV，而Co在Pd(111)和Pt(111)上的面外各向异性是已知的。XMCD测量的轨道与自旋磁矩之比表明，与Fe原子相比，Co原子表现出更强的轨道磁化分量。在理论水平上讨论了轨道力矩与MAE之间的关系，包括诱导衬底磁化的贡献。
We performed a combined theoretical and experimental investigation of the orbital magnetism and magnetocrystalline anisotropy of isolated Co and Fe adatoms on Pd(111) and Rh(111). Theoretical calculations of the spin and orbital moments are based on ab initio spin-polarized density-functional theory (DFT) including a self-consistent treatment of spin-orbit coupling. The calculations use a slab model to represent the adsorbate/substrate complex and allow for a complete structural relaxation leading to a strong inward displacement of the adatom and modest vertical and lateral relaxations in the substrate atoms. Compared to an idealized geometry where the atoms are kept on bulk lattice positions up to the surface, relaxation leads to a much stronger adatom/ligand hybridization. This is also reflected in the results for orbital moments and magnetocrystalline anisotropy energy (MAE). The enhanced hybridization leads to strong quenching of the adatom orbital moments but also to the formation of large induced spin and orbital moments in the substrate. As a consequence, we find that the substrate contribution to the MAE is much more important than estimated before on the basis of studies using an idealized geometry. We also find the surprising result that the MAE strongly depends on the adsorption site. The magnitude and even the sign of the MAE change for adatoms on face-centered cubic with respect to the ones on hexagonal close-packed hollow sites on the (111) surface. The dependence of the MAE on the combination of adatom and substrate has been analyzed in terms of the electronic structure, leading to a sound physical picture of the origin of the MAE. A fundamental problem, however, is the correct prediction of the size of the orbital moments of the adatoms. We suggest that this problem can be solved only via post-DFT corrections introducing an orbital dependence of the exchange potential. The theoretical results are compared to site-averaged, element-specific x-ray magnetic circular dichroism (XMCD) measurements. Low-temperature XMCD spectra and magnetization curves reveal weak out-of-plane anisotropy for Fe adatoms on both substrates. Interestingly, Co adatoms on Rh(111) present in-plane anisotropy with MAE of about -0.6 meV, contrary to the known out-of-plane anisotropy of Co on Pd(111) and Pt(111). The orbital to spin magnetic-moment ratio measured by XMCD shows that the Co adatoms present much stronger orbital magnetization components compared to Fe. The connection between orbital moments and MAE is discussed at the theoretical level including the contribution of the induced substrate magnetization.