Exchange interactions and spin switching at the single atom level

交换相互作用在磁学理论中具有重要的基础意义。通常，它的强度——在海森堡模型中用交换常数表示——是在实验中通过测量非弹性自旋激发（例如中子或电子）的影响间接确定的。在配备磁尖的原子力显微镜（AFM）上，也可以直接探测交换相互作用。这项技术在2007年首次被证明，被称为磁交换力显微镜。它已被用来获得磁尖和磁表面之间的交换相互作用。然而，到目前为止，还不可能直接探测与单个原子或分子的交换相互作用。另一方面，不同的研究小组利用扫描隧道显微镜（STM）的尖端对单原子进行了接触测量。最近，已经证明可以使用磁尖将STM和AFM结合起来，以便同时测量自旋极化（SP）隧道电流和交换（EX）相互作用-一种被称为SPEX成像和光谱的技术。在这个提议的项目中，我们的目标是通过密度泛函理论（DFT）计算来理解和指导使用SPEX技术进行的实验。在过去，DFT已经成为解释扫描隧道或原子力显微镜实验不可缺少的工具。在这里，我们将应用DFT来研究磁尖与磁性表面上的磁性原子和二聚体之间的交换相互作用。在与来自奈梅亨内梅亨大学的实验伙伴的合作下，我们希望提供第一个单磁性附着原子的交换能量和力与距离的曲线。这允许量化尖端尖端原子和附原子之间的交换常数，并确定其依赖于所选的附原子和尖端终止。通过获得交换力图，我们还希望更深入地了解交换耦合机制。第二个目标是通过交换力来证明单个原子的自旋开关。我们将探索基于DFT计算的有前途的系统，以指导实验工作。最后，我们想研究自旋-轨道耦合和非共线自旋态对交换力测量的影响，这一点迄今为止还没有在理论上进行过探讨。

The exchange interaction is of fundamental importance in the theory of magnetism. Typically, its strength – expressed within the Heisenberg model in terms of exchange constants – is determined indirectly in experiments by measuring the effect of inelastic spin excitations for example by neutrons or electrons. It is also possible to probe the exchange interaction directly in the setup of an atomic force microscope (AFM) equipped with a magnetic tip. This technique, which has been first demonstrated in 2007, is coined magnetic exchange force microscopy. It has been used to obtain the exchange interaction between a magnetic tip and magnetic surfaces. However, it has so far not been possible to directly probe the exchange interaction with single adatoms or molecules. On the other hand, contact measurements of single adatoms with the tip of a scanning tunneling microscope (STM) have been performed by various groups. Very recently, it has been demonstrated that one can combine an STM and an AFM using magnetic tips in order to simultaneously measure the spin-polarized (SP) tunneling current and the exchange (EX) interaction – a technique which is known as SPEX imaging and spectroscopy.In this proposed project we aim to understand and guide experiments performed using the SPEX technique by means of density functional theory (DFT) calculations. In the past, DFT has become an indispensable tool to explain scanning tunneling or atomic force microscopy experiments. Here, we will apply DFT to study the exchange interaction between a magnetic tip and magnetic adatoms and dimers on magnetic surfaces. In collaboration with our experimental partners from the Radboud University, Nijmegen, we would like to provide the first exchange energy and force vs. distance curves for single magnetic adatoms. This allows to quantify the exchange constants between the tip apex atom and the adatom and determine its dependence on the chosen adatom and on the tip termination. By obtaining exchange force maps we would also like to gain deeper insight into the exchange coupling mechanisms. A second goal is to demonstrate switching of the spin of a single adatom by exchange forces. We will explore promising systems based on DFT calculations to guide the experimental efforts. Finally, we would like to study the effects of spin-orbit coupling and of noncollinear spin states on the exchange force measurements which have so far not been theoretically explored at all.