Exchange interactions and spin switching at the single atom level

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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.