Ab initio description of skyrmion detection with tunnel junctions

Abstract

In this work, I use the Korringa-Kohn-Rostoker (KKR) method to analyze the possibility of using magnetic tunnel junctions (MTJ) to detect magnetic skyrmions. Usually, These junctions require a magnetic reference layer and then show the well-known Tunneling magnetoresistance effect (TMR). However, under the right conditions, the non-collinear magnetic structure of skyrmions can lead to a significant increase in conductance through a tunnel junction even without a magnetic reference layer. While a large part of the Overall change in conductance can be attributed to the disturbance of quantum well states (QWS) in the iron monolayer hosting the skyrmion, the majority of the effect, including the increase, is due to the scattering of electronic states from the lead at the skyrmion. This is an interesting situation which demonstrates that scattering does not necessarily reduce conductance, but can also increase conductance drastically if coherent transport is blocked by the symmetry of the electronic states in the ferromagnetic case. The combination of vanadium as lead material with a MgO barrier was identified as a material combination where coherent transport in the collinear ferromagnetic base structure is suppressed, but can be enhanced by scattering of electronic states. Beyond that, I show that the presence of a skyrmion in a tunnel barrier not only alters the conductance but also changes the Seebeck coeffcient. It turns out that the absolute value of the Seebeck coeffcient tends to be reduced by the skyrmion. Since antiferromagnetic skyrmions offer several advantages over ferromagnetic skyrmions but are much harder to handle and detect than their ferromagnetic counterparts, I investigate how useful the proposed effect might be for the detection of antiferromagnetic skyrmions. The results indicate that the electronic transport is, in principle, also sensitive to antiferromagnetic non-collinear structures. Finally, I take a brief excursion into the topic of the TMR effect, which requires a magnetic reference layer but can potentially be used to distinguish between different types of non-collinear structures by doing multiple measurements with low spatial resolution.