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Researchers Discover a New Mechanism for the Stabilization of Skyrmions

The so-called skyrmions, small magnetic whirls that can take place in materials, have an excellent potential for innovative electronic devices or magnetic memory in which they are utilized as bits to store data.

Stabilization of skyrmions by higher-order exchange interactions. The red curve shows the energy barrier for the collapse of a magnetic skyrmion (upper left) into the ferromagnetic background (lower right). At the highest point of the curve defining the barrier height one finds the transition state (upper right). The cones show the “atomic bar magnets” of individual atoms on a hexagonal lattice. Silver arrows denote cones pointing upwards while red color specifies cones pointing downwards. Lower left: Schematic structure of an atomic layer of palladium (Pd) on an atomic layer of iron (Fe) deposited on a rhodium (Rh) surface with (111) crystallographic orientation. Image Credit: © Soumyajyoti Haldar.

Stability of these tiny magnetic whirls is a basic prerequisite for any application. Now, a team of researchers from the Institute of Theoretical Physics and Astrophysics of Kiel University have shown that magnetic interactions, which were so far neglected, can play a crucial role in the stability of skyrmions and can significantly improve their lifetime.

Recently published in the Nature Communications journal on September 21st, 2020, the new study also unlocks the perspective to stabilize skyrmions in novel material systems in which the formerly considered mechanisms are not adequate.

Intensive Research on Stability at Room Temperature

The special magnetic structure of skyrmions, more specifically their topology, provides them with stability and protects them from collapsing. Thus, skyrmions are represented as knots in the magnetization. However, on the atomic lattice of a solid, this kind of protection is not perfect and there is just a limited energy barrier.

The situation is comparable to a marble lying in a trough which thus needs a certain impetus, energy, to escape from it. The larger the energy barrier, the higher is the temperature at which the skyrmion is stable.

Stefan Heinze, Professor, Kiel University

In particular, skyrmions with diameters measuring less than 10 nm and which are required for upcoming spin electronic devices have only been detected at extremely low temperatures, to date. Considering that applications are usually carried out at room temperatures, the improvement of the energy barrier is a major goal in current studies related to skyrmions.

Earlier, a typical model of the applicable magnetic interactions contributing to the energy barrier has been determined. Now, a group of theoretical physicists from the research team of Professor Stefan Heinze has revealed that one kind of magnetic interaction has been neglected so far.

In the 1920s, Werner Heisenberg was able to describe the occurrence of ferromagnetism through the quantum mechanical exchange interaction which occurs from the spin-dependent “hopping” of electrons between a pair of atoms.

If one considers the electron hopping between more atoms, higher-order exchange interactions occur.

Dr Souvik Paul, Study First Author, Kiel University

However, such interactions are relatively weaker when compared to the pair-wise exchange suggested by Heisenberg and were therefore overlooked in the research on skyrmions.

Weak Higher-Order Exchange Interactions Stabilize Skyrmions

On the basis of quantum mechanical calculations and atomistic simulations carried out on the supercomputers of the North-German Supercomputing Alliance (HLRN), the Kiel researchers have now described that such weak interactions can still make an unexpectedly huge contribution to the stability of skyrmions.

Particularly, the cyclic hopping across four atomic sites (indicated as red arrows in the above image) controls the energy of the transition state quite strongly (the highest point on the upper right of the above image), where just a few number of atomic bar magnets are tilted against one another. The simulations also revealed stable antiskyrmions which are beneficial for a few upcoming data storage ideas but usually decay too quickly.

Higher-order exchange interactions occur in several magnetic materials that are utilized for promising skyrmion applications, like iron or cobalt. Moreover, such interactions can stabilize skyrmions in magnetic structures, wherein the earlier considered magnetic interactions are either too small or cannot take place. Hence, the current study unlocks new potential routes to study these interesting magnetic knots.

Journal Reference:

Paul, S., et al. (2020) Role of higher-order exchange interactions for skyrmion stability. Nature Communications.


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