Breakthrough in understanding ultrafast magnetism

They publish their results on February 3rd in the journal Physical Review Letters.

Magic of magnets attracting or repulsing objects fascinated people since ancient time, but yet 100 years ago understanding the nature of magnetism remained to be very illusive. At that time the basics of quantum mechanics -- an essential ingredient of modern theory of magnetism -- had just been founded. The facts that every magnet consist elementary atomic magnets (spins), strongly coupled by a purely quantum mechanical force, called exchange interaction, had not been discovered yet.

Today's physicists are able to describe magnetic phenomena modeling a magnet as an ensemble of spins strongly coupled by the exchange interaction and in many cases such a model works sufficiently well. However, even nowadays quantum mechanical theory of magnetism is far from to be complete since it fails to describe magnetic phenomena if the latter evolve on a time-scale much faster than 100 ps. The main reason for it is the fact that the established theories of magnetism have been making the assumption that the exchange interaction is infinitely powerful and that after an excitation of the magnet the exchange interaction establishes equilibrium among the spins infinitely fast.

An international consortium of scientists from Sweden, Ukraine and The Netherlands have now been able to push the frontiers of knowledge further and achieve a breakthrough in theory of ultrafast magnetism, suggesting a general framework that describes magnetization dynamics on the time-scale of the coupling between the spins, thus revealing the real power of the exchange interaction at work.

The new theory leads to results that completely contradict the expectations of the established theory of magnetic phenomena. For example, because the coupling between the spins is no longer considered as infinite, different spins can evolve very different. Perhaps even more unexpected, an ultrafast excitation of a magnet will align the spins for a short period of time opposite to the alignment in the groundstate, thereby seemingly breaking the strong coupling between the spins. This has been observed recently for spins with antiparallel coupling and the new theory predicts that it is possible as well for parallel coupling using a different type of excitation. Hence, this work opens up a possibility to harness the hidden power of the exchange interaction for revolutionary new, counterintuitive approaches of recording and processing magnetically stored information at unprecedentedly fast time scales.