Spins in a magnet. |
On
the shortest of time scales magnetic spins do not behave according to
existing theory. They are not coupled and move at a different pace,
dependent on the element they’re part of, Johan Mentink from Institute
for Molecules and Materials (IMM) at Radboud University Nijmegen
theorizes. He worked on the new theory of ultrafast magnetic phenomena
with an international consortium of scientists from The Netherlands,
Sweden and Ukraine. They published their results on Feb. 3, 2012 in
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.
