Ilie Radu, Kadir Vahaplar, Christian Stamm, Torsten Kachel, Niko Pontius, Florin Radu, Radu Abrudan, Hermann Dürr, Thomas Ostler, Joe Barker, Richard Evans, Roy Chantrell, Arata Tsukamoto, Akiyoshi Itoh, Andrei Kirilyuk, Theo Rasing, Alexey Kimel
Revealing the ultimate speed limit at which magnetic order can be controlled, is a fundamental challenge of modern
magnetism having far reaching implications for magnetic recording industry. Exchange interaction is the strongest force
in magnetism, being responsible for ferromagnetic or antiferromagnetic spin order. How do spins react after being
optically perturbed on an ultrashort timescales pertinent to the characteristic time of the exchange interaction? Here we
demonstrate that femtosecond measurements of X-ray magnetic circular dichroism provide revolutionary new insights
into the problem of ultrafast magnetism. In particular, we show that upon femtosecond optical excitation the ultrafast
spin reversal of Gd(FeCo) - a material with antiferromagnetic coupling of spins - occurs via a transient ferromagnetic
state. The latter one emerges due to different dynamics of Gd and Fe magnetic moments: Gd switches within 1.5 ps
while it takes only 300 fs for Fe. Thus, by using a single fs laser pulse one can force the spin system to evolve via an
energetically unfavorable way and temporary switch from an antiferromagnetic to ferromagnetic type of ordering. These
observations supported by atomistic simulations, present a novel concept of manipulating magnetic order on different
classes of magnetic materials on timescales of the exchange interaction.
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