IMX Seminar Series - Ultrafast magnetism – terra incognita beyond the classical approximations

While magnetism is practically the strongest quantum mechanical phenomenon, modern description of magnetization dynamics and magnetization reversal relies on thermodynamics and the corresponding approximations. I will show that ultrashort (sub-100 ps) stimuli push magnetic media into a strongly non equilibrium state, where the conventional description of magnetic phenomena in terms of equilibrium thermodynamics fail and the experimentally observed ultrafast magnetization dynamics and reversal challenge the current theories. For instance, while the conventionally accepted Curie-Neumann’s principle states that “the symmetries of the causes are to be found in the effects" [1], in ultrafast magnetism the principle fails and magnetization dynamics becomes counter-intuitive.

In my talk, I will discuss two main examples of such counter-intuitive spin dynamics:

1) While according to thermodynamics and simply intuition, heat can only destroy magnetization, we will demonstrate that ultrafast (sub-100 ps) heat pulse can cause magnetization reversal without any magnetic fields [2].
2) While control of spins in antiferromagnets requires increasingly high magnetic fields, rapidly varying magnetic field at THz rates is a game-changer in the field. Picosecond pulses of THz magnetic field with the strength below 1 T can efficiently excite spins in antiferromagnets and even push spin dynamics into nonlinear regime, where new channels of spin-lattice interaction open-up [3] and the principle of superposition fails i.e. 1+1>2 [4].

[1] P. Curie, J. Phys. Theor. Appl., 393-415(1894).
[2] T.A. Ostler et al, Ultrafast heating as a sufficient stimulus for magnetization reversal in a ferrimagnet, Nature-Communications 3, 666 (2012).
[3] E. A. Mashkovich et al, THz light driven coupling of antiferromagnetic spins to lattice, Science 374, 1608-1611 (2021).
[4] T. G. H. Blank et al, Empowering control of antiferromagnetic spins by THz spin coherence, Phys. Rev. Lett. 131, 096701 (2023).

Bio: In 2002 he obtained his PhD from the Ioffe institute (St. Petersburg, Russia) and joined Radboud University as a postdoc, where he became later an assistant professor (2007), associate professor (2013) and full professor (2017). He pioneered ultrafast spin dynamics in antiferromagnetic materials already 20 years ago [PRL89, 287401 (2002), Nature 435 655–657 (2005)] and his works in a large extent defined the development of ultrafast magnetism during the last two decades. He is a co-inventor of ultrafast all-optical magnetic recording [PRL99, 047601 (2007)] and inertia of spins in antiferromagnets [Nature-Physics5, 727–731 (2009)], as a recognized world-leader in the field he obtained several prestigious research grants (Veni2004, Vidi2006, Vici2017, ERC-SG2010, Russian MegaGrant-2013, ERC-AG2022). During the last years, his group has been pushing the frontiers of the field demonstrating the fastest and the least dissipative magnetic switching [Nature 542, 71–74 (2017)], revealing temporal and spectral fingerprints of the coherent switching in antiferomagnets [Nature 569, 7756 (2019)], discovering the regime of highly anharmonic dynamics of antiferromagnetic spins [Science 374 1608-1611 (2021), PRL (2023)].