Jeudi 15 décembre 2022 à 11h00, Alexey Kimel (Radboud University, Nijmegen) donnera un séminaire dans l’Auditorium de TRT.
Titre : Ultrafast magnetism – terra incognita beyond the classical approximations
Résumé :
While magnetism is essentially 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 :
1) Although, according to thermodynamics and 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 becomes 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 [3].
Finally, we will discuss recent advances in ultrafast writing of magnetic bits. In particular, I would like to highlight that ultrafast magnetism can be the way to write information in the record fast and the least dissipative way [4].
[1] P. Curie, J. Phys. Theor. Appl., 393–415(1894) [https://en.wikipedia.org/wiki/Curie%27s_Principle]
[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] A. Stupakiewicz et al, Ultrafast nonthermal photo-magnetic recording in a transparent medium, Nature 542, 7639 (2017).
