报告题目：Ultrafast magnetism – terra incognita beyond the classical approximations
报 告 人：Prof. Alexey Kimel，Radboud University (Nijmegen, The Netherlands)
报告时间：2024年11月28日16：00
报告地点：理学院郑裕彤大讲堂
内容摘要：
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 challenge the current theories.
While conventionally accepted Curie-Neumann’s principle states that “the symmetries of the causes are to be found in the effects” [1], in ultrafast magnetism this principle fails, and magnetization dynamics becomes counter-intuitive. We will demonstrate that ultrafast (sub-100 ps) heating with the help of ultrashort laser pulses causes magnetization reversal without any magnetic fields [2], laser-induced spin dynamics is strongly non-linear, where new channels of spin-lattice interaction open-up [3,4], the principle of superposition fails i.e. 1+1>2 [5] and the approximation of macrospin is no longer adequate [6].
[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. W. J. Metzger et al, Magnon-phonon Fermi resonance in antiferromagnetic CoF2, Nature Communications 15, 5472 (2024).
[5] T. G. H. Blank et al, Empowering control of antiferromagnetic spins by THz spin coherence, Phys. Rev. Lett. 131, 096701 (2023).
[6] F. Formisano et al, Coherent THz spin dynamics in antiferromagnets beyond the approximation of the Neel vector, APL Mater. 12, 011105 (2024).
报告人简介：He obtained his PhD in 2002 from the Ioffe institute (St. Petersburg, Russia) and joined Radboud University (Nijmegen, The Netherlands) 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-StG2010, Russian MegaGrant-2013, ERC-AdG2022). 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)].
