报告题目：Ultrafast magnetism – terra incognita beyond the classical approximations

报 告 人：A.V. Kimel，Radboud University

报告时间：2023年11月20日15:30

报告地点：理科楼C302

报告摘要：

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 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 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 become 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].

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 [5].

[1] P. Curie, J. Phys. Theor. Appl., 393–415 (1894) [https://en.wikipedia.org/wiki/Curie%27s_Principle]

[2] T.A. Ostler et al, Nature-Communications 3, 666 (2012).

[3] E. A. Mashkovich et al, Science 374, 1608-1611 (2021).

[4] T. G. H. Blank et al, Phys. Rev. Lett. 131, 096701 (2023).

[5] A. Stupakiewicz et al, Nature 542, 7639 (2017).

报告人简介：

Prof. Kimel obtained his PhD from the Ioffe Institute (St. Petersburg, Russia) in 2002. He pioneered the ultrafast research on antiferromagnetic materials for over 20 years [PRL 89, 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-Physics 5, 727–731 (2009)], and a recognized world-leader in the field. During the last few 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)].