Ultrafast magnetization reversal driven by optical phonons

Andrei Kirilyuk, Radboud University

Identifying an efficient pathway to change the order parameter via a subtle excitation of the coupled high-frequency mode is the ultimate goal of the field of ultrafast phase transitions [1,2]. This is an especially interesting research direction in magnetism, where the coupling between spin and lattice excitations is required for magnetization reversal [3]. Despite several attempts [4,5] however, the switching between magnetic states via resonant pumping of phonon modes has not yet been demonstrated.
To provide resonant excitation of the phonon modes, we use pulses from FELIX (Free Electron Lasers for Infrared eXperiments, Nijmegen, The Netherlands). The IR/THz light with photon energy ranging between 25 meV and 124 meV (wavelength 10−50 μm) is typically used.
And thus we show how an ultrafast resonant excitation of the longitudinal optical phonon modes in magnetic garnet films switches magnetization into a peculiar quadrupolar magnetic domain pattern, unambiguously revealing the magneto-elastic mechanism of the switching [7]. In contrast, the excitation of strongly absorbing transverse phonon modes results in thermal demagnetization effect only. The mechanism appears to be very universal, and is shown to work in samples with very different crystallographic symmetry and magnetic properties, including weak ferromagnets and antiferromagnets [7].

[1] T. Kubacka et al, Science 343, 1333 (2014).
[2] A. Kirilyuk, A.V. Kimel, T. Rasing, Rev. Mod. Phys 82, 2731 (2010).
[3] N. Li et al., Rev. Mod. Phys. 84, 1045 (2012).
[4] T. F. Nova et al., Nature Physics 13, 132 (2017).
[5] S. F. Maehrlein et al., Science Advances 4, 5164 (2018).
[6] G.M.H. Knippels et al, Phys. Rev. Lett. 83, 1578 (1999).
[7] A. Stupakiewicz et al, Nature Physics 17, 489 (2021).
[8] P. Stremoukhov et al, New J. Physics 24, 023009 (2022).

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