Ultrafast magnetic switching by resonant excitation of optical phonons

Andrei Kirilyuk

FELIX Laboratory, Radboud University, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands

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 focused onto the sample. The pulses of FELIX have been shown to be Fourier-transform limited [6], with their bandwidth experimentally tunable in the range of 0.5-2.0%, corresponding to the typical pulse width of 1-10 ps, depending on the wavelength range.

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.

1. T. Kubacka et al. Large-Amplitude Spin Dynamics Driven by a THz Pulse in Resonance with an Electromagnon. Science 343, 1333 (2014).
2. A. Kirilyuk, A.V. Kimel, T. Rasing, Ultrafast optical manipulation of magnetic order. Rev. Mod. Phys. 82, 2731 (2010).
3. N. Li et al., Colloquium: Phononics: manipulating heat flow with electronic analogs and beyond. Rev. Mod. Phys. 84, 1045 (2012).
4. T.F. Nova et al., An effective magnetic field from optically driven phonons. Nature Physics 13, 132 (2017).
5. S.F. Maehrlein, et al., Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation. Science Advances 4, 5164 (2018).
6. G.M.H. Knippels, X. Yan, A.M. MacLeod, W.A. Gillespie, M. Yasumoto, D. Oepts, and A. F.G. van der Meer, Generation and Complete Electric-Field Characterization of Intense Ultrashort Tunable Far-Infrared Laser Pulses. Phys. Rev. Lett. 83, 1578 (1999).
7. A. Stupakiewicz, C.S. Davies, K. Szerenos, D. Afanasiev, K. S. Rabinovich, A.V. Boris, A. Caviglia, A. V. Kimel, and A. Kirilyuk, Ultrafast phononic switching of magnetization, Nature Physics 17, 489 (2021).

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