Dynamic self-organisation and pattern formation by magnon-polarons

Abstract

Magnetic materials play a vital role in energy-efficient data storage technologies, combining very fast switching with long-term retention of information. However, it has been shown that, at very short time scales, magnetisation dynamics become chaotic due to internal instabilities, resulting in incoherent spin-wave excitations that ultimately destroy magnetic ordering. Here, contrary to expectations, we show that such chaos gives rise to a periodic pattern of reversed magnetic domains, with a feature size far smaller than the spatial extent of the excitation. We explain this pattern as a result of phase-synchronisation of magnon-polaron quasiparticles, driven by strong coupling of magnetic and elastic modes. Our results reveal not only the peculiar formation and evolution of magnon-polarons at short time-scales, but also present an alternative mechanism of magnetisation reversal driven by coherent packets of short-wavelength magnetoelastic waves.

Spectroscopy of phononic switching of magnetization in the terahertz gap

“…Note that only two magnetic domains are observed, even though the sample exhibits a four-fold in-plane anisotropy. We find that this pattern is rather typical when the thin film is exposed to a macropulse, whereas a single micropulse generally gives rise to four magnetic domains [10]. We speculate that this drastically different pattern stems from the slower processes of domain formation that occur across the time scale of microseconds.…”

“…Phononic switching of magnetization-the process through which magnetization is solely reversed by the excitation of optical phonons at resonance-poses an intriguing and somewhat counter-intuitive mechanism for magnetic reversal [8,10]. The thriving research field of non-linear phononics has revealed that infrared-active transverse optical (TO) phonon modes, when driven at resonance to high amplitude [11,12], can give rise to a wealth of transient effects and functionalities such as promoting superconductivity to room temperature or transforming insulators into metals [13,14].…”