Magnon Coupling in Two-Dimensional Artificial Magneto-Elastic Crystals

June 12, 2024 (Wed.) at 1:30PM (ET)

Supriyo Bandyopadhyay

Virginia Commonwealth University

A two-dimensional artificial magneto-elastic crystal consists of a periodic array of magnetostrictive nanomagnets (100-300 nm lateral dimensions) deposited on a piezoelectric substrate. A surface acoustic wave (SAW) launched in the substrate acts as a source of phonons which excite spin waves (magnons) in the nanomagnets via magneto-elastic coupling or magnon-phonon coupling. These magnons can, in turn, couple with radiative photons and radiate electromagnetic waves in space (tripartite phonon-magnon-photon coupling)[1]. We have studied the coupling modalities and the associated features with time-resolved magneto-optical Kerr effect (TR-MOKE) spectroscopy.

Additional features can be observed in bilayer magnetic dots that have a layer of plasmonic material (e.g., Al) in them. Surface plasmons from the Al hybridize with phonons sourced from the SAW to produce a hybrid acousto-plasmonic excitation that excites spin wave frequency combs in the samples owing to strong non-linear tripartite coupling between the phonons, plasmons and magnons. We have observed two octaves of spin wave frequency combs (in the GHz regime) with TR-MOKE. There is also evidence of large parametric amplification where energy is transferred from the acousto-plasmonic mode to spin wave modes efficiently[2]. The large coupling efficiency (estimated cooperativity factor much larger than unity) hints at the formation of a new quasi-particle that we call “magnon-plasmon-polaron”.

We have also observed tripartite magnon-phonon-magnon coupling (coupling between two Kittel type spin wave modes and a magneto-elastic mode of mixed magnon-phonon character) in a two-dimensional artificial magneto-elastic crystal[3]. Again, the coupling is strong enough to form a new quasi-particle – a binary magnon-polaron. The strong coupling with cooperativity factor exceeding unity (estimated from the measured anti-crossing in the dispersion relations and linewidths of the spectral peaks) leads to complete (100%) transfer of energy from the magneto-elastic mode to the two Kittel type modes, with accompanying parametric amplification. The coupling exhibits significant anisotropy since the array does not have rotational symmetry in space. The experimental observations are in good agreement with simulations. We showed that it is possible to engineer the tripartite coupling by choosing the frequency of the surface acoustic wave exciting the magneto-elastic mode to match the frequencies of the Kittle type modes, the latter being controlled by a magnetic field. We also observe that at the point of maximal coupling between the two spin wave modes, the phase profiles of these modes within the nanomagnets exhibit a 1800 phase difference between them, which is reminiscent of dark magnon modes. Because of this phase difference, the spins in the two spin wave modes are rotating in opposite directions, but we do not know which mode is rotating clockwise and which one is rotating anti-clockwise. We cannot write their joint state as a tensor product of individual states, which is analogous to “entanglement”. This replicates some (but not all) of the features of Einstein-Podolsky-Rosen (EPR) states, except of course the system here is classical and there is no quantum non-locality at play since the “entangled” mode survives only in the presence of the coupling agent (the SAW) and vanishes if the SAW is extinguished.

[1] Advanced Science, 9(8), 2104644 (2022)

[2] Advanced Functional Materials, 33(37), 2304127 (2023)

[3] NPG Asia Materials, 15(1), 51 (2023)