The Evolution of Data Storage and the Rise of Magnonics
The story of data storage is a narrative of human progress, intertwining with the complexities of society's growth. Millennia ago, the burgeoning agricultural civilizations of Mesopotamia gave birth to the cuneiform script—wedge-shaped impressions on tablets of clay. This was one of humanity's first steps toward recording and preserving information. Fast forward to the present day, and we find ourselves amidst an informational renaissance, with spintronic devices encoding 0’s and 1’s using the spin states of electrons on magnetic materials.
However, the digital era's insatiable appetite for data demands storage solutions that are denser and faster than ever before. Enter magnonics—a next-generation approach that looks to magnons, the quantized spin waves, as the new bearers of information. Manipulating magnons has traditionally been a challenge, but a groundbreaking study by Felix Godejohann and his colleagues at the Technical University of Dortmund, Germany, hints at a future where we can control magnons through their interaction with acoustic vibrations, or phonons, in materials with finely engineered structures.
The Magnetic Attraction of Magnons
Magnons offer several advantages as carriers of information. Their high frequencies, ranging from gigahertz to terahertz, and their short wavelengths set the stage for memory elements that are not only faster but also smaller than today’s electron-based counterparts. Moreover, magnon-based systems could circumvent some of the intrinsic limitations of electronic devices, such as the heat and energy loss associated with electron flow.
Quantum Leaps in Information Technology
Perhaps even more compelling is the potential role of magnons in hybrid quantum information systems. Here, they could act as intermediaries, facilitating the transfer of coherent quantum states between various platforms—from superconducting qubits to atoms and ions. Magnons are adept at coupling with microwave photons, which are already interacting with atoms and ions, while phonons might also bridge the transition from magnons to electrons.
This dual potential for classical and quantum applications has spurred intensive research into magnon-phonon interactions. Researchers have focused on magnetostrictive materials, where magnetization interplays with mechanical strain, allowing for the coupling of lattice deformations (phonons) with spin systems (magnons). The goal is to harness this interaction so that magnons and phonons can engage in a dynamic exchange of energy, creating a magnon-polaron—a hybrid quasiparticle that captures properties of both magnons and phonons.
A Dance of Energy: The Magnon-Polaron Tango
When magnons and phonons couple sufficiently, they create a distinctive experimental marker: the lifting of their energy degeneracy and the formation of split energy bands. This splitting is a direct measure of the strength of their interaction, quantified by the cooperativity parameter (CC). Achieving a CC greater than one signifies a strong-coupling regime, a benchmark for the efficient exchange of energy between magnons and phonons.
A Milestone in Magnonics
The creation of devices capable of operating in the strong-coupling regime was a long-standing aspiration, until 2019, when our group reported the first clear observation of a strongly coupled magnon-polaron in a nanomagnet. By adjusting the magnon frequency via an external field to match the nanomagnet's vibrational resonance, we achieved a remarkable CC of 1.7 in this strong-coupling regime.
Godejohann's team built upon this achievement, enhancing the coupling strength further. They ingeniously adapted a thin film of Galfenol, a highly magnetostrictive iron-gallium alloy, and sculpted precise grooves on its surface to influence the spatial distribution of magnons and phonons. Their pump-probe experiment allowed real-time observation of magnon-phonon hybridization, leading to a significant CC of about 8 under the right conditions.
The Future Shaped by Sound and Spin
The study's insights emphasize the importance of matching the spatial distribution of magnons and phonons and balancing their strengths to foster hybridization. With these findings, we stand on the brink of a new era in data storage and quantum information processing, where the intricate dance between sound and spin could unlock unprecedented possibilities.
The elegant scheme is well suited to explore properties of magnon-polarons from both a fundamental and an applied perspective. Experiments with similar configurations could explore the transport and scattering properties of these quasiparticles in view of their potential use as quantum transducers. Furthermore, since different types of magnon-polarons can be distinguished by their spatial localization or their frequency, one can envision multiplexing schemes that encode a complex signal into multiple quasiparticles. Another interesting direction would be to involve microwave photons in the dance. Researchers have demonstrated coupling involving photons, phonons, and magnons in 3D cavities, and used it to realize effects such as electromagnetically induced transparency, parametric amplification, and phonon lasing. Compared to cavity-based schemes, the planar geometry of Godejohann and colleagues’ structure is better suited for applications in integrated optoelectronic circuits.
References
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D. A. Bozhko et al., “Magnon-phonon interactions in magnon spintronics,” Low Temp. Phys. 46, 383 (2020).
P. G. Gowtham et al., “Mechanical back-action of a spin-wave resonance in a magnetoelastic thin film on a surface acoustic wave,” Phys. Rev. B 94, 014436 (2016).
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