Broadband phonon to magnon conversion in yttrium iron garnet

We propose and experimentally demonstrate a means of broadband phonon-magnon interconversion that relies on combining magnetoelastic coupling with translational symmetry breaking in the important experimental material yttrium iron garnet (YIG). As well as being of interest for its basic physics, this quasiparticle coupling mechanism adds to the range of effects that potentially find useful application in hybrid solid-state quantum computing devices as well as low-power wave-based classical computing architectures. The magnon is a relative newcomer to the catwalk of hybrid solid-state quantum science but it has already begun to turn heads. The rich physics and ready tunability of microwave magnonic systems combined with their demonstrable compatibility with the existing tools of experimental solid-state quantum engineering suggest significant and wide-reaching opportunities, not only in device design, but also in fundamental research [1–14]. Moreover, in the context of classical computing, there has been steadily growing interest in the use of magnonic systems as a platform for wave-based information technologies that overcome the ever more pressing ‘heat death’ issues associated with conventional computing hardware [15–28]. Indeed, phase-modulated spin-waves have significant appeal as data carriers in both classical and quantum computing devices: they offer Joule-heat-free spin information transfer and their short wavelengths relative to electromagnetic waves of the same frequency (microwave-frequency spin waves have wavelengths in the millimetre to nanometre range) are highly conducive to progressive device miniaturization [24–28]. Moreover, the ability to interconvert between spin-wave or magnon signals and those in other physical domains—notably microwave photonics, spin currents, heat currents, and optics—is widely recognised as a further important dividend [27–30]. However, until now, a notable gap has existed in the catalogue of magnonic conversion effects. Though the coupling between the magnon and phonon systems of magnetic materials was, in fact, the inspiration for the original theoretical framework upon which all of spin-wave and magnon physics came to be based [31], the interconversion between magnon and phonon signals—as opposed to incoherent excitations—had yet to be practically demonstrated [32, 33]. In this paper, we propose and demonstrate the first experimental proof of principle of a novel phonon-based approach to magnon signal generation. The effect is predicated on a new quasiparticle coupling mechanism with two essential ingredients: magnetoelastic coupling of sufficient strength and appropriate symmetry in the magnonic host material; and energy-momentum matching between the phonons and the magnons. The latter is generally difficult to realise since the phonon and magnon dispersion relations overlap and hybridise only over a very narrow range of wavenumbers that would be impractical for broadband signal transfer. As explained below, we circumvent this by borrowing the artifice of translational symmetry breaking that is used to solve similar issues in other areas of wave physics. The key requirement of magnetoelastic coupling is present to exactly the right degree in the magnetic material we choose for our experiments, the popular electrically insulating ferrimagnet yttrium iron garnet or YIG (Y3Fe5O12). The magnetoelastic coupling in YIG is of sufficient magnitude to provide adequate phonon–magnon mode coupling, yet insufficiently strong to compromise the spin-wave decay length. These favourable properties arise on account of the fact that YIG’s magnetic moment comes from S-state Fe3+ ions [34] and that, as is typical of magnetic garnets, the chemical and magnetic unit cells are inequivalent [35]. © 2021 The Author(s). Published by IOP Publishing Ltd Mater. Quantum Technol. 1 (2021) 011003 To address the energy/wavenumber (k) matching issue we take our cue from the operating principle of a Yagi television aerial which phase-matches guided and free-space electromagnetic waves with very different dispersions. The Yagi does this through the expedient of breaking translational symmetry by establishing standing-wave patterns with a geometry calculated to generate spatial frequencies in the local electromagnetic disturbance that match the wavenumbers of the quasiparticles that it is required to produce. We mimic this approach by using a piezoelectric transducer to send a narrow pulsed microwave acoustic beam which impinges locally on a magnonic waveguide fashioned from a YIG film that is subject to an externally applied magnetic field. This process generates a small magnetoelastically active volume defined by the beam profile and the waveguide thickness. The local symmetry break due to the presence of the waveguide interfaces and the resultant acoustic standing wave structure allows the low k magnons to couple effectively to phonons of much higher k values, i.e. well removed from the usual hybridisation regime. Since our aim is a very broadband phonon–magnon conversion process, we generate a spatial frequency spectrum that is as white as possible by engineering the waveguide and the beam to confine the region of magnetoelastic interaction as tightly as possible in all three dimensions. Due to the magnetoelastic coupling, the acoustic displacement of the YIG film locally generates a small signal oscillating internal magnetic field contribution hme, some of whose spatial frequency components are capable of coupling to and exciting MSW modes. In principle, hme couples to an infinite family of propagating modes; however, excitation of the higher order modes is inefficient owing to their low propagation speeds, and hence only the lowest order mode is excited. For a given magnetoelastic free energy density Fme, the oscillating magnetic field is hme = − 1 Ms ∂Fme