Professor John Gregg

Abstract:

Strong coupling between distinct quasiparticles in condensed matter systems gives rise to hybrid states with emergent properties. We demonstrate the hybridization of confined phonons and finite-wavelength magnons, forming a magnon-polaron cavity with tunable coupling strength and spatial confinement controlled by the applied magnetic field direction. Our platform consists of a low-loss, single-crystalline yttrium iron garnet (YIG) film coupled to a zinc oxide (ZnO)-based surface acoustic wave (SAW) resonator. This heterostructure enables exceptionally low magnon-polaron dissipation rates below κ/2π < 1.5 MHz. The observed mode hybridization is well described by a phenomenological model incorporating the spatial profiles of magnon and phonon modes. Furthermore, we report the first observation of Rabi-like oscillations in a coupled SAW-spin wave system, revealing the dynamical formation of magnon-polarons in the time domain. These results establish a platform for engineering hybrid spin-acoustic excitations in extended magnetic systems and enable time-resolved studies of magnon-polaron states.

Abstract:

We demonstrate the efficient excitation of spin waves in the ultralow magnetic damping material yttrium iron garnet (YIG) by surface acoustic waves (SAWs). To this end, we use interdigital transducers fabricated on a piezoelectric zinc oxide (ZnO) thin film covering the YIG. This enables the excitation of coherent, propagating Rayleigh-type and Sezawa-type SAWs. We find that the ultralow magnetic damping of YIG is retained after the $\mathrm{Zn}\mathrm{O}$ deposition and we perform a comprehensive investigation of the magnetoelastic interaction due to the different SAW modes as a function of the external magnetic field magnitude and orientation. By measuring the SAW attenuation, our experiments reveal a highly anisotropic and nonreciprocal SAW–spin-wave interaction in agreement with our model calculation, while demonstrating that low-damping magnons can be efficiently excited by SAWs.

Abstract:

<jats:p>We study surface acoustic waves (SAWs) in yttrium iron garnet (YIG)/zinc oxide (ZnO) heterostructures, comparing the results of a computationally lightweight analytical model with time-resolved microfocused Brillouin light scattering (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mtext fontfamily="times">μ</a:mtext></a:math>-BLS) data. Interdigital transducers (IDTs), with operational frequencies in the gigahertz regime, were fabricated on 50-nm and 100-nm thin films of YIG prior to sputter deposition of 830-nm and 890-nm films of piezoelectric ZnO. We find good agreement between our analytical model and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><e:mtext fontfamily="times">μ</e:mtext></e:math>-BLS data of the IDT frequency response and SAW group velocity, with clear differentiation between the Rayleigh-like and Sezawa-like modes. Finally, nonreciprocal coupling between SAWs and spin waves (SWs) is shown. This work paves the way for a detailed study of the interaction between SAWs and SWs in low SW damping YIG, with the possibility of a method for future energy-efficient SW excitation.</jats:p> <jats:sec> <jats:title/> <jats:supplementary-material> <jats:permissions> <jats:copyright-statement>Published by the American Physical Society</jats:copyright-statement> <jats:copyright-year>2025</jats:copyright-year> </jats:permissions> </jats:supplementary-material> </jats:sec>

Abstract:

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.

Abstract:

Apparatus for generating spin waves comprising a body (102) of magnetic material and an elastic wave generator (120), wherein the body (102) has a surface (108) and the elastic wave generator (120) is arranged to transmit elastic waves so that they propagate through the body (102) towards the …