arXiv subjects

John F. Gregg

Publications and source records attributed to John F. Gregg.

7 recordsLinked to original sources

arXiv2026

Nonlinear frequency shift and bistability of magnon-polarons

We investigate the nonlinear dynamics of strongly coupled surface acoustic waves (SAWs) and spin waves (SWs) in a magnetoacoustic resonator based on a YIG/ZnO heterostructure by combining microwave reflection measurements with microfocused Brillouin light scattering spectroscopy. In the linear regime, the electrical response reveals clear hybridization between standing SAW cavity modes and finite-wave-vector SWs, resulting in pronounced avoided crossings. At elevated drive powers, the hybrid system exhibits a strongly field-dependent nonlinear response characterized by a positive frequency shift of the driven SW mode. Using the vector Hamiltonian formalism for nonlinear spin-wave dynamics, we show that this shift is dominated by a cross-shift term. In our resonator geometry, this contribution becomes significant because the standing SAW cavity mode simultaneously excites counterpropagating SWs with wave vectors $+k$ and $-k$. For suitable field detuning, the nonlinear shift drives the SW mode into resonance with the SAW excitation, leading to a strong enhancement of the magnon population, broadband nonlinear scattering, and bistable foldover behavior. Beyond the foldover threshold, both the magnon and phonon responses stabilize. These results establish SAW-driven $k \neq 0$ magnon-phonon hybrids as a promising platform for nonlinear magnetoacoustics and wave-based information processing.

cond-mat.mes-hall↗

arXiv2025

Magnon-polaron control in a surface magnetoacoustic wave resonator

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 $\kappa / 2\pi < 1.5\;$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.

cond-mat.mes-hall↗

arXiv2024

Generation of gigahertz frequency surface acoustic waves in YIG/ZnO heterostructures

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 micro-focused Brillouin light scattering data. Interdigital transducers (IDTs), with operational frequencies in the gigahertz regime, were fabricated on 50 and 100nm thin films of YIG prior to sputter deposition of 830nm and 890nm films of piezoelectric ZnO. We find good agreement between our analytical model and micro-focused Brillouin light scattering data of the IDT frequency response and SAW group velocity, with clear differentiation between the Rayleigh and Sezawa-like modes. This work paves the way for the study of SAW-spin wave (SW) interactions in low SW damping YIG, with the possibility of a method for future energy-efficient SW excitation.

cond-mat.mes-hall↗

arXiv2019

Onset of spin wave time-domain fractals in a dynamic artificial crystal

We report on the first observation of an exact fractal pattern in the time-domain arising spontaneously from a dynamic artificial crystal (DAC). The all-magnon process occurs in a passive, unlithographed magnetic waveguide. The DAC was created by a standing spin wave of frequency $f_{\mathrm{DAC}}$ in a region of nonlinear waveguide, resulting in a spatio-temporally periodic potential. The interaction of travelling spin waves of frequency $f'$ with the DAC resulted in a series of new modes appearing in a comb with intervals $Δf^{(0)} = \mid f_{\mathrm{DAC}}-f'\mid$. As $H$ was increased a 1st pre-fractal pattern was observed with frequency interval $Δf^{(1)} =Δf^{(0)}/2$. Finally, the onset of a 2nd pre-fractal was observed with frequency interval $Δf^{(2)} =Δf^{(1)}/2 = Δf^{(0)}/4$. The magnetic field dependence of the nonlinear signals effect matched that of the DAC. The fractal-like behavior was demonstrated for different values of $f'$.

cond-mat.mes-hall↗

arXiv2018

A Microscopic Explanation of Microwave Spin Pumping

We contend that Microwave Spin Pumping was first predicted and observed - albeit using a different and more sensitive detection mechanism than Inverse Spin Hall Effect - in the 1950's. This discovery was the founding step in the widely used analytical tool that is now known as Dynamic Nuclear Polarisation. Recognising this hitherto unsung connection between 20th Century Magnetic Resonance and 21st Century Spintronics not only helps to explain and unify contemporary metallic spin pumping observations: it is also the key to unlocking the immense and very sophisticated toolbox of Magnetic Resonance and placing it at the disposal of the future of Spintronics.

cond-mat.mes-hall↗

arXiv2018

Phase modulation and amplitude modulation interconversion for magnonic circuits

Circuit elements within magnonic computers generally operate using signals that are either amplitude or phase modulated (AM or PM). We propose a simple all-magnon circuit element capable of converting between these two types of encoding. We highlight the potential of our technique to augment existing devices and propose a novel schematic for an 'equality' gate and XNOR gate.

cond-mat.mes-hall↗

arXiv2011

Oscillatory Energy Exchange Between Waves Coupled by a Dynamic Artificial Crystal

We describe a general mechanism of controllable energy exchange between waves propagating in a dynamic artificial crystal. We show that if a spatial periodicity is temporarily imposed on the transmission properties of a wave-carrying medium whilst a wave is inside, this wave is coupled to a secondary counter-propagating wave and energy oscillates between the two. The oscillation frequency is determined by the width of the spectral band gap created by the periodicity and the frequency difference between the coupled waves. The effect is demonstrated with spin waves in a dynamic magnonic crystal.

cond-mat.mes-hall↗