Observation of thermally activated coherent magnon-magnon coupling in a magnonic hybrid system

TL;DR

This work addresses whether incoherent, thermally activated magnons can coherently couple in a YIG/Py bilayer. The authors use microfocused Brillouin light scattering to access a broad wavevector range and corroborate with microwave-based spin pumping and spin-torque FMR measurements. They observe an avoided crossing around $f \approx 5.7$ GHz and $H \approx 50$ mT between the YIG $n=1$ PSSW and the Py uniform mode, extracting a interfacial coupling strength $g_c \approx 10$ mT and showing a Py magnon band up to $k \in [0,16]$ rad/μm. A two-oscillator model with this coupling reproduces the hybridization, and a spacer experiment confirms that interfacial exchange underpins the effect. Overall, the study demonstrates coherent hybridization of thermal magnons, enabling all-magnon, energy-efficient information processing and potential quantum spin-wave platforms.

Abstract

We experimentally demonstrate strong magnon-magnon coupling by thermal spin excitations in yttrium iron garnet/permalloy (YIG/Py) hybrid structures using microfocused Brillouin light scattering - an optical technique that enables the detection of zero-wavevector and higher-order wavevector spin waves in a broad frequency range. The thermally activated magnons in the bilayer lead to a hybrid excitation between magnon modes in the conductive Py layer with a wide wavevector range and the first perpendicular standing magnon modes in the insulating YIG layer, facilitated by strong interfacial exchange coupling. To further investigate this coupling, we compare the thermal magnon spectra with the results obtained from electrical excitation and detection methods, which primarily detect the uniform Py mode. The realization of coherent coupling between incoherent (thermal) magnons is important for advancing energy-efficient magnonic devices, particularly in classical as well as quantum spin-wave computing technologies.

Figures (4)

• Figure 1: (a) Illustration of coupling between uniform Py mode and higher order YIG PSSW modes. (b) Microfocused BLS measurement approach in which the inelastic scattered probe beam is analyzed using a tandem Fabry-Pérot interferometer (FPI). (c) Spin rectification measurement where an RF current is passed through a conducting Py layer and the rectified signal is measured by a lock-in technique. (d) Spin pumping measurement where RF field is supplied through a coplanar waveguide (shown in black) and the voltage is measured between the two ends of the sample.
• Figure 2: Uniform and higher order PSSW modes for YIG and Py obtained using (a) thermal BLS, (b) spin-pumping, and (c) spin rectification techniques for a YIG(84)/Py(10) sample as a function of applied field and frequency. The observed avoided crossing is due to a coherent coupling between the Py mode and YIG ($n=1$) PSSW modes in each of the three figures. The red and yellow curves are fits obtained from Eqs. (1) and (2).
• Figure 3: (a) BLS spectrum for the YIG(86)/SiO$_2$(5)/Py(10) sample, in which SiO$_2$ disables the direct exchange interaction between YIG and Py. As a result, we do not observe any mode coupling. (b) Corresponding BLS data for YIG(84)/Py(10) without the SiO$_2$ interlayer, the same data as presented in Fig. \ref{['fig2']}(a), with the observed avoided crossing between the two coupled modes modeled by two coupled harmonic oscillator curves, which are overlaid as colored dashed lines and labeled as H$+$ and H$-$. Curves in red and yellow correspond to YIG ($n = 0$) and YIG ($n = 2$) modes, respectively.
• Figure 4: The calculated frequency-field relationship illustrating the hybridization between the first order PSSW mode of YIG and the Py spin-wave band spanning a wavevector range from $k$ = 0 to 16 rad/$\mu$m. The inset shows the calculated frequency vs. field curve overlaid on the experimental BLS spectra, highlighting the agreement between theoretical model and experimental observations.