Direct Observation of Antimagnons with Inverted Dispersion

Abstract

We report direct spectroscopic evidence of antimagnons, i.e., negative-energy spin waves identified by their signature inverted dispersion with Brillouin light scattering (BLS) spectroscopy. We investigate an ultrathin BiYIG film with a perpendicular magnetized anisotropy that compensates the demagnetizing field. By injecting a spin-orbit torque, the magnetization is driven into auto-oscillation and eventually into a non-equilibrium reversed state above a secondary current threshold ($\sim$1.2$\times$10$^7$~A/cm$^2$). The dispersion is measured by wavevector-resolved BLS and exhibits a sharp change from an upward dispersion to a downward one, in agreement with theoretical predictions and micromagnetic simulations. Around the threshold current, we observe the coexistence of conventional magnons and antimagnons. Our work establishes antimagnons with inverted dispersion and is a first step towards exploring novel phenomena and applications due to magnon-antimagnon coupling, such as magnon amplification and magnon-antimagnon entanglement, which are part of the emerging field of antimagnonics.

Figures (4)

• Figure 1: (a) Schematic illustration of an SOT device based on a BiYIG thin film with a Pt bar on top. The BiYIG film thickness $t=4$ nm. The Pt bar is about 10 $\mu$m wide. A magnetic field of 60 mT is applied along the $y$ direction. An electric direct current (dc) current $j_{\rm dc}$ is applied along the Pt bar and exerts spin-orbit torques on BiYIG underneath to drive its magnetization into auto-oscillation and eventually into a dynamically stabilized non-equilibrium state where antimagnons emerge (red arrow). The Brillouin light scattering (BLS) technique with wavevector ($k$) resolution is employed to detect magnons. The detection laser spot is placed on the Pt/BiYIG region. (b) BLS spectra measured as a function of current density $j_{\text{dc}}$. The detection wavevector is fixed at $k=10~\text{rad}/\upmu$m. Three regimes are observed with increasing current density.
• Figure 2: BLS intensity spectra measured for three different wavevectors (0, 10 and 17 rad/$\upmu$m) with (a) zero current, corresponding to thermal magnons, (b) 5.8 mA, corresponding to the auto-oscillation regime, and (c) 7.0 mA, corresponding to the antimagnon regime, where the resonance frequency exhibits a red shift at higher $k$. The intensity of the high-$k$ spectra in (b) and (c) are amplified for optimized presentation. (d-f) Magnon dispersion relations extracted from a series of BLS spectra in three characteristic regimes of (d) thermal magnons, (e) auto-oscillation and (f) antimagnons. Red solid lines represent the dispersion relations theoretically calculated for the respective cases based on Eq. (\ref{['dispersion']}). All data are measured under an applied field of 60 mT.
• Figure 3: (a) BLS spectra measured at $k=17$ rad/$\upmu$m while sweeping the current $I_{\rm{dc}}$ from 6.0 to 6.7 mA in increments of 0.05 mA. The external magnetic field is 60 mT. Around 6.35 mA (blue dashed line), the switching from the conventional magnon to antimagnon state occurs accompanied by a sharp change in the resonance frequency. (b) Three representative BLS spectra plotted for $I_{\rm{dc}}=6.30$, 6.35 and 6.40 mA around the transition [black, blue and red dashed lines in (a)]. Around 6.35 mA, two peaks are observed indicating the coexistence of the magnon and antimagnon phase.
• Figure 4: (a) Magnon dispersion obtained from micromagnetic simulations with a large SOT driving the magnetization at the dynamical stabilization threshold showing the coexistence of magnons and antimagnons in the system. (b) Snapshot of the spatial distribution of the magnetization component $M_{y}$ at steady state under critical current excitation.