Chiral Spin-Wave Velocities Induced by All-Garnet Interfacial Dzyaloshinskii-Moriya Interaction in Ultrathin Yttrium Iron Garnet Films

Spin waves can probe the Dzyaloshinskii-Moriya interaction (DMI), which gives rise to topological spin textures, such as skyrmions. However, the DMI has not yet been reported in yttrium iron garnet (YIG) with arguably the lowest damping for spin waves. In this work, we experimentally evidence the interfacial DMI in a 7-nm-thick YIG film by measuring the nonreciprocal spin-wave propagation in terms of frequency, amplitude, and most importantly group velocities using all electrical spin-wave spectroscopy. The velocities of propagating spin waves show chirality among three vectors, i.e., the film normal direction, applied field, and spin-wave wave vector. By measuring the asymmetric group velocities, we extract a DMI constant of $16\mu \mathrm{J}/{\mathrm{m}}^2$, which we independently confirm by Brillouin light scattering. Thickness-dependent measurements reveal that the DMI originates from the oxide interface between the YIG and garnet substrate. The interfacial DMI discovered in the ultrathin YIG films is of key importance for functional chiral magnonics as ultralow spin-wave damping can be achieved.

Figure 1

(a) Damon-Eshbach spin-wave chirality in thick YIG films. (b) An illustrative diagram of the chiral propagation of spin waves in an ultrathin YIG film. The group velocities of spin waves propagating in $+k$ and $-k$ directions are different. The inset shows a right-handed chirality among three vectors, i.e., the film normal direction $n$, applied field $H$, and DMI-induced drift group velocity $v_g^{\mathrm{DMI}}$. (c) A high-angle annular dark-field image is taken at the YIG/GGG interface of the 7 nm-thick YIG sample. The scale bar is 5 nm.

Figure 2

Spin-wave transmission spectra $S_{12}$ (a) and $S_{21}$ (c) measured as a function of the field angle $\theta$. The field is fixed at 44 mT. $\theta$ is defined as the angle between $H$ and $k$. Single spectra at 90° [dashed lines in (a) and (c)] are shown for $S_{12}$ (b) and $S_{21}$ (d). The peak-to-peak frequencies $\mathrm{\Delta }{f}_{+}\approx 0.15\mathrm{GHz}$ and $\mathrm{\Delta }{f}_{-}\approx 0.11\mathrm{GHz}$ are extracted for the estimation of spin-wave group velocities $v_g^{+}$ and $v_g^{-}$. The 260-nm-wide strip lines are used in the experiments [37].

Figure 3

The asymmetric group velocity $\delta v_g=v_g^{+}-v_g^{-}$ as a function of the applied field. The field is swept from $-50$ to 50 mT. Black circles are data points calculated using the values of $v_g^{+}$ and $v_g^{-}$ extracted from experiments [37]. The red line is a fit using Eq. (2). The green dashed line is the micromagnetic simulation results [37].

Figure 4

(a) The asymmetric spin-wave dispersion in the presence of an interfacial DMI. The red dots (black squares) are experimental data extracted with an applied field of $+20\mathrm{mT}$ ($-20\mathrm{mT}$). The red line (black line) is the calculated dispersion using a DMI constant of $16\mu \mathrm{J}/{\mathrm{m}}^2$ for $+20\mathrm{mT}$ ($-20\mathrm{mT}$). (b) Angle-dependent asymmetric group velocities $\delta v_g$. $\theta$ is defined in Fig. 2. The field is set at 44 mT. Black open circles are data points extracted from the experiments and the red line is a sinusoidal fitting. (c) $\delta v_g$ for samples with different thicknesses of 7 nm, 10 nm, 20 nm, 40 nm and 80 nm with an applied field of 20 mT. The red line is a linear fit to the experimental data. (d) Spin-wave amplitude nonreciprocity $\beta$ measured as a function of $\theta$ on the 10 nm-thick sample. The field is fixed at 10 mT. The 730 nm-wide strip lines are used in the experiments [37].

Figure 5

Frequency shift $\delta f$ (black squares) measured on the 10-nm-thick YIG sample in an applied field of 80 mT with BLS in reflection geometry at four different incident angles probing spin waves with wave vectors $k=4.6$, 9.1, 13.3, and $17.1\mathrm{rad}/\mu \mathrm{m}$ [37]. The red line is a linear fit to the experimental data.