Interfacial Dzyaloshinskii-Moriya interaction in perpendicularly magnetized ${\text{Pt/Co/AlO}}_x$ ultrathin films measured by Brillouin light spectroscopy

Spin waves in perpendicularly magnetized ${\text{Pt/Co/AlO}}_x/\text{Pt}$ ultrathin films with varying Co thicknesses (0.6–1.2 nm) have been studied with Brillouin light spectroscopy in the Damon-Eshbach geometry. The measurements reveal a pronounced nonreciprocal propagation, which increases with decreasing Co thickness. This nonreciprocity, attributed to an interfacial Dzyaloshinskii-Moriya interaction (DMI), is significantly stronger than asymmetries resulting from surface anisotropies for such modes. Results are consistent with an interfacial DMI constant $D_{\mathrm{s}}=-1.7\pm 0.11\text{pJ}$/m, which favors left-handed chiral spin structures. This suggests that such films below 1 nm in thickness should support chiral states such as skyrmions at room temperature.

Figure 1

Polar MOKE magnetometry for $\text{Pt/Co}\left(t\right){\text{/AlO}}_x$ samples of different thicknesses $t$. Left panel: Experimental (symbols) and fitted (red bold line) hysteresis loops of the perpendicular component of the magnetization vs in-plane applied magnetic field for the $t=0.8\text{nm}$ sample. Right panels: Hysteresis loops of some representative samples for fields applied perpendicularly to the sample plane.

Figure 2

BLS spectra measured for $\text{Pt/Co}\left(1.2\text{nm}\right)/{\text{AlO}}_x$ at two different applied field values ${\mu }_{0}H=0.4\text{T}$ $(-0.4$ T) in blue squares (red circles) and at four characteristic light incidence angles corresponding to $k_{\text{sw}}=18.09$, 15.18, 8.08, and 4.1 $\mu {\text{m}}^{-1}$. Symbols refer to the experimental data and solid lines are the Lorentzian fits.

Figure 3

(a) Measured spin-wave dispersion for various Co thicknesses $t$. Symbols show the experimental BLS data for AS (red, with $k_{\mathrm{sw}}$ inverted) and S frequencies (black). Solid lines represent the model described by Eq. (1) with effective DMI constants $D_{\mathrm{eff}}$ and magnetic parameters given in Table 1. (b) Wave vector $\left(k_{\mathrm{sw}}\right)$ dependence of experimental frequency difference $\mathrm{\Delta }f$ (symbols) compared to the DMI model (solid curves). The inset shows the measured dependence of $\mathrm{\Delta }f$ vs the applied magnetic field in the case of the $t=1.2\text{nm}$ sample at a fixed wave vector value $k_{\mathrm{sw}}=15.18\mu {\text{m}}^{-1}$.