Probing the Dzyaloshinskii-Moriya interaction in CoFeB ultrathin films using domain wall creep and Brillouin light spectroscopy

We have characterized the strength of the interfacial Dyzaloshinskii-Moriya interaction (DMI) in ultrathin perpendicularly magnetized CoFeB/MgO films, grown on different underlayers of W, TaN, and Hf, using two experimental methods. First, we determined the effective DMI field from measurements of field-driven domain wall motion in the creep regime, where applied in-plane magnetic fields induce an anisotropy in the wall propagation that is correlated with the DMI strength. Second, Brillouin light spectroscopy was employed to quantify the frequency nonreciprocity of spin waves in the CoFeB layers, which yielded an independent measurement of the DMI. By combining these results, we show that DMI estimates from the different techniques yield only qualitative agreement, which suggests that open questions remain about the underlying models used to interpret these results.

Figure 1

Differential Kerr images illustrating the expansion of a nucleated domain, where the black region indicates the area swept by the domain wall during the field pulse. The images compare motion under a pulsed perpendicular field, $H_z$, vs an additional in-plane static field, $H_x$. The scale bar indicated in (a) represents $125 \mu \mathrm{m}$ in (a)–(f) and $250 \mu \mathrm{m}$ for (g)–(l). $v_x$ is defined in (c) and $v_y$ in (a). The center is an up nucleated domain (c). (a–c) W (2-nm) underlayer under ${\mu }_0{H}_z=2.8$ mT, with a pulse duration of 5 s in (a) and (c) and 50 s in (b). (d–f) W (3-nm) underlayer under ${\mu }_0{H}_z=2.9$ mT, with a pulse duration of 1.5 s in (d) and (f) and 15 s in (e). (g–i) TaN (1-nm) underlayer under ${\mu }_0{H}_z=0.83$ mT, with a pulse duration of 0.4 s in (g) and (i) and 1.5 s in (h). (j–l) Hf (1-nm) underlayer under ${\mu }_0{H}_z=2.9$ mT, with a pulse duration of 75 ms in (j) and (l) and 250 ms in (k). The in-plane field is ${\mu }_0{H}_x=-50$ mT (a, d, g, j), ${\mu }_0{H}_x=0$ mT (b, e, h, k), and ${\mu }_0{H}_x=+50$ mT (c, f, i, l).

Figure 2

Domain wall velocity $v$ in CoFeB as a function of the perpendicular field ${\mu }_0{H}_z$ for different underlayers in zero in-plane field. Solid lines correspond to fits based on the creep model [Eq. (1)].

Figure 3

Domain wall velocity as a function of the in-plane applied field $H_x$ for propagation along (up/down domain wall; circles) and perpendicular to (squares) the field direction. The propagation takes place under a static perpendicular field, $H_z$. (a) A 2-nm-thick W underlayer at ${\mu }_0{H}_z=2.8$ mT; triangles show the wall velocity at the two opposite sides of the bubble, showing the chiral nature of the wall. (b) A 3-nm-thick W underlayer at ${\mu }_0{H}_z=2.9$ mT. (c) A 1-nm-thick TaN underlayer at ${\mu }_0{H}_z=0.83$ mT. (d) A 1-nm-thick Hf underlayer at ${\mu }_0{H}_z=2.9$ mT. The dashed vertical line indicates the offset field, $H_{\mathrm{offset}}$.

Figure 4

(a) BLS spectra measured for the 2-nm-thick W underlayer at a light incidence corresponding to $k_{\mathrm{SW}}=20.45 \mu {\mathrm{m}}^{-1}$ and an in-plane field of 1 T to saturate the magnetization. Lorentzian fit (red curve) is shown. Inverted Stokes and anti-Stokes (blue curve) are also superposed to show the nonreciprocal behavior of the spin wave. Inset: Schematic of the Damon-Eshbach configuration used. (b–e) Linear fit of the frequency difference $\mathrm{\Delta }f$ as a function of the wave vector for the different underlayers.