Interfacial control of Dzyaloshinskii-Moriya interaction in heavy metal/ferromagnetic metal thin film heterostructures

The interfacial Dzyaloshinskii-Moriya interaction (DMI) in ultrathin magnetic thin film heterostructures provides a new approach for controlling spin textures on mesoscopic length scales. Here we investigate the dependence of the interfacial DMI constant $D$ on a Pt wedge insertion layer in Ta/CoFeB/Pt(wedge)/MgO thin films by observing the asymmetric spin-wave dispersion using Brillouin light scattering. Continuous tuning of $D$ by more than a factor of 3 is realized by inserting less than one monolayer of Pt. The observations provide new insights for designing magnetic thin film heterostructures with tailored $D$ for controlling skyrmions and magnetic domain-wall chirality and dynamics.

Figure 1

Schematics of (a) a multilayer thin film and (b) BLS measurement geometry. (c) Spin-wave dispersions without (dashed line) and with (solid lines) the interfacial DMI $D>0$. (d) Schematics of spatial chiralities of DE spin waves propagating in opposite directions. Canted red arrows ${\mathbf{m}}_{\mathit{i}}$ and ${\mathbf{m}}_{\mathit{j}}$ refer to the spin rotation on the $x\text{−}y$ plane away from the external field direction $+z$ at different atomic sites $i$ and $j$, respectively. The DMI vector $\mathbf{D}$ points into the paper plane $\left(D>0\right)$ for the two spins with spatial displacement $\mathbf{r}$. The green dots represent the Ta atoms, and the dashed line indicates the three-site indirect exchange.

Figure 2

BLS spectra for spin waves of (a) the DE mode and (b) the BWVM mode recorded at fixed incident angles under oppositely oriented external magnetic fields $\mathbf{H}$. The solid lines represent fittings with Lorentzian functions. (c) The linear dependence of $f_{\mathrm{DM}}$ on $k_x$. The error bars on $f_{\mathrm{DM}}$ are determined from the deviation of Lorentzian fitting on BLS spectra, and the solid line refers to the least-squares fit. The uncertainty of $D$ is estimated from the deviation of the least-squares fit.

Figure 3

(a) The linear dependences of $f_{\mathrm{DM}}$ on $k_x$ for a Pt wedge layer with a thickness gradient where the inset illustrates the multilayer structure. The solid lines show the least-squares fits. (b) The interfacial DMI constant $D$ as a function of the Pt layer thickness $t_{\mathrm{Pt}}$. The red squares are derived from the linear fits on the $k_x$ dependences of $f_{\mathrm{DM}}$ with different $t_{\mathrm{Pt}}$'s using Eq. (2). The black circles are obtained through $f_{\mathrm{DM}}/k_x$ at fixed $k_x=16.7\mathrm{rad}/\mu \mathrm{m}$ by varying $t_{\mathrm{Pt}}$ in a smaller step size. (c) $M_{\mathrm{S}}$ and (d) $4\pi M_{\mathrm{eff}}$ as functions of $t_{\mathrm{Pt}}$. The red (black) dots in (d) are obtained from the field dependence BLS (VSM) measurements, respectively.

Figure 4

(a) Schematics of the directions of ${\mathbf{D}}_{\mathrm{Ta}}$ (pointing along $-z)$ and ${\mathbf{D}}_{\mathrm{Pt}}$ (pointing along $+z)$ on annealed Ta/CoFeB/Pt/MgO. The green dots represent the HM atoms. (b) The linear dependence of $f_{\mathrm{DM}}$ on $k_x$ extracted from a control sample, annealed Ta(5)/CoFeB(1.1)/MgO.