Emergence of Huge Negative Spin-Transfer Torque in Atomically Thin Co layers

Current-induced domain wall motion has drawn great attention in recent decades as the key operational principle of emerging magnetic memory devices. As the major driving force of the motion, the spin-orbit torque on chiral domain walls has been proposed and is currently extensively studied. However, we demonstrate here that there exists another driving force, which is larger than the spin-orbit torque in atomically thin Co films. Moreover, the direction of the present force is found to be the opposite of the prediction of the standard spin-transfer torque, resulting in the domain wall motion along the current direction. The symmetry of the force and its peculiar dependence on the domain wall structure suggest that the present force is, most likely, attributed to considerable enhancement of a negative nonadiabatic spin-transfer torque in ultranarrow domain walls. Careful measurements of the giant magnetoresistance manifest a negative spin polarization in the atomically thin Co films which might be responsible for the negative spin-transfer torque.

Figure 1

(a)–(c) $ϵ$ of the down-up (red, ${ϵ}^{+}$) and up-down (blue, ${ϵ}^{-}$) DWs with respect to ${\mu }_0{H}_x$ for strips with $t_{\text{Co}}$ of (a) 0.4, (b) 0.35, and (c) 0.3 nm. The inset of (a) illustrates the current-assisted, field-driven DW motion for $ϵ$ measurement.

Figure 2

(a) $ϵ$ (upper panel) and $v$ (lower panel) plotted with respect to ${\mu }_0{H}_x$ for the strip with $t_{\text{Pt}}=2.5\mathrm{nm}$. (b)–(d) $v$ (upper panels) and $ϵ$ (lower panels) plotted with respect to ${\mu }_0{H}_x^{*}$ for strips of $t_{\text{Pt}}$ equal to (b) 2.5, (c) 1.5, and (d) 3.5 nm. The red (blue) symbol corresponds to the down-up (up-down) DW. For $v$, the applied field ${\mu }_0{H}_z$ was (b) $\pm 1$, (c) $\pm 3.5$, and (d) $\pm 1\mathrm{mT}$.

Figure 3

(a) Micromagnetic prediction of $\lambda$ and $m_x^{\mathrm{DW}}$ for a (2.5-nm $\mathrm{Pt}/0.3\text{−}\mathrm{nm}$ $\mathrm{Co}/1.5\text{−}\mathrm{nm}$ Pt) strip. The orange dashed lines indicate ${\mu }_0{H}_D$. (b) Predicted ${ϵ}_{\mathrm{STT}}/|\beta P|$ with respect to ${\mu }_0{H}_x^{*}$. The red line shows the values from the relation ${ϵ}_{\mathrm{STT}}/|\beta P|=-\hslash /2e{M}_S\lambda$. The blue symbols show the results from the micromagnetic simulations with the STT module. (c)–(e) Decomposition of $ϵ$ into ${ϵ}_{\mathrm{STT}}$ and ${ϵ}_{\mathrm{SOT}}$ for the strips with $t_{\text{Co}}$’s of (c) 0.45, (d) 0.4, and (e) 0.35 nm. (f) Maximum values of $|{ϵ}_{\mathrm{STT}}|$ and $|{ϵ}_{\mathrm{SOT}}|$ and estimated $\lambda$’s with respect to $t_{\text{Co}}$. As a side note, the harmonic Hall measurement yields results consistent with the ${ϵ}_{\mathrm{SOT}}^{\max }$ within the same order of magnitude [24]. (g) $|\beta P|$ with respect to $\lambda$. The values were obtained for both the Bloch ($H_x^{*}=0$) and Néel ($H_x^{*}=H_D$) configurations in each strip. Because $|P|\le 1$, the $|\beta P|$ presents the lower bound of $|\beta |$.

Figure 4

(a)–(c) Scanned Kerr images of the purely current-induced DW displacements in (2.5-nm $\mathrm{Pt}/0.3\text{−}\mathrm{nm}$ $\mathrm{Co}/t_{\text{Pt}}$ Pt) strips with $t_{\text{Pt}}$ equal to (a) 1.5, (b) 2.5, and (c) 3.5 nm. The red (blue) symbol indicates the direction of $H_z^{*}$ inside the down-up (up-down) DW. (d)–(f) CPP-GMR curves of (d) stacks I, (e) II, and (f) III. The parallel and antiparallel states are indicated by P and AP, respectively.