Enhancement of spin-orbit torque by inserting ${\mathbf{CoO}}_x$ layer into Co/Pt interface

We report that the spin-orbit torque (SOT) in a Co/Pt structure is significantly enhanced by partial oxidation of the Co-side interface. The interface-oxidized and unoxidized Co/Pt samples were prepared to compare the SOTs, which were determined using the harmonic Hall measurement. Even though an insulating ${\mathrm{CoO}}_x$ layer exists at the Co/Pt interface, the dampinglike and fieldlike SOT efficiencies in the interface-oxidized sample were severalfold and nearly an order of magnitude larger than those of the unoxidized sample, respectively. The enhancement of the SOT is most likely attributed to the efficient spin current transmission across the ${\mathrm{CoO}}_x$ layer and the appropriately modulated interfacial Rashba effect.

Figure 1

(a) The layer structures for the IO and UO samples. $j_{\mathrm{s}}$ is the SHE-induced spin current from the Pt layer. (b) A photograph of the Hall cross device taken by an optical microscope. The experimental setup for the harmonic Hall measurement is schematically depicted on the device.

Figure 2

(a) The DL and (b) FL effective fields as a function of the current density flowing in the Pt layer for the IO and UO samples with similar areal magnetic moments. The solid and open symbols represent the results obtained for the $+z$- and $-z$-magnetized states, respectively.

Figure 3

(a) The SOT efficiencies for the IO and UO samples as a function of the areal magnetic moment. The results for the Co/Pt sample with a 0.8-nm MgO insertion layer are also shown. The solid and open symbols represent the DL and FL torque efficiencies, respectively. (b) The ratios of the FL to DL torque efficiency for the same samples shown in (a).

Figure 4

(a) The XAS and (b) normalized XMCD spectra around the Pt $L_3(\sim 11.57$ keV) and $L_2(\sim 13.28$ keV) edges for the same samples shown in Fig. 2. The inset shows the areal magnetic moment dependence of the orbital $\left(m_{\mathrm{orb}}\right)$ and effective spin magnetic moments $\left(m_{\mathrm{spin}}^{\mathrm{eff}}\right)$ of the Pt layer deduced from the sum rules [31, 40, 41].