Modification of interfacial spin-orbit torque in Co/Pt/oxide hybrid structures

We investigate current-induced spin-orbit torque (SOT) in Co/Pt/oxide systems by varying a sort of the oxide layer. When the Pt thickness is less than ∼2 nm, Pt at the interface of the oxide layer is magnetically polarized owing to the magnetic proximity effect. In these systems, fieldlike (FL) SOT depends significantly on the adjacent oxide material, whereas dampinglike SOT is almost irrelevant to oxides. The FL SOT efficiency of the $\mathrm{Pt}/\mathrm{Hf}{\mathrm{O}}_2$ sample is 2.2 times greater than that of the Pt/MgO sample at the maximum. X-ray magnetic circular dichroism spectroscopy reveals that the anisotropy of the Pt orbital magnetic moment varies with the oxide material, suggesting that the modulation of the electronic structure at the Pt/oxide interface contributes to SOT enhancement.

Figure 1

(a) Cross-sectional view of layer structures used for SOT experiments. (b) Pt thickness dependence of areal saturation magnetic moment. Inset shows dc Hall resistance measured by sweeping perpendicular magnetic field for Pt(0.74 nm)/$\mathrm{Hf}{\mathrm{O}}_2$ sample. (c) Pt thickness dependence of Pt resistivity. Solid line shows resistivity calculation [27].

Figure 2

(a) Schematic illustration of device structure and experimental setup for harmonic Hall measurements. (b) First harmonic and (c) second-harmonic Hall resistance in the Pt(3.0 nm)/$\mathrm{Hf}{\mathrm{O}}_2$ sample at in-plane magnetic field of 0.3 T and ac charge current with the magnitude of 5.5 mA. Black solid lines show fitting result estimated using Eqs. (2) and (3). Red and blue solid lines in (c) show DL and FL terms, respectively. (d), (e) Pt thickness dependence of DL and FL SOT efficiencies for three Pt/oxide samples.

Figure 3

[(a), (b)] Pt thickness dependence of change ratio of DL (FL) SOT efficiency for the Pt/$\mathrm{Hf}{\mathrm{O}}_2$ and Pt/${\mathrm{Al}}_2{\mathrm{O}}_3$ samples.

Figure 4

(a) Spin magnetic moment $m_{\mathrm{S}}$ and average of out-of-plane ($m_{\mathrm{L}}^{\perp }$) and in-plane ($m_{\mathrm{L}}^{\parallel }$) components of orbital magnetic moment ${\overline{m}}_{\mathrm{L}}=(m_{\mathrm{L}}^{\perp }+m_{\mathrm{L}}^{\parallel })/2$; (b) anisotropy of orbital magnetic moment $\mathrm{\Delta }{m}_{\mathrm{L}}=m_{\mathrm{L}}^{\perp }-m_{\mathrm{L}}^{\parallel }$ for three Pt(0.74 nm)/oxide samples. (c) Areal PMA energy for Pt(0.74 and 2.0 nm) with different oxide capping layer.

Figure 5

XRD profiles of the Pt(0.74 and 3.0 nm)/$\mathrm{Hf}{\mathrm{O}}_2$ samples, where the XRD intensity of the Pt(3.0 nm)/$\mathrm{Hf}{\mathrm{O}}_2$ sample is resized by 1/20. Black arrows indicate the peaks obtained around $2\theta ={43}^{\circ }$.