Spin-Orbit Torques in Co/Pd Multilayer Nanowires

Current induced spin-orbit torques have been studied in ferromagnetic nanowires made of 20 nm thick Co/Pd multilayers with perpendicular magnetic anisotropy. Using Hall voltage and lock-in measurements, it is found that upon injection of an electric current both in-plane (Slonczewski-like) and perpendicular (fieldlike) torques build up in the nanowire. The torque efficiencies are found to be as large as 1.17 and 5 kOe at ${10}^8\mathrm{A}/{\mathrm{cm}}^2$ for the in-plane and perpendicular components, respectively, which is surprisingly comparable to previous studies in ultrathin ($\sim 1\mathrm{nm}$) magnetic bilayers. We show that this result cannot be explained solely by spin Hall effect induced torque at the outer interfaces, indicating a probable contribution of the bulk of the Co/Pd multilayer.

Figure 1

(a) TEM image of a Co/Pd multilayer film. (b) SEM image of the device with the measurement scheme. (c) Hysteresis loop of the Co/Pd multilayer thin film before patterning. (d) The first and second harmonics of the Hall voltage measured for $I_{\mathrm{ac}}=1\mathrm{mA}$.

Figure 2

(a) First harmonic loop amplitude versus ac currents at different dc currents. (b) Second harmonic amplitude as a function of the ac current for $I_{\mathrm{dc}}=0$, 1 mA with a curve fit. The first harmonic (c) and second harmonic (d) loop amplitude at different dc currents for $I_{\mathrm{ac}}=1\mathrm{mA}$ with a corresponding curve fit.

Figure 3

The changes in the first harmonic amplitude as a function of ac (a) or dc (b) currents at different bias fields. The first derivative of the first harmonic loop amplitude with respect to the ac (c) or dc (d) currents as a function of bias fields with curve fits. The insets show results at a larger range of the bias field.