Origin of fieldlike spin-orbit torques in heavy metal/ferromagnet/oxide thin film heterostructures

We report measurements of the thickness and temperature $\left(T\right)$ dependencies of current-induced spin-orbit torques, especially the fieldlike (FL) component, in various heavy metal (HM)/normal metal (NM) spacer/ferromagnet (FM)/oxide (MgO and $\mathrm{Hf}{\mathrm{O}}_x/\mathrm{MgO})$ heterostructures. The FL torque in these samples originates from spin current generated by the spin Hall effect in the HM. For a FM layer sufficiently thin that a substantial portion of this spin current can reach the FM/oxide interface, T-dependent spin scattering there can yield a strong FL torque that is, in some cases, opposite in sign to that exerted at the NM/FM interface.

Figure 1

(a) ST-FMR spin torque efficiency ${\xi }_{\mathrm{FMR}}$ as a function of the FeCoB thicknesses $t_{\mathrm{FeCoB}}$ for two in-plane magnetized samples W(4)/Hf(0.25,1)/FeCoB/MgO. (b) Dampinglike and fieldlike spin torque efficiencies ${\xi }_{\mathrm{DL}}$ and ${\xi }_{\mathrm{FL}}$ as a function of Hf thickness, determined using fits to Eq. (1) for $t_{\mathrm{FeCoB}}$ spanning the range 2–7 nm. The solid (dashed) orange lines are fits to the two sets of data (see Ref. [27]). The signs of both ${\xi }_{\mathrm{DL}}$ and ${\xi }_{\mathrm{FL}}$ are negative. The red solid line indicates a small residual FL spin torque efficiency $\left(-0.010\pm 0.004\right)$.

Figure 2

(a) The current-induced effective fields $\mathrm{\Delta }{H}_{\mathrm{DL}}/\mathrm{\Delta }{J}_e$ and $\mathrm{\Delta }{H}_{\mathrm{FL}}/\mathrm{\Delta }{J}_e$ as a function of $t_{\mathrm{FeCoB}}$ for a Ta/FeCoB/MgO and a $\mathrm{Ta}/\mathrm{FeCoB}/\mathrm{Hf}{\mathrm{O}}_x(0.2)/\mathrm{MgO}$ sample. (b) $\mathrm{\Delta }{H}_{\mathrm{DL}}/\mathrm{\Delta }{J}_e$ and $\mathrm{\Delta }{H}_{\mathrm{FL}}/\mathrm{\Delta }{J}_e$ as a function of the Hf passivation layer thickness. (c) $\mathrm{\Delta }{H}_{\mathrm{FL}}/\mathrm{\Delta }{J}_e$ as a function of the inverse of the anisotropy field $H_a$. Inset: $H_a$ for different Hf passivation layer thicknesses.

Figure 3

Spin-orbit torque effective fields as a function of temperature for different samples: (a) Ta(4)/FeCoB(0.8)/MgO, (b) $\mathrm{Ta}\left(6\right)/\mathrm{FeCoB}\left(0.8\right)/\mathrm{Hf}{\mathrm{O}}_x(0.2)/\mathrm{MgO}$, (c) Ta(4)/Hf(1)/FeCoB(1)/MgO and Pt(4)/Hf(0.5)/FeCoB(1)/MgO, and (d) W(4)/Hf(1)/FeCoB(1)/MgO and Hf(4)/FeCoB(1)/MgO. The dashed lines are fits to the linear portion of the $\mathrm{\Delta }{H}_{\mathrm{FL}}\left(T\right)/\mathrm{\Delta }{J}_e$ variation.