We study the device size dependence of spin-orbit torque induced magnetization switching in a Ta/CoFeB/MgO structure with perpendicular easy axis. The miniaturization of the device from micrometer-sized wire to 80-nm dot results in the increase of the threshold current density Jth by one order, whereas Jth increases only slightly with further reducing the device size down to 30nm. No significant increase in Jth is seen, as the current pulse width decreases from 100ms down to 3ns. We reveal that the switching in devices at reduced size is reasonably well explained by the macrospin model, in which the effects of both the Slonczewski-like torque and field-like torque are included.
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