Current-induced effective magnetic fields in Co/Cu/Co nanopillars

We present a method to measure the effective field contribution to spin-transfer-induced interactions between the magnetic layers in a bilayer nanostructure, which enables spin current effects to be distinguished from the usual charge-current-induced magnetic fields. This technique is demonstrated on submicron Co/Cu/Co nanopillars. The hysteresis loop of one of the magnetic layers in the bilayer is measured as a function of current while the direction of magnetization of the other layer is kept fixed, first in one direction and then in the opposite direction. These measurements show a current-dependent shift of the hysteresis loop which, based on the symmetry of the magnetic response, we associate with spin transfer. The observed loop shift with applied current at room temperature is reduced in measurements at 4.2 K. We interpret these results both in terms of a spin-current-dependent effective activation barrier for magnetization reversal and a spin-current-dependent effective magnetic field. From data at 4.2 K, we estimate the magnitude of the spin-transfer-induced effective field to be ∼1.5 × 10^-7 Oe cm²/A, about a factor of 5 less than the spin torque.