  We show that dipole-field induced antiferromagnetic coupling, or RKKY
ferromagnetic coupling, between Co layers can strongly affect the low magnetic
field switching behavior of Co/Cu/Co nanopillars. Whereas current-assisted
switching at low fields in uncoupled nanopillars is always hysteretic, strong
coupling of either kind can change the switching to non-hysteretic
(reversible). These differences can be understood with a simple picture of
current-assisted thermal activation over a barrier.
