Bipolar high-field excitations inCo∕Cu∕Conanopillars

Current-induced magnetic excitations in $\mathrm{Co}∕\mathrm{Cu}∕\mathrm{Co}$ bilayer nanopillars ($\sim 50\mathrm{nm}$ in diameter) have been studied experimentally at low temperatures for large applied fields perpendicular to the layers. At sufficiently high current densities excitations, which lead to a decrease in differential resistance, are observed for both current polarities. Such bipolar excitations are not expected in a single domain model of spin transfer. We propose that at high current densities strong asymmetries in the spin accumulation cause spin-wave instabilities transverse to the current direction in bilayer samples, similar to those we have reported for single magnetic layer junctions.