Current-Driven Excitations in Symmetric Magnetic Nanopillars

We study experimentally the current-driven magnetic excitations in symmetric $\mathrm{C}\mathrm{o}/\mathrm{C}\mathrm{u}/\mathrm{C}\mathrm{o}$ nanopillars. In contrast with all the previous observations where the current of only one polarity is capable of exciting a multilayer system saturated by an externally applied magnetic field, we observe that both polarities of the applied current trigger excitations in a symmetric multilayer. This may indicate that in symmetric structures the current propels high-frequency magnetic oscillations in all magnetic layers. We argue, however, that only one layer is excited in our multilayers but, interestingly, currents of opposite polarities excite different layers. This hypothesis is supported by modeling the spin accumulation in symmetric magnetic multilayers.