Quantitative Studies of Spin-Momentum-Transfer-Induced Excitations in Co/Cu Multilayer Films Using Point-Contact Spectroscopy
William H. Rippard, Matthew R. Pufall, Thomas J. Silva
We have measured current-induced magnetic excitations in a variety of exchange-coupled Co/Cu multilayers using point-contact spectroscopy. A step in the DC resistance and corresponding peak in dV/dI are observed at a critical current Ic whose value linearly depends applied magnetic field B, in agreement with Slonczewski's theory. These features are observed for both in- and out of plane fields. Excitations in ferromagnetically-coupled films occur even without an applied field. The spin transfer efficiency is determined from the slope and intercept of Ic vs. Bapp and varied from contact-to-contact. These variations are within the range predicted by Slonczewski's theory, as bounded between the diffusive and ballistic limits for electron propagation through a point contact.