Quantitative studies of spin-momentum-transfer-induced excitations in Co/Cu multilayer films using point-contact spectroscopy

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 I-c whose value depends linearly on applied magnetic field B-app(') 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 I-c vs B-app and varied from contact to contact. For out of plane magnetized samples, the deduced spin transfer efficiency values are in good agreement with the predictions of Slonczewski.