Materials dependence of the spin-momentum transfer efficiency and critical current in ferromagnetic metal/Cu multilayers

We have quantitatively compared spin-momentum transfer effects in FM/Cu multilayers, with FM=Co, Co90Fe10, Ni80Fe20, Ni40Fe10Cu50, and Fe. All of the measured multilayers exhibited an abrupt step up in resistance at a critical current I-c, the value of which was a linear function of applied magnetic field H-app. The spin-transfer efficiencies determined from these data were in substantial agreement with their predicted values over the range of materials studied. I-c(H-app), when evaluated at zero net field, was found to increase with the exchange energy density of the ferromagnet, demonstrating the tunability of the critical current by materials selection.