Spin-transfer torque efficiency measured using a Permalloy nanobridge

We report magnetoresistance, focused Kerr effect, and Lorentz microscopy experiments performed on a nanoscale Permalloy bridge connecting microscale pads. These pads can be switched from a parallel to antiparallel state through the application of small fields, causing a detectable magnetoresistance. We show that this switching field H-sw is modified by the application of a high current density (J(dc)) through spin-transfer torque effects, caused by the spin-current interacting with the magnetization gradients generated by the device geometry, yielding an estimate for the spin-transfer torque efficiency xi=dH(sw)/dJ(dc)=0.027 +/- 0.001 Oe/MA cm(-2). (C) 2010 American Institute of Physics. [doi:10.1063/1.3520144]