Low frequency magnetization excitations in magnetic nanopillars induced by a dc spin-polarized current

We present observations of low frequency, (fundamentals <= 1 GHz) current-driven magnetization dynamics at 293 K in partially patterned Py (24 nm)/Cu(10)/Py(6), elliptical, nanopillars, with the top Py(6) magnetic layer and part of the Cu layer shaped into an similar to 70 X 140 nm(2) nanopillar and the rest left extended. The magnetoresistance curves for applied current I approximate to 2 mA, and the spin-transfer-torque curves for applied magnetic field H approximate to 0, both behave as expected for simple, uncoupled magnetic layers. We describe several behaviors similar to those previously seen and associated with excitation and dynamics of vortices: sharp, low frequency (f) peaks at H=0, with df/dI similar to 40 MHz/mA; hysteretic behavior with the direction of sweep of current I; disappearance of the peaks for parallel field larger than 10 mT and for perpendicular field larger than 400 mT; nonmonotonic power production as a function of I. Two new behaviors are the "simultaneous" presence of such peaks for both I>0 and I<0, and strong asymmetry of the I<0 peaks when H is reversed. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3068627]