Influence of the bias sign on the dynamic modes induced by spin-transfer in exchange biased spin-valves

In this study, spin-transfer induced magnetization dynamics in exchange-biased spin-valves, for both polarities of current and field are studied. The structure of the samples included a synthetic pinned layer and a soft free layer, patterned into square pillars with a lateral size of the order of 100 nm. The resistance of the pillars was of the order of 5Omega, and the measured giant magnetoresistance reached 2.5%. The coercivity of the free layer varied between 5 and 150 Oe and a magnetostatic coupling of the same order of magnitude was measured in most cases, favoring the antiparallel state. The critical current densities were of the order of 10⁷A/cm². Microwave spectra obtained for negative currents and showed similar trends to those evidenced by simple Co/Cu/Co nanopillars and the small angle FMR signals can be reasonably well predicted using Kittes formula.