Current-induced magnetization switching in exchange-biased spin valves for current-perpendicular-to-plane giant magnetoresistance heads

In contrast to earlier studies performed on simple $\mathrm{Co}∕\mathrm{Cu}∕\mathrm{Co}$ sandwiches, we have investigated spin-transfer effects in complex spin-valve pillars with a diameter of 130 nm developed for current-perpendicular-to-plane magnetoresistive heads. The structure of the samples included an exchange-biased synthetic pinned layer and a free layer, both laminated by insertion of several ultrathin Cu layers. Despite the small thickness of the polarizing layer, our results show that the free layer can be switched between the parallel (P) and the antiparallel (AP) states by applying current densities of the order of ${10}^{7}\phantom{\rule{0.3em}{0ex}}\mathrm{A}∕{\mathrm{cm}}^{2}$. A strong asymmetry is observed between the two critical currents ${I\phantom{\rule{0.1em}{0ex}}}_{c}^{\mathrm{AP}\text{\ensuremath{-}}\mathrm{P}}$ and ${I}_{c}^{\mathrm{P}\text{\ensuremath{-}}\mathrm{AP}}$, as predicted by the model of Slonczewski. Due to the use of exchange-biased structures, the stability phase diagrams could be obtained in the four quadrants of the $(H,I)$ plan. The critical lines derived from the magnetoresistance curves measured with different sense currents, and from the resistance versus current curves measured for different applied fields, match each other very well. The main features of the phase diagrams can be reproduced by investigating the stability of the solutions of the Landau-Lifshitz-Gilbert equation including a spin-torque term, within a macrospin model.