A novel spin transfer torque effect in Ag2Co granular films

We studied a spin transfer torque (STT) effect in Ag2Co granular films, induced by a high current density 108-9 A cm-2 injected via a point contact. The system consists of single domain Co nanoparticles randomly embedded in Ag matrix, with a mean distance corresponding to the typical quantity of layer thicknesses used for the nonmagnetic space in multilayer nanopillars. A large giant magnetoresistance (GMR) effect of 55% measured at 4.2 K is an indication for high spin scattering anisotropy which is required for STT observations. Supposedly, a certain amount of large-sized Co particles saturated in external magnetic field H acts here as the spin polarizer for the injected current I and small-sized particles unsaturated at 4.2 K and Hmax (90 kOe) even act as the detector for switching. A novel STT effect was observed thereby as I rises across a threshold value Ic, showing a sharp decrease in R (ΔR/R = 130% with two steps), which arises accordingly from further alignment of the small-sized Co granules. The behavior is polar and hysteretic, similar to properties measured for multilayer nanopillars. The two-step behavior could be an effect related to a lognormal particle size distribution. Depending on the spin polarization, Ic is found to be field disproportional, indicating a larger STT efficiency at a higher H