Abstract
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.
Highlights
Size distribution of the nanoparticles [20, 21]
grazing-incidence small-angle x-ray scattering (GISAXS) data measured for the Ag2Co film are presented in figure 1, showing a diffuse scattering near 2θ = 0.025 caused by the correlation of the Co particles with the electron
The field asymmetry of spin transfer torque (STT) observed in the granular films is dissimilar to the situation in multilayers of Co/Cu/Co [14], where, for I > Ic, additional R changes induced by STT come out as well at relatively larger H compared to giant magnetoresistance (GMR), but are nearly symmetrical with respect to H
Summary
The Ag2Co films with a thickness of about 150 nm were prepared by dc-magnetron-sputtering from an alloy target. The deposition temperature was chosen because of the largest GMR effect measured, concerning a nearly complete separation of the components and an optimized size distribution of the Co precipitates. Magnetotransport measurements with an in-plane magnetic field (H) up to 90 kOe were performed both by standard 4-point probe geometry and by a single point contact technique (schematic in the inset of figure 3) to show a normal GMR and a potential STT effect, respectively. Current scans within ±0.3 A and field scans within ±85 kOe (Hmax) were done alternatively at low temperatures to determine the dependence of the critical current Ic on the magnetic field. To describe the current polarity of STT, the sign of the electric current flowing from the tip into the film is defined as positive
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