Spin-dependent intergranular hopping transport in very thin highly spin-polarized CoS$_{2}$ thin films

The Co$_{1-x}$Fe$_{x}$S$_{2}$ alloy system has been shown to exhibit high, composition tunable, spin polarization (-56 {\%} $< \quad P <$ +85 {\%}) in bulk, demonstrating great promise for fundamental studies in spintronics. Incorporation in heterostructures requires reliable thin film deposition routes, which have recently been developed. We present here a detailed study of the thickness ($t)$ dependence of the structural, magnetic, and electronic properties of polycrystalline CoS$_{2}$ thin films (70 -- 1600 {\AA}). As $t$ is decreased, we observe a suppression in magnetic properties accompanied by a metal-insulator transition. A distinct 3D to 2D crossover is evident in the conductance-voltage curves and intergranular tunneling magnetoresistance. At $t$ of order 70 {\AA} we observe granular metal conduction, in the presence of a Coulomb charging penalty. We demonstrate quantitative agreement between experiment and proposed models. The very thin film data are understood in terms of enhanced grain boundary resistance, due to S accumulation, which is evidenced via several modes of structural characterization.