Structure and magnetic properties of facing-target sputtered Co–C granular films

We studied the structure and magnetic properties of as-deposited and subsequently annealed Co_xC_100−x granular films fabricated by a DC facing-target magnetron sputtering system at room temperature using atomic force microscopy, x-ray diffraction (XRD), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy and a vibrating sample magnetometer. The average roughness, R_α, of the as-deposited Co_xC_100−x granular films is smaller than that of the Si(100) substrates. XRD and TEM analyses indicate that the as-deposited Co_xC_{100 − x} granular films are composed of ∼2 nm amorphous cobalt grains dispersed in an amorphous carbon matrix, and their morphology is composition independent. The high resolution TEM image of the as-deposited Co₃₀C₇₀ film shows that cobalt and carbon have already separated during the deposition, even if the aggregation of cobalt is not complete. Annealing at 300–450°C causes the crystallization of amorphous cobalt followed by an increase in grain size and the graphitization of the amorphous carbon matrix. The constriction arising from the structural environment results in the coexistence of the hcp and fcc Co phases at temperatures higher than the phase transformation point of 425°C. Magnetic measurements reveal that the coercivity of the as-deposited Co_xC_100−x granular films decreases with the increase in cobalt concentration, and increases with decrease in film thickness. The enhanced coercivity can be attributed to the weakened intergrain interaction because of the increased percolation threshold and/or the destruction of long-range domain structures caused by the reduction in film thickness.