Observation of multiple magnetic phases and complex nanostructures in Co implanted amorphous carbon films

Room temperature implantation of 30 keV Co ions into an amorphous carbon film with a high fluence of 1.2×10¹⁷ Co/cm² results in formation of magnetic nanostructures displaying multiple magnetic phases. Cross-sectional TEM images show formation of Co containing nanoparticles at the surface and near-surface regions of the implanted films. EDXS measurements suggest the nanoparticles to be composed primarily of Co and O at the surface and Co and C in deeper regions. These nanoparticles with differing compositions were observed to be segregated by a thin layer devoid of Co. Magnetic measurements reveal the presence of superparamagnetic behavior from small Co_xC nanoclusters with a blocking temperature of 5 K. There is a small fraction of larger Co_xC nanoclusters that show magnetic hysteresis even at room temperature. The saturation magnetic moment is as high as 0.51 μ_B/Co at 2 K and 0.32 μ_B/Co at room temperature. Spin-disorder is seen with a range of spin glass temperatures below ∼70 K. Our high fluence Co implantation into amorphous carbon has resulted in the formation of complex magnetic nanostructures composed of cobalt, oxygen, and carbon. These nanostructures give rise to multiple magnetic phases such as superparamagnetism, spin glass, ferromagnetism, and possibly antiferromagnetism.