Superparamagnetic behavior of granular Co-C films consisting of nanocrystalline cobalt encapsulated in carbon

Nanocomposite Co-C thin films of about 15 nm thick were prepared by pulsed filtered vacuum arc deposition. The films were characterized by x-ray photoelectron spectroscopy, non-Rutherford backscattering spectrometry, x-ray diffraction, magnetic force microscopy and magnetic measurements. The as-deposited films were amorphous. After annealing at 350°C for one hour in vacuum (< 10^-3 Pa), the films were found to consist of nanocrystalline Co grains encapsulated in carbon. The superparamagnetism of the annealed Co₃₆C₆₄ film was demonstrated by the measurement of DC susceptibility and magnetic hysteresis using a SQUID magnetometer. The superparamagnetic relaxation blocking temperature was marked to be about 12K by the peak of the zero-field-cooled magnetization under a field of 100 Oe. The magnetic properties of these annealed granular Co-C films transform from superparamagnetism to ferromagnetism when the Co concentration increases.