Microstructure And Magnetic Properties Of (Co, Fe)-C Granular Thin Films

(Co, Fe)-C granular films were fabricated by a DC facing-target magnetron sputtering system at room temperature and were subsequently annealed at different temperatures. The surface morphology, composition, microstructure and magnetic properties were investigated by using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and vibrating sample magnetometer (VSM). AFM results indicate that the average surface roughness of as-deposited Co-C granular films is smaller than that of Si (100) substrates. However, the average surface roughness of as-deposited Fe-C granular films is larger than that of Si (100) substrates and increases with the increase of annealing temperature. XRD, HRTEM, SAED, and XPS analyses show that the as-deposited Co-C granular films consist of ~ 2 nm amorphous Co granules embedded in amorphous C matrix. With the increase of annealing temperature, the amorphous Co-C granular films crystallize and the size of Co granules increases. Due to the constrictions of crystallographic environment, when the anneal temperature reaches 450 oC, hcp-Co phase appears, which is generally considered to be stable at temperatures below 425 oC. The as-deposited Fe48C52 granular thin films are composed of ~ 4 nm amorphous Fe granules dispersed in amorphous C. Annealed at temperatures above 400 oC, the amorphous Fe crystallizes to α-Fe and its size increases with the anneal temperature. Compared with Co-C granular films, a more complete crystallization of amorphous Fe granules can be observed in the HRTEM image of Fe-C granular films annealed at the same conditions. Magnetic measurements indicate that the coercivity of as-deposited Co-C granular films decreases with Co concentration and film thickness, which can be interpreted as the enhanced inter-granule interaction through the Co atoms dispersed in amorphous C matrix and that arising from the decrease of percolation threshold with the film thickness. The as-deposited Fe48C52 thin films are superparamagnetic, and their magnetization curves measured at room temperature can be described by a Langevin function taking a log-normal size distribution into account. With the increase of annealing temperature and time, the coercivity of annealed Fe48C52 granular films increases and the magnetic reversal mechanism changes from domain wall motion to Stoner-Wohlfarth rotation, which can be attributed to the decrease of inter-particle interaction caused by improved phase separation.