Microstructure and Magnetic Properties of Thin Iron Carbide Films Formed Under Plasma-Enhanced Chemical Vapor Deposition

The microstructures of plasma-deposited Fe _7C_3 and Fe _3C films were investigated by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and Auger electron spectroscopy. According to X-ray diffraction, single phase Fe_7 C_3 and Fe_3 C films were successfully produced on glass substrates at temperatures of 300^circC and 400^circC, respectively. Electron diffraction of films deposited on carbon coated copper grids showed the presence of Fe_7C _3, Fe_3C, and Fe_3O_4 at all deposition temperatures, suggesting that the films grown on glass probably contain small quantities of secondary phases with extremely fine grains. Two morphologies were observed with high resolution transmission electron microscopy in films deposited on carbon-coated grids at 300^circC. The first morphology exhibited half-micron size platelets of both Fe_7C_3 and Fe_3C surrounded by 0.1-0.2 μm crystals on continuous film regions of the carbon support film on the copper grids. The second morphology observed at the edges of torn carbon films consists of grape-like clumps of 100-200 A size crystals each surrounded with a uniform graphitic coating. The magnetic properties of the Fe_7 C_3 and Fe_3 C films were determined using a SQUID magnetometer. The magnetic moments of films were found to be aligned perpendicular to the film surface. After annealing the Fe_7 C_3 film an increase in coercivity and saturation magnetization was observed. In contrast, the coercivity of the Fe_3C film decreased after annealing. The effect of annealing on coercivity is related to the particle sizes in the Fe _7C_3 and Fe _3C films. Further, an approximate zero coercivity measured for the Fe_7C _3 film at 300K is analogous to that of superparamagnetic behavior of free particles.