Coercivity and unit particle size of metal pigment

Iron based metal pigments exhibiting coercivity as high as 1000-1400 Oe, and iron nitride (Fe4N) pigments exhibiting coercivity as high as 500- 800 Oe for magnetic recording media was prepared from acicular particles of iron-cobalt oxalate, iron oxalate, iron oxides or iron oxyhydrates by reduction in hydrogen or following nitridation in ammonia. The particles of the pigment maintained the characteristic original shape of the oxalates, oxides or oxyhydrates. In addition, the particles contained a number of pores in them, and the particles consisted of small unit particles linked together to form a stereo-network structure. The coercivity of the pigment was related to the pore size, the specific surface area and the crystallite size. A study of the unit particle which is essentially related to the single domain particle was made. The size of the unit particle was determined by an electron microscope (D1), calculated from the pore size (D2), calculated from the specific surface area (D3) and determined by X-ray diffractometry (Lc). At the higher coercivity level of the metal pigments, D2, D3 and Lc are all in good agreement, that is, they all have a nearly spherical single crystallite as large as 200-300 Å which plays the role of a single unit particle. In the case of iron nitride (Fe4N) pigment, D2 and Lc are in good agreement, at the value of 300-400 Å.