A technique for controlling primary particle diameter, secondary particle length and magnetic properties of ultrafine iron particles prepared by pyrolysis of Fe(CO)5 has been studied. A relation between fractal dimension as a parameter of shape of the secondary particles and magnetic properties has been also studied. The length of secondary particles is closely related in the primary particle diameter; the larger the diameter, the longer the length. It is affected by the feed concentration, residence time and pre-decomposition ratio of Fe(CO)5. The coercive force of the iron particle powder reaches its maximum value of 120kAm-1 at the primary diameter of 20-25nm. The squareness Br/Bm of video tapes is improved by using longer secondary particles. It is also improved by using particles with smaller fractal dimension. The shape of secondary particles can be characterized by using fractal dimension; the smaller the dimension, the larger the squareness of video tapes.
