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The digital recording density of magnetic media can be improved by the development of thin ferromagnetic coatings having a high coercive force. An experimental study of the influence of zinc on the coercive force of chemically reduced cobalt-phosphorus films is presented. Incorporation of zinc increases the coercive force of cobalt-phosphorus films on Mylar substrates by a factor of 2 to 60. Cobalt-zinc-phosphorus films exhibited coercivities ranging from 300 to 1200 Oe, depending on the preparation conditions. The influence of substrate and solution parameters, such as hypophosphite ion concentration, zinc concentration, and pH are discussed with respect to the hysteresis properties of the films. The influence of zinc on the coercivity is attributed to increased magnetic isolation of the cobalt particles and to a modification of particle size.