A soft magnetic CoNiFeS film as a write head core material for the next generation was prepared by electrodeposition. In this system, thiourea was used as an additive in the CoNiFe ternary alloy plating bath. The most suitable magnetic properties were obtained at the film composition of (Co73Ni12Fe15)(99.1)S-0.9 [atomic percent (a/o)] with a high saturation magnetic flux density (B-s) of 1.7 T, a high resistivity (rho) of 51 mu Omega cm, and a low saturation magnetostriction (lambda(s)) of 4.4 x 10(-6). The film consisted of fine minute crystal grains 5 to 10 nm in diameter. The S content of ca. 0.9 a/o is believed to be responsible for the formation of small crystal grains with low coercivity and high resistivity.