A new soft magnetic CoNiFeB thin film with high saturation magnetic flux density, B-S and good corrosion resistance has been prepared by using an electroless deposition method. The electroless CoFeB thin film has high B-S but a poor corrosion resistance compared with the electrodeposited NiFe (80-permalloy) film. The addition of Ni to the CoFeB film improves corrosion resistance without affecting its soft magnetic properties. One of the best CoNiFeB films prepared had a composition of Co77Ni13Fe9B1 with excellent magnetic and electrical properties (high B-S = 1.5-1.7 T, low coercivity, H-C = 1.2 Oe, low magnetostriction, lambda (S) = 2.0 x 10(-6), and moderate resistivity, rho = 40 mu Ohm cm). The new CoNiFeB thin film is suitable for use as a core material for the next generation of merged MR-heads.