Electroless Co alloy films for use as perpendicular magnetic recording media were investigated with respect to the thickness dependence of their magnetic properties. The read/write characteristics, especially the dependence of the overwrite on the recording density, were very different for Co-Ni-Re-Mn-P films and Co-Ni-Re-P films. Since those films displayed different thickness dependences of perpendicular coercivity Hc (⊥), efforts were made to control the thickness dependence of the Hc(⊥) of Co-Ni-Re-P alloy films. The thickness dependence of the Hc(⊥) of Co-Ni-Re-P alloy films can be easily controlled by adjusting the sodium malonate concentration in the plating bath. When films are very thin, such as 0.1 µm thick, the Hc (⊥) value and degree of c-axis orientation increased with increasing sodium malonate concentration. However, when the film thickness was increased to 0.5 µm, these properties were nearly the same for different sodium malonate concentrations.
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