PURPOSE: To provide an electroless plating method of stably manufacturing an isotropic semi-hard magnetic film so as to replace a semi-hard magnetic film large in saturation magnetization, possessed of a coercive force of 30-300Oe and suitable for a base layer used in a vertical recording medium but having such a problem that magnetic anisotropy occurs causing coercive force to vary.

CONSTITUTION: An in-plane magnetic isotropic thin film at least composed of Co, Mn, and B as component elements is provided. The film is formed through an electroless plating bath which is obtained by adding citric acid groups or malic acid groups as complexing agent to an aqueous solution which contains cobalt ions and manganese ions as metal ions and dimethylamine volan or diethylamine volan as reducing agent for the metal ions concerned.