1) The PH ranges which regulate three types of the Goethite to be produced when the ferrous sulfate solution is mixed with sodium hydroxide soluton, are determined as fottows ;
at PH = 5.4～1.0 ; pale yellow and needle-like crystalline.
at PH = 13.2～5.4, orange yellow and bar-or needle-like crystalline.
at PH = 13.6～13.2, pale yellow and needle-like crystalline.
where, in each PH, the first terms indicate those PH value that two solutions are mixed, and the second terms are PH values gained when the oxydizing reactions of those mixed solutions by air-blowing are ended.
2) The optimum conditions to gain the needle-like and 0.5×0.1μ size crystalline for tape-racording are given.
3) The cobalt-ferrite crystalline obtained by wett method and treated at 600°～700℃ gives much higher Br and Hc values.