The invention relates to a modified strontium ferrite of the general chemical formula: Sr1-xLaxFe12-yCoyO19; in which x=y=0.01-1.00 or x=0.15 and y=x/1.6n to =x/2.6n, where n is the Fe2O3:SrO ratio used and may be 5-6. According to the invention, lanthanum and cobalt are added in an amount such that 0.14≦x≦0.145 and 0.14≦y≦0.145. The modified strontium ferrite thus prepared has excellent magnetic properties with regard to the remanence BR, the coercive force HCJ and the ratio of HK to HCJ.