Magnetic properties of permanent magnets based on M-type strontium ferrite with Sr <sup>2+</sup> substituted by Sm <sup>3+</sup> and Gd <sup>3+</sup> and Fe <sup>3+</sup> substituted by Zn <sup>2+</sup> and Co <sup>2+</sup>
Sr <sub>1-x</sub>Ln <sub>x</sub>Fe <sub>12-x</sub>M <sub>x</sub>0 <sub>19</sub> (Ln <sup>3+</sup> - Sm <sup>3+</sup>, Gd <sup>3+</sup>; M <sup>2+</sup> - Co <sup>2+</sup>, Zn <sup>2+</sup>) and (Ba <sub>0,5</sub>Sr <sub>0,5</sub>) <sub>1-x</sub>Sm <sub>x</sub>Fe <sub>12-x</sub>Co <sub>x</sub>0 <sub>19</sub> (x = 0; 0,1; 0,2; 0,3) ferrites have been prepared by solid-state method in air at 1473 K. It has been shown that isotropic permanent magnets based on Sr <sub>0,9</sub>Sm <sub>0,1</sub>Fe <sub>11,9</sub>Co <sub>0,1</sub>О <sub>19</sub>, (Ba <sub>0,5</sub>Sr <sub>0,5</sub>) <sub>0,9</sub>Sm <sub>0,1</sub>Fe <sub>11,9</sub>Co <sub>0,1</sub>О <sub>19</sub> ferrites show higher values of magnetic induction (B <sub>r</sub>), coercive force ( <sub>B</sub>H <sub>c</sub>) and maximum energy product (BH) <sub>max</sub> than ones based on SrFe <sub>l2</sub>0 <sub>19</sub> ferrite. It has been found that using SrFe <sub>9,5</sub>Al <sub>2,5</sub>0 <sub>19</sub> as an additive increases the coercive force ( <sub>B</sub>H <sub>c</sub>) and maximum energy product (BH) <sub>max</sub> for isotropic permanent magnets from the solid solutions of Sr <sub>0,9</sub>Sm <sub>0,1</sub>Fe <sub>11,9</sub> Co <sub>0,1</sub>О <sub>19</sub>, (Ba <sub>0,5</sub>Sr <sub>0,5</sub>) <sub>0,9</sub>Sm <sub>0,1</sub>Fe <sub>11,9</sub>Co <sub>0,1</sub>О <sub>19</sub> ferrites.