$Co^{3+} = [Ar] 3d^6$, Unpaired $e^-(n) = 4$. Spin magnetic moment = $\sqrt{4(4+2)} = \sqrt{24} BM$. $Cr^{3+} = [Ar] 3d^3$, Unpaired $e^-(n) = 3$. Spin magnetic moment = $\sqrt{3(3+2)} = \sqrt{15} BM$. $Fe^{3+} = [Ar] 3d^5$, Unpaired $e^-(n) = 5$. Spin magnetic moment = $\sqrt{5(5+2)} = \sqrt{35} BM$. $Ni^{2+} = [Ar] 3d^8$, Unpaired $e^-(n) = 2$. Spin magnetic moment = $\sqrt{2(2+2)} = \sqrt{8} BM$.