Fe 5 SiB 2 has been synthesized and magnetic measurements have been carried out, revealing that M sat = 0.92 MA/m at T = 300 K. The M vs T curve shows a broad peak around T = 160 K. The anisotropy constant, K 1 , estimated at T = 300 K, is 0.25 MJ/m 3 . Theoretical analysis of Fe 5 SiB 2 system has been carried out and extended to the full range of Fe 5 Si 1−x PxB 2 , Fe 5 P 1−x SxB 2 , and (Fe 1−x Cox) 5 SiB 2 compositions. The electronic band structures have been calculated using the Full-Potential Local-Orbital Minimum-Basis Scheme (FPLO-14). The calculated total magnetic moments are 9.20, 9.15, 9.59 and 2.42µ B per formula units of Fe 5 SiB 2 , Fe 5 PB 2 , Fe 5 SB 2 , and Co 5 SiB 2 , respectively. In agreement with experiment, magnetocrystalline anisotropy energies (MAE's) calculated for T = 0 K changes from a negative (easy-plane) anisotropy −0.28 MJ/m 3 for Fe 5 SiB 2 to the positive (easy-axis) anisotropy 0.35 MJ/m 3 for Fe 5 PB 2 . Further increase of the number of p-electrons in Fe 5 P 1−x SxB 2 leads to an increase of MAE up to 0.77 MJ/m 3 for the hypothetical Fe 5 P 0.4 S 0.6 B 2 composition. Volume variation and fixed spin moment calculations (FSM) performed for Fe 5 SiB 2 show an inverse relation between MAE and magnetic moment in the region down to about 15% reduction of the spin moment. The alloying of Fe 5 SiB 2 with Co is proposed as a practical realization of magnetic moment reduction, which ought to increase MAE. MAE calculated in virtual crystal approximation (VCA) for a full range of (Fe 1−x Cox) 5 SiB 2 compositions reaches the maximum value of 1.16 MJ/m 3 at Co concentration x = 0.3, with the magnetic moment 7.75µ B per formula unit. Thus, (Fe 0.7 Co 0.3 ) 5 SiB 2 is suggested as a candidate for a rare-earth free permanent magnet. For the stoichiometric Co 5 SiB 2 there is an easy-plane magnetization, with the value of MAE = −0.15 MJ/m 3 .

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