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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 .

Magnetic and magnetocaloric properties of high-purity, giant magnetocaloric polycrystalline and singlecrystalline Fe 2 P are investigated. Fe 2 P displays a moderate magnetic entropy change, which spans over 70 K and the presence of strong magnetization anisotropy proves this system is not fully itinerant but displays a mix of itinerant and localized magnetism. The properties of pure Fe 2 P are compared to those of giant magnetocaloric (Fe,Mn) 2 (P,A) (where A = As, Ge, Si) compounds helping understand the exceptional characteristics shown by the latter, which are so promising for heat pump and energy conversion applications.

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The compound FeMnP 0.5 Si 0.5 has been studied by magnetic measurements, Mössbauer spectroscopy and electronic structure and total energy calculations. An unexpected high magnetic hyperfine field for Fe atoms located at the tetrahedral Me(1) site in the Fe 2 P structure is found. The saturation moment derived from magnetic measurements corresponds to 4.4 µ B /f.u. at low temperatures, a value substantially higher than previously reported, but in accord with the results from our electron structure calculations. This high saturation moment, a first order nature of the ferromagnetic transition and a tunable transition temperature make the Fe 2−x Mn x P 1−y Si y system promising for magnetocaloric applications.PACS numbers: 75.30.Cr, 75.30.Sg, 31.15.A-Fe 2 P based compounds are promising materials for magnetocaloric applications.A study of the FeMnP 1−y Si y series by Cam Thanh et al. showed strong magnetocaloric effects 1 . It was found that FeMnP 1−y Si y crystallizes in the hexagonal Fe 2 P-type structure which persists for a Si content from y ≈ 0.24 up to y ≈ 0.65 and undergoes a first order para-to ferromagnetic phase transition with T C tunable around room temperature. The low temperature saturation moment for FeMnP 0.50 Si 0.50 was of the order 3.8 µ B per formula unit (f.u.) 1 .In the hexagonal Fe 2 P-type and the closely related orthorhombic Co 2 P-type (y ≤ 0.24) structures two equally populated metal sites are present, the tetragonally coordinated Me(1) and the pyramidally coordinated Me(2) site 2 . The initial compound Fe 2 P has a saturation moment of 2.94 µ B /f.u. with the site specific magnetic moments of µ(Fe(1),Fe(2)) = (1.03, 1.91)µ B and a magnetic hyperfine field of B hf (Fe(1),Fe(2)) = (-11.4, -18.0) T, Ref. 3. By substitution of Fe for Mn antiferromagnetic ordering and a structural phase transition into the orthorhombic Co 2 P-type structure are induced. In pure FeMnP, the Fe atoms preferentially populate the Me(1) site and the Mn atoms the Me(2) site 4 . Substitution of P for Si restores the hexagonal Fe 2 P-type structure and stabilizes ferromagnetic ordering. At the border region between the orthorhombic and hexagonal structure, y ≈ 0.25, the compound with hexagonal structure could either be in an antiferromagnetic or a ferrimagnetic state depending on the heat treatment after the synthesis 5 .In this study of the FeMnP 0.5 Si 0.5 compound it is found that the magnetic hyperfine field for Fe atoms located at the tetrahedral Me(1) site is strongly enhanced in comparison to other compounds within the Fe 2−x Mn x P 1−y Si y system. The enhanced magnetic hyperfine field is accompanied by a correspondingly high saturation moment of 4.4 µ B /f.u. at 5 K. An enhanced saturation moment can promote the magnetocaloric properties and make this material suitable for applications. In addition, the FeMnP 1−y Si y series is favorable for applications due to its cheap, nonhazardous and environmentally friendly element composition.Samples of stoichiometric FeMnP 0.5 Si 0.5 were prepared by the drop synthesis...