Four High-Temperature Ferromagnets in the Hf-Fe-Sn System

We report the synthesis and characterization of four new ferromagnetic compounds discovered using Sn flux: Hf1.823(16)Fe5Sn3.815(14), HfFe2-xSnx, and two polymorphs of Hf1-xFe2Snx. All are closely related to HfFe2 Laves phase parent structures. HfFe2-xSnx (x approximate to 0.3-0.4) adopts the MgZn2-type (C14) crystal structure, whereas Hf1-xFe2Snx (x approximate to 0.1-0.4) adopts both the MgCu2-type (C15), and MgNi2-type (C36) structures. They crystallize in P6(3)/mmc, Fd3m, and P6(3)/mmc, respectively, with measured unit-cell parameters of a = 4.9238(7) angstrom and c = 7.9643(12) angstrom; a = 7.068(2) angstrom; and a = 4.9944(4) angstrom and c = 16.2604(15) angstrom, although phase width leads to a range of unit cell edge lengths. Hf1.823(16)Fe5Sn3.815(14) adopts a more complicated, incommensurately modulated structure in the superspace group Xmmm(00 gamma)000 with an orthorhombic subcell a = 9.7034(12) angstrom, b = 16.823(2) angstrom, and c = 8.4473(10) angstrom, three centering vectors of (1/2 0 0 1/2), (0 1/2 0 1/2), and (1/2 1/2 0 0), and a single-component modulation vector q = 0.2768(8)c*. The structure is composed of alternating slabs of the Fe-bonded Kagome nets observed in the HfFe2 parent structures alternated with Sn-rich Th2Zn17-type slabs, with Hf atoms primarily occurring at the interfaces between the slabs. All four compounds are ferromagnetic metals at room temperature, with Curie temperatures ranging from 467(2) to 658(2) K. Their coercive fields are remarkably low, between 2(1) and 15(2) Oe. Interestingly, in two of three cases the addition of nonmagnetic Sn atoms in place of magnetic Hf or Fe atoms in the HfFe2 structure seems to strengthen rather than weaken magnetic coupling and increase TC. Fits to electrical resistivity data for the compound suggest that electron scattering in the Laves phase polymorphs shows substantial contributions from electron-magnon and/or electron-electron scattering, while the electrical behavior of Hf1.823(16)Fe5Sn3.815(14) is dominated by electron-phonon scattering, as is the case in most metals.