Structure and magnetic properties of the C14 Laves phase (Fe1−xMnx)2Nb intermetallics

X-ray diffraction and DC magnetization have been used to investigate the magnetic properties of the (Fe1−xMnx)2Nb intermetallics with the Nb content kept fixed. The hexagonal C14 Laves phase (space group P6/mmc) is found in all the studied alloys coexisting with a tetragonal phase (P4/mmm) which increases in proportion with the Mn content. The unit cell volume exhibits a parabolic behavior for the hexagonal phase, while steadily growing with the increase of the Mn concentration. At low temperatures the onset of a ferromagnetic ordering has been observed in the Laves phase. The Curie temperature attains a maximum (60 K) for a cell volume of 161.21 Å3. The tetragonal phase shows ferromagnetism above x = 0.33 with a Curie temperature that increases with the Mn content. Antiferromagnetic correlations have been detected in both structures within the studied composition range, coexisting with ferromagnetism. At low Mn concentration a competition mechanism between ferromagnetic and antiferromagnetic spin correlations leads to frustrated couplings ending in a re-entrant spin-glass like behavior at low temperature. At 4.2 K the Mn-richer alloys exhibit a superconducting behavior.