Crystal growth of iron-based compounds by the cold-crucible Czochralski technique

Abstract

Bulk single crystals of NbFe2, HoFe2 and (TbDy)Fe2 have been grown from a melt with a cold-crucible Czochralski technique. The as-grown crystals were characterised by X-ray diffraction, optical and scanning electron microscopy. The microstructural defects of the pseudo-binary (TbDy)Fe2 crystals have been studied by transmission electron microscopy. The as-grown NbFe2 crystals with C14 hexagonal structure contain many thermal cracks, but no secondary phases. Both X-ray lattice parameter measurement and EDX/SEM analysis show consistent stoichiometric composition. However, both HoFe2 and (TbDy)Fe2 have C15 cubic Laves phase structure and form by peritectic reactions with a narrow separation between the liquidus and solidus lines. By adopting a slow growth rate, a planar growth front has been maintained and bulk single crystals have been achieved. The as-grown crystals of (TbDy)Fe2 contain many needle-shaped Widmanstatten precipitates formed by a solid state reaction. Microstructural study by transmission electron microscopy revealed the presence of many {111} plane stacking faults and a few twin boundaries. A large number of interfacial dislocation networks have been observed on the Widmanstatten precipitates. The presence of thermal cracks, microstructural defects and Widmanstatten precipitates in the Laves phase compounds will strongly affect the measurement of the magnetic properties of the crystals.