Evolution of magnetic properties and microstructure of Hf₂Co₁₁B alloys

Amorphous Hf₂Co₁₁B alloys produced by melt-spinning have been crystallized by annealing at 500-800°C, and the products have been investigated using magnetization measurements, x-ray diffraction, and scanning electron microscopy. The results reveal the evolution of the phase fractions, microstructure, and magnetic properties with both annealing temperature and time. Crystallization of the phase denoted HfCo₇, which is associated with the development of coercivity, occurs slowly at 500°C. Annealing at intermediate temperatures produces mixed phase samples containing some of the HfCo₇ phase with the highest values of remanent magnetization and coercivity. The equilibrium structure at 800°C contains HfCo₃B₂, Hf₆Co₂₃, and Co, and displays soft ferromagnetism. Maximum values for the remanent magnetization, intrinsic coercivity, and magnetic energy product among the samples are approximately 5.2kG, 2.0 kOe, and 3.1 MGOe, respectively, which indicates that the significantly higher values observed in crystalline, melt-spun Hf₂Co₁₁B ribbons are a consequence of the non-equilibrium solidification during the melt-spinning process. Application of high magnetic fields during annealing is observed to strongly affect the microstructural evolution, which may provide access to higher performance materials in Zr/Hf-Co hard ferromagnets. The crystal structure of HfCo₇ and the related Zr analogues is unknown, and without knowledge of atomic positions powder diffraction cannot distinguish among proposed unit cells and symmetries found in the literature.