A study on the order-disorder phase transformations of rapidly solidified Sm-Co-based permanent magnets

The structural transformation from the metastable, disordered TbCu7-type SmCo7 structure to the equilibrium, ordered Th2Zn17-type Sm2Co17 structure was revealed by X-ray diffraction analysis using Rietveld refinement. The magnetic properties depended strongly on the stage of the transformation, as the coercivity depended on the annealing temperature. The as-solidified alloys in the TbCu7-type structure exhibited coercivity as high as 7.85 kOe, which increased to greater than 9 kOe after heat treatment. The magnetization processes were also strongly influenced by the structural state. Initially it was dominated by nucleation processes, which gave way to domain wall pinning-controlled magnetization processes with the development of the Sm2Co17 structure. Transmission electron microscopy revealed the development of antiphase domains during heat treatment, which apparently served as the domain-wall pinning sites during magnetization reversal.