Maximizing loop squareness by minimizing gradients in the microstructure

Enhanced energy product and higher operating fields can be achieved by improving the squareness of the demagnetization loop. In isotropic materials, increasing loop squareness can be achieved by minimizing microstructural gradients and maximizing homogeneity. Achieving homogeneity by melt spinning can be difficult due to localized changes in cooling conditions from extrinsic factors such as gas entrapment between the melt and the chill surface and intrinsic effects resulting from the stochastic nature of nucleation on a surface. These localized variations are reduced by altering the solidification kinetics of the alloy, thus reducing its sensitivity to localized changes in cooling conditions. This was achieved through development of a nine-element modified Nd-Fe-B alloy which solidified with a uniform glass structure at much lower cooling rates. After crystallization, enhanced microstructural homogeneity was verified by electron microscopy and described by parameters based on the loop shape. The nine-element alloy had squarer loops and uniformly high hard magnetic properties (>12 MG Oe) for cooling rates over 10 m/s. (C) 1999 American Institute of Physics. [S0021-8979(99)47708-4].