Origin of high coercivities in as-cast SmFe4-xNixB alloys

Large coercive fields have been observed in as-cast SmFe
4-x
Ni
x
B alloys. The coercive fields increase drastically at lower temperatures and complete hysteresis loops cannot be measured at cryogenic temperatures. Microstructure studies show the presence of two types of grain with slightly different chemical composition (Fe-rich or Ni-rich) but with the same crystal structure. These differences in chemical composition may also occur at the microscopic level and can cause fluctuations in exchange and magnetic anisotropy that can pin the domain walls.