  To identify the possible mechanism of coercivity (Hc) degradation of Nd-Fe-B
sintered magnets, we study the roles of the exchange field acting on the 4f
electrons in Nd ions and theoretically investigate how the variation of the
exchange field affects the values of the magnetic anisotropy constants K1 and
K2. We find that, with decreasing exchange field strength, both values decrease
as a result of the lower asphericity of the 4f electron cloud, indicating that
the local anisotropy constants might become small around the grain boundaries
where the exchange fields are decreased owing to the smaller coordination
number.

Materials Science