Coercivity of 2:17 based permanent magnets

High-quality permanent magnets either are based on large nucleation fields or strong pinning forces of domain was (dws). In the case of 2:17 based magnets depending on the temperature range considered both hardening mechanisms have been discussed. The dominant hardening mechanisms at temperatures below around 700 K are repulsive or attractive pinning of domain walls at the cell walls (1:5 structure) between the nanostructured pyramidal cells (2:17 structure). Micromagnetic calculations of the pinning forces sensitively depend on the shape of the interaction potential between the dw and the microstructure. In the case of the 2:17 based magnets these calculations become rather complex if the domain wall width is of the order of the potential width. In this case the domain wall is modified due to the space dependent local anisotropy which changes from K-1(2:17) to K-1(1:5) of the cell boundary phase over a distance D of the intergranular phase boundary. Using the K-1(r)-profiles as determined from the chemical composition obtained by high-resolution EDX measurements the coercive field due to domain wall pinning is determined self-consistently as a function of the pinning-potential parameters taking into account the modification of the domain wall by the space dependent material parameters. It is shown that the coercive field of the high-temperature magnets as a function of the width of the cell boundary phase varies between 3 T for a sharp boundary (D = 0 nm) and 1 T for a wide boundary (D = 4 nm). On the basis of micromagnetism the condition for the dominance of pinning or nucleation is investigated. It is shown that in the case of 2:17 based high-temperature magnets with increasing temperature the hardening mechanisms change from repulsive to attractive pinning and nucleation starts at around 700 K. The temperature ranges where the transitions take place sensitively depend on the Cu content and on the annealing parameters.