The coercive field of permanent magnets decreases with increasing grain size.
The grain size dependence of coercivity is explained by a size dependent
demagnetizing factor. In Dy free Nd$_2$Fe$_{14}$B magnets the size dependent
demagnetizing factor ranges from 0.2 for a grain size of 55 nm to 1.22 for a
grain size of 8300 nm. The comparison of experimental data with micromagnetic
simulations suggests that the grain size dependence of the coercive field in
hard magnets is due to the non-uniform magnetostatic field in polyhedral
grains
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