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 Nd2Fe14B 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.
Cite
@article{arxiv.1603.08239,
title = {Grain-size dependent demagnetizing factors in permanent magnets},
author = {Simon Bance and Bernhard Seebacher and Thomas Schrefl and Lukas Exl and Michael Winklhofer and Gino Hrkac and Gergely Zimanyi and Tetsuya Shoji and Masao Yano and Noritsugu Sakuma and M Ito and Akira Kato and Akira Manabe},
journal= {arXiv preprint arXiv:1603.08239},
year = {2016}
}