The size of a grains, which is frequently expressed as the diameter of the grain. Iron-oxide grains with diameters smaller than 40 nm exhibit superparamagnetic behaviour. Between 40 nm and 1 \mum grains have a single magnetic domain or pseudo single magnetic domain (e.g. flower state). Grains larger than 1 \mum frequently contain more than 1 magnetic domain, hence we call them multi domain.
![Chart with grain sizes based on coercitivity and nanoparticles radius](https://mimatom.org/wp-content/uploads/2022/03/grain_sizes.png)