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.