The effect of exchange coupling on switching fields in magnetic recording

It is known that increased levels of exchange generally reduce the coercivity of thin-film granular magnetic media. However, this behavior cannot be derived from simple mean field theory. Irrespective of whether grains switch individually or are located on the edge of a domain propagating through the medium, a critical grain tends to see equal numbers of positively and negatively polarized grains in its vicinity. The mean exchange field is thus zero and cannot apparently influence the remanent coercivity.