Although Fe3O4 is more magnetic than Fe2O3, it is less
effective for shielding than Fe2O3. This suggests that the
shielding in Fe2O3 is not just due to magnetic dipoles.
Electric dipoles may contribute. Both Fe2O3 and Fe3O4 are
more effective than fly ash for shielding. The resistivity is
slightly lowered and the reflectivity is enhanced by replacing
fly ash by either type of iron oxide, suggesting the role
of a nonmagnetic mechanism in the shielding enhancement