Two ions in adjacent sites can have magnetic interactions. If
the interaction is antiferromagnetic, the intensity of light absorption
by Fe3+ ions can be significantly changed.
Comparison of the spectra of hematite, Fe2O3, and andradite garnet, Ca3Fe2(SiO4)3. The spectra are presented for equal amounts of iron. The absorption by hematite at 860 nm is much greater due to antiferromagnetic interactions.
The graph on the right shows that the lower the magnetic moment (that is to say, the stronger the antiferromagnetic interaction) the more intense (greater e) will be the absorption for the band near 860 nm.