NMR analysis of ferromagnets: Fe oxides

A short historical review is given on internal field NMR of ferromagnets, illustrated with recent pulsed NMR spectra of the elemental ferromagnets Fe, Co and Ni and the Fe-oxides magnetite, maghemite and hematite, which, with the exception of maghemite, have resonance frequencies first reported over 45 years ago. Since the magnetic hyperfine field at the nucleus is not known a priori, the original search frequency motivations are discussed along with the mechanisms for the initially much larger than expected (～10³) NMR signals that were observed. The ⁵⁷Fe spectra of the three principal Fe-oxide ferromagnets, magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃) and hematite (α-Fe₂O₃), obtained here under uniform spectroscopic conditions, are then discussed in more detail, with a focus on the influence of particle size and vacancy content on the hyperfine fields