The spin structure of maghemite investigated by 57Fe NMR

Maghemite (gamma-Fe2O3) is a ferrimagnet that has long been studied; however, its exact spin structure and hyperfine field are still controversial. Maghemite has an inverse spinel structure where magnetic Fe ions occupy either the interstitial octahedral or tetrahedral sites of the oxygen lattice. The nuclear magnetic resonance (NMR) spectrum shows double peaks in zero external field. We split these peaks clearly by applying an external magnetic field and compared the peak intensities. The result showed that the hyperfine field at the nuclear spins in the octahedral site (50.2 T) was larger than that in the tetrahedral site (49.3 T). The field dependence of resonance frequency indicates that the spins at the octahedral site are antiparallel with external field, while those at the tetrahedral site are parallel but canted by 42 +/- 10 degrees.