$^{57}Fe$ NMR Study of magnetic structures of $\alpha$- and $\gamma$- phase iron oxides ($Fe_2O_3$)

Abstract

In this study, the magnetic properties of $\alpha$- and $\gamma$- phases of iron oxides ($Fe_2O_3$) were investigated by the nuclear magnetic resonance (NMR) method. First, we investigated the magnetic properties of maghemite ($\gamma - Fe_2O_3$) powder sample. As a result, we found 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). Moreover, we confirmed that the field dependence of resonance frequency indicates that the spins at the octahedral site are nearly antiparallel with external field, while those at the tetrahedral site are parallel but canted. Second, we performed NMR experiment on 40 nm sized maghemite nanoparticles. We confirmed that the spins are canted with different angles from those of bulk sample. The canting angles of the nanoparticle and the bulk at the tetrahedral site are the same within experimental error. However, the canting angle of the nanoparticle at the octahedral site shows slight increase which exceeds the experimental error range. Last, we investigated the magnetic properties of a natural haematite ($\alpha - Fe_2O_3$) crystal at the temperature of liquid helium using magnetometry and $^{57}Fe$ nuclear magnetic resonance (NMR) methods. The magnetization curve shows that the net magnetization in the (111) plane vanishes at the Morin temperature (260 K) but weakly reappears as the temperature decreases below 40 K. A comparison of the magnetization and NMR results indicates that the spin state, direction and canting angle, at a low temperature is identical to that in a weak-ferromagnetic state above the Morin temperature. Its volume, however, occupies only 3 % of the entire sample. The rest of the spins align in the [111] direction antiferromagnetically. The relaxation of magnetization with time, the difference of the zero-field-cooled and field-cooled magnetization, and the rise of the NMR echo intensity with increasing magnetic field exhi...