NMR in magnetite below and around the Verwey transition

Abstract

NMR on ${}^{57}\mathrm{Fe}$ nuclei was measured in two single crystals of magnetite between 4.2 and 135 K (temperature of the Verwey transition ${T}_{V}=121.5$ and 123.9 K). We were able to register all lines detected at 4.2 K also at higher temperatures up to ${T}_{V}.$ The spectra are compatible with the space group $Cc,$ which predicts 8 and 16 lines for iron on tetrahedral (A) and octahedral (B) sites, respectively. One of the $\mathrm{Fe}(A)$ lines is doubly degenerate for all $T<{T}_{V},$ which restricts the possible models of the ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ordering on the B sites. Above ${T}_{V}$ no remnant of the low-temperature spectrum was detected. At 4.2 K the NMR spin-spin and spin-lattice relaxation times were determined.