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 $T_V=121.5\mathrm{}$ 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 $\mathrm{Fe}\mathrm{}\left(A\right)\mathrm{}$ lines is doubly degenerate for all $T<\mathrm{}{T}_V,$ 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.