Verwey transition of nano-sized magnetite crystals investigated by $^{\mathrm{57}}$Fe NMR

It is well known that magnetite crystals undergo a metal-insulator transition at the Verwey transition temperature, T$_{\mathrm{V}} \quad =$ 123 K. In this work, we studied the Verwey transition of nano-sized crystals with $^{\mathrm{57}}$Fe NMR. In the metallic state above Tv, the NMR spectrum shows a single sharp peak, which broadens below T$_{\mathrm{V}}$ indicating the Verwey transition. We measured the spectra of the nano-crystals with radii of 16 nm, 25 nm, and 40 nm and compared with that of a bulk. The transition temperature obtained from the NMR spectra depends on both the crystal size and crystallinity. When the crystal size decreases from bulk to 16 nm, the transition temperature drops from 123 K to 100 K. The transition temperature of the samples kept dry air decrease due to aging.