Microscopic states and the Verwey transition of magnetite nanocrystals investigated by nuclear magnetic resonance

⁵⁷Fe Nuclear Magnetic Resonance (NMR) was measured for magnetite nanocrystals ranging in size from 7 nm to 7 μm. The linewidth of the NMR spectra changes drastically around 120 K, showing microscopic evidence of the Verwey transition. In the region above the transition temperature, the linewidth of the spectrum increases and the spin-spin relaxation time decreases as the nanocrystal size decreases. The linewidth broadening indicates that the magnetic structure of the nanocrystals is significantly deformed and the charge order is markedly reduced compared to bulk even when structural distortion is unobservable. The reduction of the spin-spin relaxation time is attributed to the enhanced electron-phonon coupling in the quantum-confinement regime and thus to the suppressed polaron hopping conductivity in ferromagnetic metals. The results show that the magnetic distortion occurs in the entire nanocrystal, contrary to the prediction of the frequently adopted core-shell binary model.