Microscopic states and the verwey transition of magnetite ($Fe_3O_4$) nanocrystals investigated by nuclear magnetic resonance

Abstract

Magnetite is the oldest magnetic material known to mankinds. Recently, mangeite nanocrystals receive attention due to the variety of potential applications. It is important to investigate how physical properties of nanocrystal changes compared to bulk. Nuclear magnetic resonance (NMR) of magnetite nanocrystals of size from 7 nm to 7 $\mu m$ is measured. At temperature around 120 K, the line width of the NMR spectra changes rapidly, showing the microscopic evidence of the Verwey transition. In the region above the transition temperature, the line width of the spectrum increases and the spin−spin relaxation time decreases as the nanocrystal size decreases. The line-width broadening indicates the significant deformation of magnetic structure and reduction of charge order compared to bulk crystals, even when the structural distortion is unobservable. The reduction of the spin−spin relaxation time is attributed to the suppressed polaron hopping conductivity in ferromagnetic metals, which is a consequence of the enhanced electron−phonon coupling in the quantum-confinement regime. Our results show that the magnetic distortion occurs in the entire nanocrystal and does not comply with the simple model of the core−shell binary structure with a sharp boundary. The Verwey transition in nanocrystal shows size-dependent thermal hysteresis. The width of hysteresis reached maximum when crystal/magnetic domain of nanocrystal becomes single domain. We suggest this giant thermal hysteresis is intrinsic feature of Verwey transition.