Microscopic States and the Verwey Transition of Magnetite Nanocrystals Investigated by Nuclear Magnetic Resonance

Abstract

Fe nuclear magnetic resonance (NMR) of magnetite nanocrystals ranging in size from 7 nm to 7 μm is measured. The line width of the NMR spectra changes drastically around 120 K, showing 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.

A Short Review on Verwey Transition in Nanostructured Fe3O4 Materials

“…The VT is generally masked in magnetization data of superparamagnetic nanocrystals below the blocking temperature; therefore, it was reassured that the heat capacity of nanocrystals shows a lambda-like anomaly similar to the single crystal Fe 3 O 4 , confirming a truly thermodynamic transition. 57 Fe nuclear magnetic resonance (NMR) of Fe 3 O 4 nanocrystals (7 nm-7 μm) was also measured [19]. The line width of NMR spectra changes drastically around 120 K (see Figure 3(d)), showing microscopic evidence of the Verwey transition.…”

“…Size-dependent characterization of the VT: temperature dependence of the first derivative of conductance (a), magnetization data measured at 100 Oe (b), and total heat capacity divided by temperature (c). The full width at quarter maximum (FWQM) of the Fe NMR spectra versus temperature[19]. The inset shows the Fe NMR spectral intensity of the 25 nm sample obtained at temperatures around T V (d).…”

Slow oxidation of magnetite nanoparticles elucidates the limits of the Verwey transition

“…NMR spectra of all samples were measured using a home-made solid-state NMR spectroscopy instrument with a cryostat as in the ref. 12 . The single peak located at 69.5 MHz splits into multiple peak structures through the Verwey transition for the fresh sample 12 , but the split NMR peaks merge and become a broad peak in the spectra of aged samples as oxidation progresses (Supplementary Fig.…”

Anomalous electric transport across Verwey transition in nanocrystalline Fe3O4 thin films

“…7 As the grain boundary volume is not only a factor in electric transport of nano-crystalline Fe 3 O 4 thin films, the different post-annealing treatments are involved that could also affect the Fe +3 /Fe +2 stoichiometry, residual thermal stresses, and B-site cation and oxygen vacancies differently. [1][2][3]32 To elucidate this aspect, we have taken room temperature Raman spectra (within the spectral range 150-800 cm −1 ) of two representative samples (T g = RT) for each reduction process, as shown in Fig. 4.…”

“…Verwey transition, particularly, in nano-sized strongly correlated magnetite [Fe 3 O 4 : (Fe +3 ) A (Fe +2 , Fe +3 ) B O 4 ] materials, continues to draw the attention of researchers involved with spintronics and the physics associated with it. [1][2][3] Electron hopping between Fe +2 and Fe +3 ions at octahedral B-sites facilitates room temperature electric conductivity σ = 200 (Ω cm) −1 in Fe 3 O 4 . Otherwise, most of the ferrites derived from it are insulators.…”