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Microscopic States and the Verwey Transition of Magnetite Nanocrystals Investigated by Nuclear Magnetic Resonance
journal contribution
posted on 2018-02-20, 00:00 authored by Sumin Lim, Baeksoon Choi, Sang Young Lee, Soonchil Lee, Ho-Hyun Nahm, Yong-Hyun Kim, Taehun Kim, Je-Geun Park, Jisoo Lee, Jaeyoung Hong, Soon Gu Kwon, Taeghwan Hyeon57Fe 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.