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.
Mössbauer measurements are performed on RE2(Fe1−xCox)17, RE = Y, Gd, Dy compounds. The spectra are analyzed using 7 (8) subspectra for samples with Th2Zn17(Th2Ni17) structure. The differences between Fe sites, caused by the Co substitution, are neglected. A maximum value of the hyperfine field is obtained at x ≈ 0.3. This agrees with the concentration dependence of the mean 3d moment and will be discussed by means of the model of two common, partially filled 3d subbands and a model involving a portion of negative exchange. From the concentration dependence of the isomer shift charge transfers to the Fe atoms may be excluded. This is supported by the results obtained for the quadrupole splittings for 0 < x < 0.5. For 0.5 ≦ x ≦ 0.9 a rotation of the easy axis into the c‐direction is obtained from the ratio of he quadrupole splitting, which agrees very well with the theoretically predicted ratio of (−1) :2.
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