Abstract
Here we report on the impact of reducing the crystalline size on the structural and magnetic properties of γ-Fe2O3 maghemite nanoparticles. A set of polycrystalline specimens with crystallite size ranging from ~2 to ~50 nm was obtained combining microwave plasma synthesis and commercial samples. Crystallite size was derived by electron microscopy and synchrotron powder diffraction, which was used also to investigate the crystallographic structure. The local atomic structure was inquired combining pair distribution function (PDF) and X-ray absorption spectroscopy (XAS). PDF revealed that reducing the crystal dimension induces the depletion of the amount of Fe tetrahedral sites. XAS confirmed significant bond distance expansion and a loose Fe-Fe connectivity between octahedral and tetrahedral sites. Molecular dynamics revealed important surface effects, whose implementation in PDF reproduces the first shells of experimental curves. The structural disorder affects the magnetic properties more and more with decreasing the nanoparticle size. In particular, the saturation magnetization reduces, revealing a spin canting effect. Moreover, a large effective magnetic anisotropy is measured at low temperature together with an exchange bias effect, a behavior that we related to the existence of a highly disordered glassy magnetic phase.
Highlights
Even before the advent of “nanoscale science” and “nanotechnology” as recognized fields of science and engineering, small particles, colloids, and high-surface-area materials have been critical in applications ranging from catalysis to waste management [1,2]
Here we report on the impact of reducing the crystalline size on the structural and magnetic properties of γ-Fe2O3 maghemite nanoparticles
Many studies are devoted to the control and the understanding of the synthesis to produce nanoparticles with tailored characteristics, [6,7,17,18,19,20,21,22], while more limited are the correlations with their crystal structure and microstructure [19,23,24,25]
Summary
Even before the advent of “nanoscale science” and “nanotechnology” as recognized fields of science and engineering, small particles, colloids, and high-surface-area materials have been critical in applications ranging from catalysis to waste management [1,2]. Many studies are devoted to the control and the understanding of the synthesis to produce nanoparticles with tailored characteristics, [6,7,17,18,19,20,21,22], while more limited are the correlations with their crystal structure and microstructure [19,23,24,25] Whether this can origin from the formation of complex architectures [7,19,26,27], a general limitation is given by the complexity of resolving the crystal structures of γ-Fe2O3 maghemite and Fe3O4 magnetite, since both phases have a cubic structure with very similar lattice parameter, which are more difficult to resolve when the crystal size is small
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