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We combine powder neutron diffraction, magnetometry and 57Fe Mössbauer spectrometry to determine the nuclear and magnetic structures of a strongly interacting weberite-type inorganic-organic hybrid fluoride, Fe2F5(Htaz). In this structure, Fe2+ and Fe3+ cations form magnetically frustrated hexagonal tungsten bronze layers of corner-sharing octahedra. Our powder neutron diffraction data reveal that, unlike its purely inorganic fluoride weberite counterparts which adopt a centrosymmetric Imma structure, the room-temperature nuclear structure of Fe2F5(Htaz) is best described by a non-centrosymmetric Ima2 model with refined lattice parameters a=9.1467(2) Å, b=9.4641(2)Å and c=7.4829(2) Å. Magnetic susceptibility and magnetization measurements reveal that strong antiferromagnetic exchange interactions prevail in Fe2F5(Htaz) leading to a magnetic ordering transition at TN =93 K. Analysis of low-temperature powder neutron diffraction data indicates that below TN, the Fe2+ sublattice is ferromagnetic, with a moment of 4.1(1) ƒÊB per Fe2+ at 2K, but that an antiferromagnetic component of 0.6(3) ƒÊB cants the main ferromagnetic component of Fe3+, which aligns antiferromagnetically to the Fe2+ sublattice. The zero-field and in-field Mossbauer spectra give clear evidence of an excess of high-spin Fe3+ species within the structure and a non-collinear magnetic structure. [ABSTRACT FROM AUTHOR]

Beyond its various properties, the model multiferroic BiFeO 3 (BFO) displays a rich magnetic structure illustrated in the bulk by its long period (∼62 nm) spin cycloidal modulation. Here, BFO nanoparticles are produced by a facile hydrothermal method and show average size of 8 nm and a narrow size distribution, as determined using x-ray diffraction analysis and transmission electron microscopy images. Mössbauer spectrometry (MS) unambiguously reveals that a cycloidal modulation does still exists with particles about 5 times smaller than the bulk cycloid. Combining macroscopic magnetic measurements and in situ Mössbauer spectrometry, we demonstrate that a critical magnetic field of ∼0.2 T destabilizes the cycloidal modulation to lead to a homogenous antiferromagnetic state, as the result of magnetic anisotropy due to magnetoelastic and surface-confinement effects. More interestingly, further increasing of the external magnetic field up to 8 T does not change the average magnetic hyperfine field and results into multiple Mössbauer sextets we propose to explain by a flexomagnetic effect i.e. magnetic anisotropies resulting from strain gradients due to a continuous variation of the coupling between magnetization and the structural distortion from the surface to the particle core. • Successful synthesis of 8 nm BFO nanoparticles by a facile hydrothermal process. • XRD analysis and TEM characterization. • Demonstration of the destabilization of cycloidal modulation by combining macroscopic magnetic measurements and in situ Mössbauer spectroscopy. [ABSTRACT FROM AUTHOR]