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Magnetic Hyperfine Interactions in the Mixed-Valence Compound Fe7(PO4)6 from Mössbauer Experiments

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journal contribution
posted on 24.07.2018, 00:00 by Alexey V. Sobolev, Alena A. Akulenko, Iana S. Glazkova, Alexei A. Belik, Takao Furubayashi, Larisa V. Shvanskaya, Olga V. Dimitrova, Igor A. Presniakov
Hyperfine parameters of Fe7(PO4)6 phosphate were examined using 57Fe Mössbauer spectroscopy in a wide temperature range 4.2 ≤ T ≤ 300 K. Four different iron sites were successfully detected: two for the high-spin Fe2+ and two for Fe3+ ions. Mössbauer spectra below TN1 ≈ 47 K were analyzed by diagonalization of the full nuclear-interaction Hamiltonian. The noticeable difference between the saturated hyperfine fields (Bhf) for the 5-fold-coordinated site (∼5.0 T) and the octahedral site (∼10.1 T) was related to the symmetry of the local crystal field. Using crystal field calculations of the energy levels based on the point symmetry of the (Fe2+O5) and (Fe2+O6) sites, we estimated the intra-atomic magnetic dipolar and electron orbital current contributions to the Bhf field. The observed line broadening below the second magnetic phase transition at TN2 ≈ 16 K is approximated by the bimodal distribution of the hyperfine fields p(Bhf) that is characteristic of spin-modulated magnetic systems.