Hyperfine interactions and local environment of 57Fe probe atoms in perovskite CaMn7O12

Moessbauer spectroscopy has been applied for studying the local environment of 57Fe probe atoms within iron-doped CaMn7O12 manganite with a perovskite-like structure. The 57Fe spectra recorded in the paramagnetic temperature range 90 K<T<380 K, where CaMn7O12 has a rhombohedral structure (R3), were discussed supposing Fe3+ probe cations to replace manganese in the octahedral (Mn3+O6) and (Mn4+O6) polyhedra. A very small (<3%) part of Fe3+ probe cations characterized by very large quadrupole splitting (∼2 mm/s) suggests that a few amounts of 57Fe probe can be substituted for Mn3+ in the (9e) positions with approximately a square planar surrounding. In the temperature range 380 K<T<450 K, the 57Fe Moessbauer spectra undergo a sharp change which can be due to a formation (at TCO∼380 K) and a gradual growth with temperature of the cubic manganite phase (Im3). At T≥450 K, the 57Fe spectrum reveals single Fe3+ sites in an oxygen symmetric octahedral surrounding. This can be well explained by a fast electron exchange Mn3+↔Mn4+, leading to the crystallographic equivalence of all MnO6 positions. At the temperature range T<TM2 (=90 K), an appearance of the distributions of the hyperfine magnetic field values at the 57Fe nucleus up to ∼350 kOe (at 77.4 K) was observed. This result is an independent experimental evidence of magnetic ordering in the manganese sublattice