Hyperfine interactions and local environment ofFe57probe atoms in perovskiteCaMn7O12

Abstract

M\ossbauer spectroscopy has been applied for studying the local environment of $^{57}\mathrm{Fe}$ probe atoms within iron-doped $\mathrm{Ca}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ manganite with a perovskite-like structure. The $^{57}\mathrm{Fe}$ spectra recorded in the paramagnetic temperature range $90\phantom{\rule{0.3em}{0ex}}\mathrm{K}lTl380\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, where $\mathrm{Ca}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ has a rhombohedral structure $(R\overline{3})$, were discussed supposing ${\mathrm{Fe}}^{3+}$ probe cations to replace manganese in the octahedral $({\mathrm{Mn}}^{3+}{\mathrm{O}}_{6})$ and $({\mathrm{Mn}}^{4+}{\mathrm{O}}_{6})$ polyhedra. A very small $(l3%)$ part of ${\mathrm{Fe}}^{3+}$ probe cations characterized by very large quadrupole splitting $(\ensuremath{\sim}2\phantom{\rule{0.3em}{0ex}}\mathrm{mm}∕\mathrm{s})$ suggests that a few amounts of $^{57}\mathrm{Fe}$ probe can be substituted for ${\mathrm{Mn}}^{3+}$ in the $(9e)$ positions with approximately a square planar surrounding. In the temperature range $380\phantom{\rule{0.3em}{0ex}}\mathrm{K}lTl450\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the $^{57}\mathrm{Fe}$ M\ossbauer spectra undergo a sharp change which can be due to a formation (at ${T}_{\mathrm{C}\mathrm{O}}\ensuremath{\sim}380\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) and a gradual growth with temperature of the cubic manganite phase $(\mathrm{Im}\overline{3})$. At $T\ensuremath{\ge}450\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the $^{57}\mathrm{Fe}$ spectrum reveals single ${\mathrm{Fe}}^{3+}$ sites in an oxygen symmetric octahedral surrounding. This can be well explained by a fast electron exchange ${\mathrm{Mn}}^{3+}\ensuremath{\leftrightarrow}{\mathrm{Mn}}^{4+}$, leading to the crystallographic equivalence of all $\mathrm{Mn}{\mathrm{O}}_{6}$ positions. At the temperature range $Tl{T}_{\mathrm{M}2}$ $(=90\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, an appearance of the distributions of the hyperfine magnetic field values at the $^{57}\mathrm{Fe}$ nucleus up to $\ensuremath{\sim}350\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ (at $77.4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) was observed. This result is an independent experimental evidence of magnetic ordering in the manganese sublattice.