Fe57 Mössbauer spectroscopy study of cycloidal spin arrangements and magnetic transitions in BiFe1−xCoxO3

We report results of a $^{57}\mathrm{Fe}$ M\"ossbauer study focused on Co-substituted $\mathrm{BiF}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x}{\mathrm{O}}_{3}$ ($x=0.05$). The experimental spectra were analyzed assuming the space-modulated cycloidal magnetic structure with high values of the anharmonicity parameter $m=0.47\ensuremath{-}0.92$ (at 4.7 K). The $m$ parameter increases with increasing temperature to reach its maximum value ($m\ensuremath{\approx}1$) for $x=0.10$, 0.15 corresponding to the transition to a collinear antiferromagnetic structure of easy-plane type (${\ensuremath{\mu}}_{\mathrm{Fe}}\ensuremath{\perp}{c}_{h}$). Such an unusual trend of the anharmonicity parameter $m$(T) increasing with temperature was well reproduced by the molecular field model in which the low-spin to intermediate-spin transition of $\mathrm{C}{\mathrm{o}}^{3+}$ is assumed. At low cobalt content, $\mathrm{BiF}{\mathrm{e}}_{0.95}\mathrm{C}{\mathrm{o}}_{0.05}{\mathrm{O}}_{3}$, the cycloidal structure is retained but with a very high degree of anharmonicity, corresponding to the nearly collinear spin order.