Origin of the giant oxygen-isotope effect in the manganiteLa0.8Ca0.2MnO3

Emission Mössbauer effect measurements for the internal hyperfine magnetic field(s), superparamagnetic behavior, and Debye-Waller factors at the Mn site for the ${}^{16}\mathrm{O}$- and ${}^{18}\mathrm{O}$-substituted ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}\mathrm{Mn}{(}^{57}\mathrm{Co}){\mathrm{O}}_3$ compounds are reported. Surprisingly, the exchange interactions between ${\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{4+}$ were observed to be about the same for both compounds at 78 K even though the Curie temperatures differ by about 25 K. Also, very significantly, the population of the softer phonons in the ${}^{18}\mathrm{O}$ compound was found to be about 12%–15% larger than that for the ${}^{16}\mathrm{O}$ compound. The high degree of anharmonicity in $\mathrm{Mn}{-}^{18}\mathrm{O}-\mathrm{Mn}$ vibrations at higher temperatures further enhances the population of soft phonons, which results in a breakdown of ferromagnetic order at a lower temperature.