Origin of the giant oxygen-isotope effect in the manganite La0.8Ca0.2MnO3

Emission Mossbauer effect measurements for the internal hyperfine magnetic field(s), superparamagnetic behavior, and Debye-Waller factors at the Mn si te for the O-16- and O-18-substituted La0.8Ca0.2Mn(Co-57)O-3 compounds are reported. Surprisingly, the exchange interactions between Mn3+/Mn4+ were ob served to be about the same for both compounds at 78 K even though the Curi e temperatures differ by about 25 K. Also, very significantly, the populati on of the softer phonons in the O-18 compound was found to be about 12%-15% larger than that for the O-16 compound. The high degree of anharmonicity i n Mn-O-18-Mn vibrations at higher temperatures further enhances the populat ion of soft phonons, which results in a breakdown of ferromagnetic order at a lower: temperature. [S0163-1829(99)14119-5].