Emission Mössbauer study of CMR manganite La0.8Ca0.2MnO3. I. Anomalous ferromagnetism

Using ${}^{57}\mathrm{Co}$ emission Mössbauer technique, we present clear evidence that in Ca-doped manganite, the magnetic and paramagnetic phases coexist below $T_C,$ with the abundance of the latter increasing with temperature. In contrast with the regular ferromagnetic materials, the variation of the hyperfine internal magnetic field $H_{\mathrm{int}}$ with temperature deviates considerably from the Brillouin relation, and exhibits an abrupt drop at $T_C.$ These features characterize the magnetic transition as a first-order transition. The non-Brillouin behavior of $H_{\mathrm{int}}\mathrm{(T)}$ and the temperature dependence of the shape of the magnetically split sextet indicate the presence of spin fluctuations in this material well below $T_C.$