Evidence for breakdown of ferromagnetic order below $T_C$ in the manganite ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_3$

We report an emission Mössbauer study of the ferromagnetic metal ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{M}\mathrm{n}(}^{57}{\mathrm{C}\mathrm{o})\mathrm{O}}_3,$ where a few parts-per-million of Mn are replaced by ${}^{57}\mathrm{Co}.$ The behavior observed is dramatically different than that for a conventional ferromagnet. The data give strong evidence for an anomalous gradual crumbling of the ferromagnetic order, and the formation of spin clusters below as well as above the Curie temperature ${(T}_C).\mathrm{}$ Application of a magnetic field, even well above $T_C,$ results in a dramatic coalescence of small spin clusters into larger clusters with more ordered spins. The cluster size is found to be strongly dependent on the magnitude of the applied field.