A well characterized homogeneous ceramic sample of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{M}\mathrm{n}(}^{57}{\mathrm{C}\mathrm{o})\mathrm{O}}_3$ shows two ferromagnetic phases below $T_c$ in contrast to the 20% Ca compound where only one is observed. One of the phases exhibits stronger exchange interactions with more rapid electron transfer between ${\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{4+}.$ At lower temperatures, there is a tendency for a single phase to exist, with well ordered spins. In both 20% and 30% Ca materials, the long-range ferromagnetic order breaks down above ${T/T}_c=0.7\mathrm{}$ and small spin clusters with superparamagneticlike behavior are formed. Above $T_c,$ the spin clusters persist giving predominantly a single phase (as seen by the Mössbauer probe ${}^{57}\mathrm{Fe}$ with a Larmor period of $\sim {10}^{-8}\sec ).\mathrm{}$ Under the influence of an external magnetic field, the small spin clusters coalesce to give larger clusters with better alignment of spins exhibiting negative magnetoresistivity by percolation. Our work raises the question whether superparamagnetic behavior is a prerequisite for observing negative magnetoresistivity.

• Received 9 December 1999

DOI:https://doi.org/10.1103/PhysRevB.62.5316