Abstract
We have studied the magnetic behavior of a single crystal of the underdoped manganite ${\mathrm{La}}_{0.9}{\mathrm{Ca}}_{0.1}{\mathrm{MnO}}_{3},$ by ac susceptibility measurements and emission M\ossbauer spectroscopic studies. We do not find any evidence of the presence of antiferromagnetic species as reported by some groups. The breakdown of the long-range ferromagnetic order occurs only above ${0.93T}_{c}.$ This behavior is in sharp contrast to that observed for polycrystalline ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3}$ and ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ where the breakdown starts occurring at as low a temperature as ${0.7T}_{c}.$ It seems that the local distortions introduced by aliovalent substitutions lead to enhanced anharmonicity of Mn-O-Mn vibrations and torsional oscillations between Mn-O octahedra, which in turn induce anomalous breakdown of long-range order even below ${T}_{c}.$ However, we observe strong short range order in magnetic polarons above ${T}_{c}.$
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