Specific heat and magnetic order of La_0.2 Ca_0.8 MnO₃

Measurements of specific heat of the electron-doped La_0.2 Ca_0.8 MnO₃ manganite are reported for 2<T<400 K. It was found that La_0.2 Ca_0.8 MnO₃ exhibits an orbital ordering at TOO =204 K and a long-range antiferromagnetic order below T_N =178 K. The entropy changes (ΔS) around both T_OO [ΔS (T_OO) ≈0.38 J/mol K] and T_N [Δ S (T_N) ≈3.58 J/mol K] were determined. The entropy change associated with the orbital ordering was found to be much smaller than that for charge ordered manganites. The low temperature specific heat indicates that La _0.2 Ca_0.8 MnO₃ has rather high Debye temperature D=685_K in comparison with existing results for electron-doped La_1-x Ca_x MnO₃ manganites. Moreover, the presence of a large 1/ T² hyperfine contribution and of a considerable linear term, unexpected for La_0.2 Ca_0.8 MnO₃ with antiferromagnetic insulating ground state, was revealed. The possible relationship between the behavior of the heat capacity and the phase separation is discussed.