Low-temperature permittivity of insulating perovskite manganites

Measurements of the low-frequency (f≤100 kHz) permittivity (ε) and conductivity (σ) at T≲150 K are reported for La_1-xCa _xMnO₃ (0≤x≤1) and Ca_1-ySr _yMnO₃ (0≤y≤0.75) having antiferromagnetic, insulating ground states covering a broad range of Mn valencies from Mn ³⁺ to Mn⁴⁺. Static dielectric constants are determined from the low-T limiting behavior. With increasing T, relaxation peaks associated with charge-carrier hopping are observed in the real part of the permittivities and analyzed to determine dopant binding energies. The data are consistent with a simple model of hydrogenic impurity levels and imply effective masses m*/m_e∼3 for the Mn⁴⁺ compounds. Particularly interesting is a large dielectric constant (ε₀∼100) associated with the C-type antiferromagnetic state near the composition La _0.2Ca_0.8MnO₃.