Colossal magnetoresistance accompanied with magnetorelaxor behavior in phase-separated Ca_1-x Ce_x MnO₃ thin films and CaMnO₃ / Ca_0.92 Ce_0.08 MnO₃ superlattices

We report on the transport properties of electron-doped manganite Ca _1-x Ce_x MnO₃ (CCMO, 0≤x≤0.08) films and superlattices composed of insulating layers CaMnO₃ (CMO) and Ca _0.92 Ce_0.08 MnO₃ (CCMO8), deposited on nearly lattice-matched NdAlO₃ substrates. The CCMO (x=0.06 and 0.07) films show colossal magnetoresistance (CMR) accompanied with magnetorelaxor behavior, which can be ascribed to the phase separation of canted G -type antiferromagnetic metal and C -type antiferromagnetic insulator. The (CMO) _m / (CCMO8) _n superlattices with 4≤m, n≤8 (unit cells) resemble the solid-solution CCMO (x=0.06 and 0.07) films in CMR and magnetorelaxor behavior, suggesting that the phase separation takes place in the superlattices. The CMR and magnetorelaxor behavior of the (CMO) _m / (CCMO8) _n superlattices strongly depend on the thicknesses of constituent CMO and CCMO8 layers. The origin of the phase separation in the superlattices is discussed in terms of the charge transfer and the phase competition at the interfaces.