Strain-controlled electronic properties and magnetorelaxor behaviors in electron-doped CaMnO$_{3}$ thin films and superlattices

We present a systematic study on electronic properties of the \textit{electron-doped} manganite Ca$_{1-x}$Ce$_{x}$MnO$_{3}$ (CCMO,$_{ }$0 $\le x\le $ 0.08) single-layer films and superlattices composed of alternating stacks of non-doped CaMnO$_{3}$ (CMO) and CCMO($x$ = 0.08) layers. The transport properties of the CCMO films are found to be very sensitive to the epitaxial strain. Metallic characteristic observed in the CCMO(0.04 $\le x\le $ 0.06) bulk polycrystal can be realized only in the practically strain-free CCMO epitaxial films on the NdAlO$_{3}$ (NAO) substrate. A large magnetoresistance accompanied with magnetorelaxor-like behavior is observed in the CCMO($x$ = 0.06) film, which can be explained in terms of the phase separation and the irreversible growth of metallic domain in antiferromagnetic insulating matrix. The metallic property is also realized in the superlattices, indicating a charge transfer at the interfaces between CMO and CCMO($x$ = 0.08) layers. When the CCMO($x$ = 0.08) layer in the superlattice is thicker than 8 unit cells, the superlattice exhibits magnetorelaxor-like phenomenon This can be attributed to a phase competition between different antiferromagnetic orderings at the interfaces, resulting in the phase separation