Correlation of structural distortion with magnetic properties in electron-doped Ca_0.9 R_0.1 MnO₃ perovskites (R=rare -earth)

A series of electron-doped orthorhombic-perovskite manganites Ca _0.9 R_0.1 MnO₃ (R=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb) are synthesized for a systematic study of their crystal structure and magnetic properties. The structural distortions, in terms of the average Mn-O-Mn bond angle θ_Mn-O-Mn and Mn-O bond length d _Mn-O, are characterized as a function of the A-site ionic size. Two degenerate vibration modes Q₂ and Q₃ are used for describing the bond length splitting and the evolution of the octahedral-site distortion. With R³⁺ doping, the magnetization increases markedly at low temperatures, which can be attributed to the formation of ferromagnetic clusters in the antiferromagnetic matrix. Both low temperature magnetization and paramagnetic susceptibility vary with the radius of R³⁺ ion and enhanced ferromagnetic domain is found in Ca0.9 Ho0.1 MnO₃. The Ńel temperature T_N, varying from 100 to 116 K, is strongly dependent on the crystal structural distortions and can be well described as functions of three structural parameters θ_Mn-O-Mn,d _Mn-O, and A -site cation size variance σ². The best size matching between Dy³⁺ and Ca²⁺ leads to the highest T_N in Ca_0.9 Dy_0.1 MnO₃.