Structure, magnetism and transport of the perovskite manganites Ln _0.5Ca_0.5MnO₃ (Ln=Ho, Er, Tm, Yb and Lu)

It was found that the manganese perovskite oxides Ln_0.5Ca _0.5MnO₃ (Ln=Ho, Er, Tm, Yb and Lu) have an orthorhombic structure (space group Pnma). The Mn-O-Mn angles were calculated to be ∼148-150°, revealing an existence of a large crystallographic distortion in these oxides. Electrical resistivity measurements indicated both an insulating nature and a small magnetoresistance effect, both of which are owing to narrow bandwidths of the Mn-3d electrons arising from the crystallographic distortion. DC magnetization measurements showed the three characteristic temperatures, which could be assigned to charge-order, antiferromagnetism of Mn moments, and possible glassy states. All of these temperatures were decreased for the heavier Ln ions, which is explained in connection with both a difference of ionic radii of Ln³⁺ and Ca²⁺, and a lowering of electron transfer. The charge-ordering transition was not clearly observed only for Lu_0.5Ca_0.5MnO₃ containing the smallest lanthanide ion, plausibly due to a large randomness of magnetic interactions arising from the ionic radii difference of Lu³⁺ and Ca²⁺. In addition, preliminary measurements of AC dielectric response suggested that these manganites belong to a so-called multiferroic system.