Ionic size effect on magnetic and electronic properties of La0.70A0.05Sr0.25CoO3 (A=La,Nd, Gd,Ho, Y)

The structural, magnetic and electronic transport properties of La_0.70A_0.05Sr_0.25CoO₃ (A=La, Nd, Gd, Ho, Y) have been investigated by X-ray diffraction and the magnetization and resistivity measurements. All the samples crystallize into hexagonal lattice with the space group R-3C (A167) at room temperature. Small doped ions shrink the lattices almost linearly and isotropically, and bend the Co–O–Co bond angle. Although the Co atoms in all compounds are stabilized in intermediate-spin electronic configuration, the (residual) resistivity (T→0$T\to 0$) increase significantly with doping of the small ions. At low temperatures, the compounds change from metallic state of La_0.70A_0.05Sr_0.25CoO₃ (A=La, Nd, Gd) to insulated state of La_0.70A_0.05Sr_0.25CoO₃ (A=Ho, Y). The 5–7% magnetoresistance (MR) near Curie temperature can be interpreted in terms of the external field suppression of the spin fluctuation, but the relative large MR in the insulator state of La_0.70A_0.05Sr_0.25CoO₃ (A=Ho, Y) (∼20% up to 50-kOe field at 5 K) probably results from the field-induced transfer integral and the tunneling effect.