Magnetoresistance and field-induced spin-state transition of Pr and Nd substituted La0.5Sr0.5CoO3La0.5Sr0.5CoO3

The AC susceptibility and magnetoresistance (MR) of (La1-xLnx)0.5Sr0.5CoO3(La1-xLnx)0.5Sr0.5CoO3 (Ln=NdLn=Nd, Pr, x=0x=0–0.2) polycrystals were studied. The substitution of Nd or Pr for La results in a slight decrease of the Curie temperature TcTc. For all samples, the cluster-freezing behavior is observed in AC susceptibility measurement, and the freezing temperature TfTf shifts to higher temperature with increasing x. The resistivity and negative MR are significantly enhanced by both Pr and Nd doping and the effect of Nd substitution on them is stronger than that of Pr substitution. These results suggest that the spin-disorder scattering is enhanced by substitution of Nd and Pr for La. A rapid decrease in MR is observed at about 90 K under 1 T, while at 70 K under 7 T. This behavior could arise from the transition induced by magnetic field from high-spin or intermediate-spin state of trivalent Co ions to low-spin state. The transition temperature is nearly the same for both of Nd and Pr cases and independent on the dopants.