Doping-Induced Phase Transitions in Polycrystalline La_0.49Sr_0.51(Mn_1-xNb_x)O₃

Observations of temperature dependences of ac susceptibility X_ac, resistivity (ρ) and dc magnetization M for polycrystalline La_0.49Sr_0.51(Mn_1-xNb_x)O₃ (x = 0, 0.05, 0.15, and 0.25) show that due to substitution of Mn⁴⁺ by diamagnetic Nb⁵⁺, there appear to be (i) the decrease of X_ac as T> T_N, (ii) the occurrence of a maximum (insulator-metal-like transition) for ρ near T_C, (iii) the decline of M as x>0.05, and (iv) the lowering of T_C. The results might be ascribed to: (i) the rearrangement of Mn³⁺ and Mn⁴⁺ ions, (ii) the decrease of the number of Mn³⁺ and Mn⁴⁺ pairs involved in double exchange interaction, (iii) the delocalization of the electrons and holes, (iv) the reduction of the randomness of the delocalized carrier scattering by thermal spin fluctuation due to magnetic ordering, and (v) the compensation effect of Nb⁵⁺ ions for Sr²⁺ ions.