Effect of annealing conditions on the physical properties of Nd_0.67Sr_0.33MnO_3-δ

A study of the influence of annealing in atmospheres with defined oxygen partial pressures (1.5×10^-6 Pa≤P_O(2)≤10⁺⁵ Pa) on the electrical resistivity ρ(T), magnetic susceptibility χ(T), and heat capacity c_p(T) of ceramic samples of Nd_0.67Sr_0.33MnO_3-δ is presented. ρ(T) at 1000 °C shows no appreciable change for 10⁺² Pa≤P_O(2)≤10⁺⁵ Pa. For samples annealed in air a sharp peak is found in c_p(T) and in ρ(T) at the ferromagnetic transition (T_C≈238 K). The insulator-metal transition is observed below T_C as a broad peak in ρ(T). Annealing at lower P_O(2) leads to an increase of ρ whereas T_C decreases only by ≈2 K. Annealing at P_O(2)≤6.3×10^-2 Pa suppresses T_C and results in spin canting and a vanishing of the peak in c_p(T). The specific heat (2-300 K) for samples annealed in air and in Ar/5% H₂ is determined and thermodynamic standard values are calculated. The problem of the separation of c_p(T) into lattice, electronic, and Mn and Nd magnetic contributions is discussed. c_p(T<20 K) is dominated by a Schottky-like anomaly from the Zeeman-split ground-state doublet of Nd³⁺. A large quasi-linear term is mimicked by a broadening of the Schottky-like anomaly and by magnetic contributions from Mn in oxygen-deficient samples. The results are discussed in the context of oxygen deficiency δ, carrier concentration, defect chemistry, and disorder.