The effect of annealing conditions on the physical properties of Nd0.67Sr0.33MnO3-δ

A study of the influence of annealing in atmospheres with defined oxygen partial pressures (1.5x10-6 Pa ≤ PO2 ≤ 10+5 Pa) on the electrical resistivity ρ (T), magnetic susceptibility χ (T), and heat capacity cp(T) of ceramic samples of Nd0.67Sr0.33MnO3-δ is presented. ρ (T) at 1000 deg. C shows no appreciable change for 10+2 Pa ≤ PO2 ≤ 10+5 Pa. For samples annealed in air a sharp peak is found in cp(T) and in ρ (T) at the ferromagnetic transition (TC approx. 238 K). The insulator-metal transition is observed below TC as a broad peak in ρ (T). Annealing at lower PO2 leads to an increase of ρ whereas TC decreases only by approx. 2 K. Annealing at PO2 ≤ 6.3x10-2 Pa suppresses TC and results in spin canting and a vanishing of the peak in cp(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 cp(T) into lattice, electronic, and Mn and Nd magnetic contributions is discussed. cp(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. (author)