Influence of A-site cation size on the magnetic and transport properties of (Nd1-yYy)0.7Sr0.3MnO3 (0≤y≤0.42)

The effects of Y-doping on the Nd-sites in the perovskite (Nd1-yYy)(0.7)Sr0.3MnO3 have been investigated by means of magnetic AC-susceptibility and resistivity measurements. With increasing y, corresponding to decreasing the A-site average ionic radius , the ferromagnetism and metallicity are strongly suppressed. Increasing y in the range 0less than or equal toyless than or equal to0.21 rapidly lowers T-C, however, there still exists a metallic-insulating transition at temperatures close to T-C. Although the ferromagnetic-state conductivity is strongly depressed, within 0less than or equal toyless than or equal to0.14, the paramagnetic-state conductivity is not significantly affected and seems to follow the variable-range hopping model. The nearest-neighbor hopping mechanism becomes dominant at higher doping levels. At the critical average cation size = 1-19Angstrom (according to y = 0.28) and lower, the ferromagnetic-paramagnetic as well as the metallic-insulating transitions are no longer observed, but spin-glass-like and insulating behavior are observed instead. These results are discussed in terms of the double-exchange mechanism and the GdFeO3-type lattice distortion. (C) 2003 Elsevier Science B.V. All rights reserved.