Evolution of A-site disorder-dependent structural and magnetic transport properties in La2/3-xEuxCa1/3-ySryMnO3

We systematically investigate the local disorder effect induced by size mismatch of A-site cation ions on crystallographic structure and magnetic transport behaviors of co-doped La2/3-xEuxCa1/3-ySryMnO3. We have prepared a series of Eu and Sr co-doped samples by keeping the average A-site cation radius preserved at 1.204 angstrom and varying A-site cation disorder sigma(2) = Sigma(i)x(i)r(i)(2) - < r(A)>(2), where x(i) is the fractional occupancy and ri is the ionic radii of i cation, respectively. Rietveld refinements show that with increasing doping level, the averaged lattice parameters vary slightly while the local atomic configurations in the unit cell vary obviously. This confirms that with doping single orbital disordered state is extended which is homogenous on macroscales, but inhomogenous at microscales due to lattice strain, as a consequence of random distribution of ions locating at A-site. Further resistance measurements show that the metal-insulator transition temperature (T-MI) decreases linearly with the change of A-site disorder, whereas, larger and more broadening magnetoresistance occur. Analysis of inverse magnetic susceptibility has identified the coexistence of FM metallic and insulating clusters in compounds, thus, contributing to the magnetic inhomogeneity when disorder is present. In addition, magnetocaloric effect is greatly suppressed as a function of disorder. This is ascribed to the gradual loss of first-order magnetic transition. Our experimental findings could be coherently explained in the framework of phase separation. (C) 2008 Elsevier B.V. All rights reserved.