Universal effect of Mn-site doping on charge ordering in La1/3Ca2/3MnO3

Comparison of doping effect on the charge ordering in La1/3Ca2/3Mn1−yMyO3 (M: Fe, Ga, Cr, Ni, Cu, Ru, Mg and 0≤y≤0.07) has been investigated by combining magnetic and transport measurements and local compositional analysis in situ in transmission electron microscope. The effect of Mn-site doping on the charge ordering temperature TCO is shown for the first time to be universal for a given dopant valence and y≤0.05. TCO is governed by the effective relative concentration nMn3+=Mn3+/(Mn3++Mn4+) of Mn3+ ions, TCO~CnMn3+, where the coefficient C depends on the valence but not on d-shell filling of a dopant ion. This dependence indicates that any of the considered dopants do not participate in charge transfer during charge ordering formation, however, indirectly affects the charge redistribution between manganese ions. Consideration of Mn-site doping effect on charge ordering temperature using a simple order-disorder model supports the concept of universality for a given dopant valence and dominating role of the factor of entropy over energy change. Analysis of the remanent magnetization together with the obtained behavior of TCO(nMn3+) testify that it is the dopant valence (Ru5+), but not formation of Ru-based magnetic clusters, that is responsible for antiferromagnetic (AFM) insulator-ferromagnetic (FM) 'bad-metal' phase transition observed in the Ru-doped system at y=0.07.