Double exchange suppression in La 0.7(CaCd) 0.3MnO 3 manganites

Abstract

We present results concerning the magnetic and structural properties of Cd-doped La 0.7(Ca 0.3− x Cd x )MnO 3 (0⩽ x⩽0.3) manganites. All compositions were synthesised using the sol–gel technique, and were indexed in the orthorhombic (Pnma) space group. However, the Cd 0.3 composition was indexed as a combination of trigonal (R-3c) and orthorhombic (Pnma) space groups. The substitution of Ca by Cd has great influence on the magnetic properties of this family of compounds, decreasing continously both the magnetic moment (from 3.5 μ B for the Ca 0.3 classical composition to 1.59 μ B for the fully doped Cd 0.3 one) and the Curie temperature (from 270 to 90 K, for the same previous compositions).Values of the magnetoresistance measured between 0 and 6 T applied field have been also determined. The samples corresponding to x=0 and 0.1 compositions show a semiconductor–metallic transition at temperatures close to the Curie ones. In fact, the x=0.1 composition shows a 95% magnetoresistance at 165 K. On the contrary, the x=0.2 and 0.3 compositions show insulating behaviour in the whole temperature range studied. The observed weakening of the double-exchange mechanism as the Cd doping level in these samples increases is discussed in terms of structural properties, cationic disorder and Mn 3+/Mn 4+content ratio.