Importance of structural tuning in manganites

Abstract

Properties of substituted La-manganites are known to be very sensitive to La site substitution. The critical parameters appear to be (a) the charge and concentration of the substituted ions, which determine the Mn³⁺/Mn⁴⁺ ratio, (b) the average ionic radius at theA site and (c) the tolerance factor,t. The latter two parameters determine the distortion and structural tuning of the perovskite structure. Here we report the influence of minor variations in the above parameters on the magnetotransport of compounds around the basic composition La_0·6M_0·07Ca_0·33MnO₃ (M=Er, Yb and Bi). The influence of the method of synthesis on different properties is also discussed. The ionic radii of Er³⁺, Yb³⁺ and Bi³⁺ ions are 1·00, 0·985 and 1·11 Å, respectively, compared to 1·32 Å for La³⁺ and this provides a systematic variation for the investigation of the effects ofA site substitution. The zero field resistivity data are essentially similar above the maxima which corresponds, at least approximately, to the Curie temperature as determined by magnetization measurements. However, there are variations below the transition temperature, where there is magnetic ordering, and large variations are often seen in the magnetic data between samples which are very similar in composition. Substantial similarities between field dependent magnetization and resistivity measurements appear to be related to domain wall displacement and rotation. These results will be discussed in the framework of structural tuning which has a strong influence on exchange parameters such as the Mn-O-Mn bond length and angle.