Effects of A-Site Doping on Structural, Magnetic, and Electrical Properties of La0.8−x A x Sr0.2MnO3 (0 ≤ x ≤ 0.6) Manganites (A = Pr, Nd, and Gd)

Abstract

The effects of rare-earth doping were investigated on the structural, magnetic, and electrical properties of La0.8−xAxSr0.2MnO3 (0 ≤ x ≤ 0.6) manganites. Results showed that the Pr- and Nd-doped samples were single phase and crystallized in the rhombohedral structure with an R-3C space group. The Gd-doped samples were also single phase for values of x equal to 0 and 0.1 but secondary phases were detected for higher doping levels (i.e., increasing mismatch of the ionic lattice). Moreover, the crystal structure was observed to transform from a rhombohedral (x = 0, 0.1) to an orthorhombic (x ≥ 0.2 samples) one with the Pnma space group. In all the samples, reducing ionic radius of the dopant ion led to a reduction in the unit cell volume. ac magnetic susceptibility measurements showed that in the Pr- and Nd-doped samples, the transition temperature decreased from a paramagnetic (PM) to a ferromagnetic (FM) one with decreasing ionic radius of dopant elements. A frequency-dependent peak was observed in the ac susceptibility for x = 0.4, 0.5, and 0.6, revealing the presence of a spin-glass state at low temperatures. The Curie temperature (TC) for the Gd-doped samples changed nonuniformly due to the formation of secondary phases. However, no spin-glass behavior was observed in the Gd-doped samples. The temperature dependence of resistivity indicated that the Nd- and Pr-doped samples with low doping levels contained an FM metal, while those with higher doping levels exhibited an FM-insulator behavior. In the Gd-doped series, the FM-metallic phase was observed in all the doped samples. In order to understand the transport mechanism, several theoretical models such as small polaron hopping and variable-range hopping were explored to fit the resistivity data; the results confirmed our experimental data.