Effects of A-site disorder on magnetic, electrical and thermal properties of La 0.5− xLn xCa 0.5− ySr yMnO 3manganites

Abstract

The magnetic, electrical and thermal properties in the La 0.5− x Ln x Ca 0.5− y Sr y MnO 3 (Ln=Pr, Nd, Sm) bulk system were investigated. Detailed dc magnetization and linear ac susceptibility measurements reveal that the samples first undergo phase transition from paramagnetic to ferromagnetic phase and then to an antiferromagnetic phase upon further cooling. It is found that both the Curie and Neel temperatures decrease systematically with increasing A-site disorder in these manganites. The electrical resistivity exhibits semiconducting behavior throughout the temperature range investigated and the electronic conduction mechanism can be conveniently described within the framework of the variable range hopping model above T=150 K. The Seebeck coefficient ( S) in the magnetically ordered regime infers that the complicated temperature dependence of S is an indication of electron–magnon scattering. Specific heat measurements depict a broadened hump in the vicinity of T C , indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The temperature dependence of thermal conductivity, κ( T), reveals a positive d κ/d T in the paramagnetic region, which may be related to the local anharmonic lattice distortions associated with small polarons.