Magnetic and electrical properties of (La_1-xDy_x)_0.7Ca_0.3MnO₃ perovskites

Abstract

The effect of substituting La with Dy in the ferromagnetic perovskite _0.7Ca_0.3MnO₃ has been studied in the series (La_1-xDy_x)_0.7Ca_0.3MnO₃ (x=0, 0.114, 0.243, and 0.347). Magnetic and transport properties have been measured using ac susceptibility, dc magnetization, neutron-depolarization, neutron-diffraction, and resistivity techniques. These studies show that an increase in Dy concentration suppresses the low-temperature ferromagnetic ground state, driving the system (for x = 0.114 and 0.243) first into a randomly canted ferromagnetic state with a reduced metal-insulator transition temperature and then (for x = 0.347) to an “insulating” cluster-spin-glass state. Such deterioration of ferromagnetism and metallic conduction with Dy substitution is explained on the basis of a decrease in the transfer integral, t_o (describing the hopping of e_g electrons between Mn³⁺ and Mn⁴⁺, known as ferromagnetic double-exchange interaction), resulting from an increase in the structural distortion. The randomly canted ferromagnetic state and the cluster-spin-glass state for x = 0.243 and 0.347 compounds, respectively, have been confirmed by neutron-depolarization measurements. These observed features have been attributed to a competition between the coexisting ferromagnetic double-exchange and the antiferromagnetic superexchange interactions in the distorted structures with reduced geometrical tolerance factor t and the randomness resulting from the random substitutions of La³⁺ with Dy³⁺.