A systematic study of four series of electron-doped rare earth manganates, Ln_xCa_1−xMnO₃ (Ln=La, Nd, Gd and Y) over the x=0.02-0.25 composition range

Abstract

Electrical and magnetic properties of four series of manganates Ln_xCa_1−xMnO₃ (Ln=La, Nd, Gd and Y) have been studied in the electron-doped regime (x=0.02-0.25) in order to investigate the various inter-dependent phenomena such as ferromagnetism, phase separation and charge ordering. The general behaviour of all four series of manganates is similar, with some of the properties showing a dependence on the average radius of the A-site cations, <r_A> and cation size disorder. Thus, all the compositions show an increase in magnetization at 100-120 K (T_M) for x<x_max, the magnetization increasing with increasing x. The value of x_max increases with decreasing <r_A>, probably due to the increased phase separation induced by site disorder. This is also reflected in the larger width of the hysteresis loops at T<T_M for small x or <r_A>. In this regime, the electrical resistivity decreases with increasing x, but remains low and nearly constant, T>T_M. The percolative nature of the conduction mechanism at T<T_M is substantiated by the fit of the conductivity data to the scaling law, σ∝|x_c−x|^p, where p is in the 2-4 range. When x>x_max, the materials become antiferromagnetic (AFM) and charge-ordered at a temperature T_CA, accompanied by a marked increase in resistivity. The value of T_CA increases with increasing <r_A> and x (up to x=0.3). Thus, all four series of manganates are characterized by a phase-separated regime between x=0.02 and 0.1-0.15 and an AFM charge-ordered regime at x>0.1-0.15.