Abstract:
Electrical and magnetic properties of four series of manganates Ln(x)Ca(1-x)MnO(3) (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, sigma proportional to \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.
