Novel effects of size disorder on the electronic and magnetic properties of rare earth manganates of the type La_0.7−xLn_xBa_0.3MnO₃ (Ln=Pr,Nd, Gd or Dy) with large average radius of the A-site cations

Abstract

The electronic and magnetic properties of four series of rare earth manganates of the general formula La_0.7−xLn_xBa_0.3MnO₃ (Ln=Pr, Nd, Gd or Dy) have been investigated to examine the effect of large size disorder in a system where the average radius of the A-site cations, <r_A>, remains high (1.216-1.292 Å) and the Mn³⁺/Mn⁴⁺ ratio is kept constant. The size disorder, as measured by the Attfield σ² parameter, has been varied over a wide range of 0.001 and 0.03 Ų. As x is increased, the materials exhibit a decrease in the ferromagnetic Curie temperature, T_c, or lose ferromagnetism entirely. This is accompanied by an insulator-metal (I-M) transition, with T_IM decreasing with increasing x, or an entirely insulating behaviour. The insulating behaviour and loss of ferromagnetism occurs when σ² is close to 0.02 Ų. Thus, in the Ln=Nd, Gd and Dy series, the non-magnetic insulating behaviour occurs at x values of 0.7, 0.5 and 0.3 respectively. Where an I-M transition occurs, T_IM<T_c, indicating the presence of a ferromagnetic insulating regime. The absence of long-range ferromagnetism in some of the compositions is accompanied by a divergence between the zero-field-cooled and the field-cooled magnetization data. The ferromagnetic or non-magnetic insulating state is due to phase separation wherein ferromagnetic clusters are present in an insulating matrix. The non-magnetic insulating compositions can be rendered ferromagnetic and metallic by decreasing σ² while keeping <r_A> constant. This extraordinary display of the effect of size disorder on the properties of the rare earth manganates is noteworthy.