Abstract:
The electronic and magnetic properties of four series of rare earth manganates of the general formula La(0.7-x)Ln(x)Ba(0.3)MnO(3) (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 angstrom) and the Mn3+/Mn4+ ratio is kept constant. The size disorder, as measured by the Attfield sigma(2) parameter, has been varied over a wide range of 0.001 and 0.03 angstrom(2). 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 sigma(2) is close to 0.02 angstrom(2). 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 nonmagnetic insulating compositions can be rendered ferromagnetic and metallic by decreasing sigma(2) while keeping < r(A)> constant. This extraordinary display of the effect of size disorder on the properties of the rare earth manganates is noteworthy.
