| Popis: |
The electronic and magnetic properties of four series of rare earthmanganates of the general formula $La_{0.7-x}Ln_{x}Ba_{0.3}Mn_{03}$ (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 $Mn^{3+}/Mn^{4+}$ 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 where in ferromagnetic clusters are present in an insulating matrix. The nonmagnetic insulating compositions can be rendered ferromagnetic and metallic by decreasing $\sigma^{2}$ whilekeeping $(r_{A})$ constant. This extraordinary display of the effect of size disorder on the properties of the rare earth manganates is noteworthy. |
