Effect of oxygen ordering on the structural and magnetic properties of the layered perovskites PrBaCo2O5+δ

The influence of oxygen ordering on the structural and magnetic properties of the layered Co-based perovskites PrBaCo₂O_5+δ (0.16<δ<0.80) was investigated by means of both x-ray diffraction and bulk magnetic measurements. For this purpose, two series of samples were prepared, following different routes: in the first group, annealing was followed by slow cooling at a rate of 6 °C h^-1; in the second group, the samples were rapidly quenched into liquid metal gallium from different annealing temperatures. Clear correlations between the thermal treatment and the degree of oxygen ordering could be established. Furthermore, we found that only the slowly cooled (\delta \in [0.44\mbox {--}0.5] ) samples show clear signs of long-range oxygen ordering. The magnetic properties confirm the extreme sensitivity of the δ~0.5 region to the sample preparation method: while for δ<0.35 an antiferromagnetic state is found independently of the cooling rate, compounds with oxygen stoichiometry close to δ~0.5 display magnetization curves which depend strongly upon the synthesis history. For \delta

\in [0.35\mbox {--}0.7] , the temperature and field dependences of the magnetization measurements point toward the appearance of a cluster glass, in which oxygen-rich and oxygen-poor clusters coexist. The long-range ferromagnetic order, observed for larger oxygen contents (δ>0.7), could thus be arising from the percolation of oxygen-rich ferromagnetic clusters.