Abstract:
The charge ordering in Nd0.5Sr0.5MnO3 (〈rA〉 = 1.24 Å), which occurs on cooling the ferromagnetic metallic ground state, is readily destroyed on application of a magnetic field of 6 T. For Y0.5Ca0.5MnO3 (〈rA〉 = 1.13 Å), for which the ground state is charge ordered, on the other hand, magnetic fields have no effect on the charge ordering. In order to understand such a marked difference in charge-ordering behaviour of the manganates, we have investigated the structure as well as the electrical and magnetic properties of Ln(0.5)Ca(0.5)MnO(3) compositions (Ln = Nd, Sm, Gd and Dy) wherein 〈rA〉 varies over the range 1.17-1.13 Å. The lattice distortion index, D, and charge-ordering transition temperature, TCO, for the manganates increase with the decreasing 〈rA〉. The charge-ordered state is transformed to a metallic state on applying a magnetic field of 6 T in the case of Nd0.5Ca0.5MnO3 (〈rA〉 = 1.17 Å), but this is not the case with the analogous Sm, Gd and Dy manganates with 〈rA〉 less than 1.17 A. In order to explain this behaviour, we have examined the 〈rA〉-dependence of the Mn-O-Mn bond angle, the average Mn-O distance and the apparent one-electron bandwidth, obtained from these structural parameters. It is suggested that the extraordinary sensitivity of the charge ordering to 〈rA〉 arises From factors other than those based on the Mn-O-Mn bond angle and average Mn-O distances alone. It is possible that the competition between the covalent mixing of the oxygen O: 2p sigma orbital with the A-site and B-sire cation orbitals plays a crucial role. Strain effects due to size mismatch between A-site cations could also cause considerable changes in TCO.
