A 57Fe Mössbauer study of charge ordering and phase separation in the rare-earth manganates, Nd 0.5Ca 0.5MnO 3 and Nd 0.5Sr 0.5MnO 3

Abstract

Mössbauer studies of 2% 57Fe-doped Nd 0.5Ca 0.5MnO 3 and Nd 0.5Sr 0.5MnO 3 have been carried out over the 4.2–300 K range after ensuring that such doping does not change their basic properties. The charge-ordering transition in these manganates is marked by abrupt changes in the quadrupole splitting. In the case of Nd 0.5Ca 0.5MnO 3, two phases manifest themselves on cooling below the charge-ordering transition temperature. The evolution of the spectra as a function of temperature shows that long-range magnetic order does not occur sharply. The observed evolution with temperature is different in the two materials studied. In Nd 0.5Ca 0.5Mn 0.98 57Fe 0.02O 3, it resembles that of a disordered magnetic material, whereas the temperature dependence of line shape of Nd 0.5Sr 0.5Mn 0.98 57Fe 0.02O 3 is typical of a superparamagnetically relaxed magnetic system. Although both the manganates show well-resolved magnetic hyperfine spectra at 4.2 K, the lines are slightly broad indicating possible coexistence of phases at low temperatures. A weak paramagnetic signal is also seen in the spectra of both the manganates at 4.2 K.