Comparative studies of the behaviour of La0.8Ca0.2MnO3with different oxygen isotope substitutions

Emission M?ssbauer effect investigations were done simultaneously with resistivity measurements on two similar samples of La0.8Ca0.2Mn(57Co)O3, in one of which 18O was substituted for 16O. Only a few parts per million of 57Co were substituted for Mn with minimal perturbation of the system. For both compounds, the long range ferromagnetic spin order decreases anomalously below the transition temperature (Tc) and we observe three broad categories of electronic phases. Surprisingly, we find that 18O substitution brings about a stronger exchange interaction between Mn3+/Mn4+, and the internal field (Hint) is larger than that for the 16O compound at all normalized temperatures (T/Tc) observed. The stronger exchange interaction in the 18O compound is in apparent contradiction with the observation that the ferromagnetism in this compound starts crumbling at a lower T/Tc, forming small spin clusters exhibiting superparamagnetic-like behaviour. However, the 18O compound is more amenable to spin ordering under the influence of a magnetic field near Tc.