I. Isaac

Comparative studies of the behaviour of La0.8Ca0.2MnO3 with 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.

A Study of the Paramagnetic to Ferromagnetic Transition in the Optimally Doped CMR Compound La0.65Ca0.35MnO3 and Its Dependence on Oxygen Mass

We present a study of the paramagnetic to ferromagnetic transition in the CMR compound La0.65Ca0.35MnO3 and its dependence on magnetic field and oxygen mass. The transition is characterized by two temperatures, the thermodynamic transition temperature at Tc, obtained from specific heat and thermal expansion data, and the resistive transition obtained from the resistivity maximum. The resistive transition occurs well within the paramagnetic range. The magnetic susceptibility in the paramagnetic range is isotope dependent up to 400 K. The magnitude of the Curie-Weiss constant indicates the presence of small clusters of about 4–5 unit cells. The resistive transition occurs when the percolation limit for these clusters is reached.