Anomalous post-annealing effects on magnetic and electrical properties of La0.8Ca0.2MnO3

We report anomalous post-annealing effects of La0.8Ca0.2MnO3 under O2 and Ar atmospheres. We have found that the critical temperature (TC) of the as-prepared sample increases after Ar-annealing and decreases upon O2 annealing. The temperature-dependent magnetization data of the as-prepared compound agree well with the sum of those of the O2- and Ar-annealed samples. Colossal magnetoresistance (CMR) below 60 K that reaches to almost 100% is observed in the O2-annealed sample. These unusual results are discussed in terms of global homogenization over the sample and magnetic inhomogeneity induced by the Mn deficiency.