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

We report anomalous post-annealing effects of La0.8KC0.2MnO3 under O-2 and Ar atmospheres. We have found that the critical temperature (T-C) of the as -prepared sample increases after Ar-annealing and decreases upon O-2 anneal ing. The temperature-dependent magnetization data of the as-prepared compou nd agreed well with the sum of those of the O-2- and Ar-annealed samples. C olossal magnetoresistance (CMR) below 60 K that reaches to almost 100% is o bserved in the O-2-annealed sample. These unusual results are discussed in terms of global homogenization over the sample and magnetic inhomogeneity i nduced by the Mn deficiency. (C) 2001 Published by Elsevier Science Ltd.