Magnetic and magnetotransport properties of misfit cobaltate Ca3Co3.93O9+δ

Samples of Ca3Co3.93O9+delta, prepared by classical ceramic route (CCR) and spark plasma sintering (SPS), were characterized by means of magnetic, electrical, and thermal experiments in a broad temperature range. The low temperature specific heat measured down to 0.3 K, reveals two important electronic contributions: the first one, dominated by magnetic excitations, that can be suppressed by an external magnetic field, and the second, due to band-like carriers, characterized by gamma similar to 50 mJ mol(-1) K-1 for both samples. The ferrimagnetism, associated with the long-range magnetic order of IS Co3+ species in rock-salt blocks detected below T-F similar to 15 K in the CCR samples, is absent in the SPS samples. Similarly, the specific heat peaks detected for the CCR sample at 410 and 840 K are not reproduced in the case of the SPS sample. In both samples the low temperature thermoelectric power is quantitatively explained by supposing the strongly correlated carriers are carrying the spin entropy. The lower hole concentration in the SPS than in the CCR sample is likely responsible for its higher thermopower, namely, at high temperatures where the configuration entropy of hopping carriers dominates. (C) 2012 American Institute of Physics. [doi:10.1063/1.3677312]