Specific heat of La1-xCaxMnO3-δ

Detailed specific heat measurements from 4.5 to 300 K are reported on a series of polycrystalline La1-xCaxMnO3-delta samples, with x = 0.10, 0.33 and 0.62. The samples were also investigated using electrical transport, magnetic measurements and low temperature NMR. The x = 0.10 composition does not show a well defined transition in the specific heat at the ferromagnetic ordering temperature, but the low temperature specific heat shows a ferromagnetic T-3/2 spin wave term, and no electronic linear term as expected for an insulator. The x = 0.33 composition undergoes a transition to a ferromagnetic metal with a well defined peak in the specific heat, suggestive of an underlying first order transition at T-c; at low temperatures a linear electronic specific heat term is observed but no spin wave ferromagnetic term. High temperature specific heat data on the x = 0.62 sample shows only one anomaly, suggesting simultaneous onset of charge ordering and antiferromagnetism. Low temperature data indicates that the magnetic phase is an A-type antiferromagnetic insulator with no electronic contribution. The NMR data at 4.2 K on this compound shows a motionally narrowed peak indicating short range ferromagnetic interactions or possibly magnetic polarons at 4.2 K. (C) 1998 Elsevier Science B.V. All rights reserved.