Effect of high temperature sintering schedule for enhanced CMR properties of La0.67Ca0.33MnO3 close to room temperature

Magnitude of [Delta rho (T, H)/rho (0)] and insulator to metal transition t emperature (Tat) in the colossal magnetoresistance (CMR) bulk ceramics resp onse of La0.67Ca0.33MnO3 (LCMO) manganites are observed to be changed signi ficantly as a function of higher sintering temperature (close to melting) a nd long hours sintering schedule. Magnetoresistance and magnetization (up t o 5 T) of LCMO are found to be decreased considerably as the sintering temp erature is increased from 1400 to 1600 degreesC. Further reduction of CMR r esponse is also observed for long time (a few tens of hours in O-2) at high er temperature. However, on the other hand the value of T-IM is significant ly enhanced and reaching close to room temperature under the same sintering schedule to achieve drastic reduction of CMR response. Microstructural ana lysis revealed the presence of clean and clear intergrain strong connectivi ty at 1600 degreesC sintering temperature for period of 17 h. This process enhances the physical densification of the bulk and promotes the creation o f new conduction channels and reduction of intergrain effective contact are a for transport current under magnetic held. Hence, drastic reduction of re sistance associated with the CMR response is due to this reason. The disrup tion in the crystalline order observed by TEM at the grain (formulation of anti-phase) boundaries promotes local spin disorder which may alter the con ductivity of polarized electrons. It is also observed that short-range loca l magnetic order is disrupted by the presence of grain boundaries which may be responsible for the reduction of magnetoresistance and increase of T-IM point towards room temperature. (C) 2001 Elsevier Science B.V. All rights reserved.