Thermal cycle effect in two prototypical phase separated manganites

JOAQUIN SACANELL; M. QUINTERO; J. CURIALE; R. S. FREITAS; L. GHIVELDER; C. ACHA; G. POLLA; G. LEYVA; F. PARISI; P. LEVY

Brasilia, Brasil

Workshop; Workshop on High magnetoresistance materials; 2001

We report on a study of the effect of thermal cycles on transport and magnetic properties of polycrystalline samples of the phase separated compounds La1/2Ca1/2MnO3 (LCMO) and La5/8-yPryCa3/8MnO3 (y=0.3) (LPCMO). This effect is most likely related to the crossing of a structural phase transition ( at ~200 K in LCMO, at 190 K in LPCMO), which induces stress due to different volume of coexisting phases.  We suggest that this memory effect is related to a training microstuctural process which favors the increase of the insulating phase when these regions were already formed in a previous thermal cycle. The thermal behavior of the PS compounds LCMO and LPCMO allows to delineate the following scenario: 1)  On cooling from room temperature these compounds undergo a sequence of phase transitions, including a first order one which involves structural changes. Microstructural tracks originated at this transitions can give origin to the global irreversibility of the compounds; 2) The effect of a thermal cycles is always towards an increase of the amount of the COO phase in the next cycle, irrespective of the actual ground state of the sample (COO for LCMO, FM for LPCMO);3) Changes induced by thermal cycling can be partially erased by applying magnetic field and hydrostatic pressure, this last result enforcing the image of the structural nature of the effect; 4) A qualitative account of the r vs cycle behavior can be obtained considering a model in which the FM phase decrease at cycle n proportionally to the amount it did at cycle n-1.