Instability of metal-insulator transition against thermal cycling in phase separated Cr-doped manganites

Pr0.5Ca0.5Mn_1-xCrₓO₃ $(x=0.015--0.05)$ undergoes an insulator-metal (I-M) transition below a temperature Tₚ driven by percolation of ferromagnetic metallic clusters in a charge ordered insulating matrix. Surprisingly, the I-M transition in these compounds is unstable against thermal cycling: Tₚ decreases and the resistivity at Tₚ increases upon temperature cycling between a starting temperature TS and a final temperature TF and changes are larger for smaller x. The resistivity transition to the low temperature metallic state in $x=0.015$ can be completely destroyed by thermal cycling in absence of magnetic field as well as under H=2 T. Magnetization measurements suggest that the ferromagnetic phase fraction decreases with increasing thermal cycling. Contrary to the thermal cycling induced effect, isothermal aging causes a slow decrease of resistivity as a function of time. We suggest that increase of strains in the ferromagnetic-charge ordered interface during thermal cycling could be a possible origin of the observed effect.