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

Pr0.5Ca0.5Mn1−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. We suggest that the increase of strains in the ferromagnetic-charge ordered interface could be a possible origin of the observed effect. PACS NO: 75.30.Vn, 71.30+h, 75.30. +Kz, 77.50. -y.