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

Abstract

${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3$ $(x=0.015\mathrm{}–0.05)$ undergoes an insulator-metal (I-M) transition below a temperature $T_p$ 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: $T_p$ decreases and the resistivity at $T_p$ increases upon temperature cycling between a starting temperature $T_S$ and a final temperature $T_F$ and changes are larger for smaller x. The resistivity transition to the low temperature metallic state in $x=0.015\mathrm{}$ can be completely destroyed by thermal cycling in absence of magnetic field as well as under $H=2\mathrm{}\hspace{0.5em}\mathrm{T}.$ 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.