${\mathrm{Nd}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ nanoparticles (average diameter $\sim 20$ and $40\mathrm{nm}$) are synthesized by the polymeric precursor sol-gel method and characterized by various physico-chemical techniques. Quite strikingly, in the $20\mathrm{nm}$ particles, the charge-ordered (CO) and the antiferromagnetic phases observed in the bulk below $250\mathrm{K}$ and $160\mathrm{K}$, respectively, are completely absent. Instead, a ferromagnetic (FM) transition is observed at $95\mathrm{K}$ followed by an insulator-to-metal transition at $75\mathrm{K}$. The $40\mathrm{nm}$ particles show a residual CO phase but a transition to the FM state also occurs, at a slightly higher temperature of $110\mathrm{K}$.

• Received 10 July 2006

DOI:https://doi.org/10.1103/PhysRevB.74.144416