Competing Instabilities and Metastable States in(Nd,Sm)1/2Sr1/2MnO3

Competition between the ferromagnetic double-exchange interaction and the antiferromagnetic charge-ordering instability gives rise to thermally as well as magnetic field (H) induced insulator-metal transitions in distorted perovskites $({\mathrm{Nd}}_{1\ensuremath{-}y}{\mathrm{Sm}}_{y}{)}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}$ with a controlled one-electron bandwidth. A systematic study on the metal-insulator phase diagram in the $T\ensuremath{-}H$ plane with varying y as well as a high-pressure study has revealed the metastable nature of the undercooled ferromagnetic metallic state.