First-order metal-insulator transitions in manganites: Are they universal?

Conductivity data for La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (x=0.6) show a first-order transition from an orbital/charge-ordered insulator to a metal as the temperature falls below $\sim $160 K. The change in conductivity of $\sim $10000 is 100 times larger than that seen previously in any single-phase manganite in zero field. The metallic low-temperature state is similar to that found at x=0.58, but x=0.58 shows no evidence of orbital/charge order. This result, and further analysis, supports a conclusion that strongly coupled magnetic/conductive transitions are universally of first order.