Insulator-metal transition and giant magnetoresistance in La1-xSrxMnO3

Insulator-metal phenomena depending on band filling (doping degree), temperature, and external magnetic field have been investigated for prototypical double-exchange ferromagnets, namely, crystals of La_1-xSr_xMnO₃ (0<=x<=0.6). The electronic phase diagram in the plane of the temperature vs nominal hole concentration (x) has been deduced from the magnetic and electrical measurements on the melt-grown crystals. Around the ferromagnetic transition temperature T_C, large negative magnetoresistance was observed. Irrespective of temperature, reduction of the resistivity is scaled with the field-induced magnetization (M) as -Δρ/ρ=C(M/M_s)² for M/M_s<~0.3, where M_s is the saturated magnetization. The coefficient C strongly depends on x, i.e., C~=4 near the compositional insulator-metal phase boundary (x_c~0.17), but decreases down to ~=1 for x>=0.4, indicating the critical change of the electronic state.