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 ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{MnO}}_3$ (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 ${\mathit{T}}_{\mathit{C}}$, 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/${\mathit{M}}_{\mathit{s}}$ ${)}^2$ for M/${\mathit{M}}_{\mathit{s}}$≲0.3, where ${\mathit{M}}_{\mathit{s}}$ is the saturated magnetization. The coefficient C strongly depends on x, i.e., C≊4 near the compositional insulator-metal phase boundary (${\mathit{x}}_{\mathit{c}}$∼0.17), but decreases down to ≊1 for x>=0.4, indicating the critical change of the electronic state.

An electronic (metal-to-insulator) phase transition of the first order, which can be caused by an external magnetic field, was discovered in Nd _1/2 Sr _1/2 MnO ₃ . A clear hysteresis was observed during the increase and decrease of an external magnetic field at a fixed temperature. The hysteretic field region was observed to depend critically on temperature and to drastically expand with a decrease of temperature, perhaps as a result of suppression of the effect of thermal fluctuations on the first-order phase transition. Although it has seldom been observed, this is thought to be a generic feature of the first-order phase transition at low temperatures near 0 kelvin.

Field-induced insulator-to-metal transitions have been found in ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x\mathrm{Mn}{\mathrm{O}}_3(0.3\mathrm{}<~x<~0.5)$ single crystals, which are accompanied by a melting of the insulating charge-ordered (i.e., ${\mathrm{Mn}}^{3+}$/${\mathrm{Mn}}^{4+}$ ordered) state. The transition is of the first order with a hysteresis and is even irreversible at low temperatures. The $x$-dependent metal-insulator phase diagrams in the $H-T$ plane indicate that a deviation of $x$ from 0.5 modifies the robustness of the charge-ordering state, which is argued in terms of the effect of discommensuration of the charge concentration on the charge-ordered state.

The perovskite-type manganese oxide, ${\mathrm{Pr}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ $(x=0.5\mathrm{})$, shows a first-order phase transition from a ferromagnetic (FM) metal to antiferromagnetic (AFM) nonmetal at 140 K, which accompanies the transition to the charge-ordered state (generalized Wigner lattice). The nonmetal-to-metal transition temperature can be decreased down to near 0 K with a magnetic field up to 70 kOe. The temperature-field phase diagram for the charge-ordered state is presented together with a discussion on the mechanism of this field-induced phase transition.