Current-driven magnetic domain wall (DW) motion has been extensively studied not only theoretically, but also experimentally. The DW motion is induced by spin-transfer torque, that is, the transfer of spin angular momentum from conduction electrons to localized electrons. The velocity of DW motion is proportional to the spin polarization [Pa = (σ↑ − σ↓)/(σ↑ + σ↓)] of electrical conductivity (σ) and its direction is the same as electron current when Pσ > 0. The reverse DW motion is thus expected in ferromagnetic materials with negative spin polarization (Pσ < 0) compared to those with positive spin polarization, because minority spin dominates the electrical conduction. Thereby, spintronics devices composed of both a positive Pσ material and a negative Pσ material, are of fundamental interest. We have paid a lot of attention to ferromagnetic Fe4N epitaxial films for application to spintronics devices because it is theoretically expected to have a large negative spin polarization (Pσ = −1.0).2 Very recently, we confirmed its negative spin polarization by experiment.3,4