Control of domain wall position in L-shaped Fe₄N negatively spin polarized ferromagnetic nanowire

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 [P_a = (σ_↑ - σ_↓)/(σ_↑ + σ_↓)] of electrical conductivity (σ) and its direction is the same as electron current when P_σ > 0. The reverse DW motion is thus expected in ferromagnetic materials with negative spin polarization (P_σ < 0) compared to those with positive spin polarization, because minority spin dominates the electrical conduction. Thereby, spintronics devices composed of both a positive P_σ material and a negative P_σ 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 (P_σ = -1.0).² Very recently, we confirmed its negative spin polarization by experiment.^3,4