  We propose a spin valve device based on the interplay of a modulated
spin-orbit interaction and a uniform external magnetic field acting on a
quantum wire. Half-metal phases, where electrons with only a selected spin
polarization exhibit ballistic conductance, can be tuned by varying the
magnetic field. These half-metal phases are proven to be robust against
electron-electron repulsive interactions. Our results arise from a combination
of explicit band diagonalization, bosonization techniques and extensive DMRG
computations.
