  The magnetic-field-induced 90-degree flop of ferroelectric polarization P in
a spin-spiral multiferroic material TbMnO3 is theoretically studied based on a
microscopic spin model. We find that the direction of the P-flop or the choice
of +Pa or -Pa after the flop is governed by magnetic torques produced by the
applied magnetic field H acting on the Mn spins and thus is selected in a
deterministic way, in contradistinction to the naively anticipated
probabilistic flop. This mechanism resolves a puzzle of the previously reported
memory effect in the P direction depending on the history of the magnetic-field
sweep, and enables controlled switching of multiferroic domains by externally
applied magnetic fields. Our Monte-Carlo analysis also uncovers that the
magnetic structure in the P||a phase under H||b is not a so-far anticipated
simple ab-plane spin cycloid but a conical spin structure.
