  Recent progress in the field of multiferroics led to the discovery of many
new materials in which ferroelectricity is induced by cycloidal spiral orders.
The direction of the electric polarization is typically constrained by spin
anisotropies and magnetic field. Here, we report that the mixed rare-earth
manganite, Gd$_{0.5}$Dy$_{0.5}$MnO$_3$, exhibits a spontaneous electric
polarization along a general direction in the crystallographic ac-plane, which
is suppressed below 10 K but re-emerges in an applied magnetic field. Neutron
diffraction measurements show that the polarization direction results from a
large tilt of the spiral plane with respect to the crystallographic axes and
that the suppression of ferroelectricity is caused by the transformation of a
cycloidal spiral into a helical one, a unique property of this rare-earth
manganite. The freedom in the orientation of the spiral plane allows for a fine
magnetic control of ferroelectricity, i.e. a rotation as well as a strong
enhancement of the polarization depending on the magnetic field direction. We
show that this unusual behavior originates from the coupling between the
transition metal and rare-earth magnetic subsystems.
