  We present a resonant x-ray diffraction study of the magnetic order in
NdFe$_3$(BO$_3$)$_4$ and its coupling to applied electric fields. Our
high-resolution measurements reveal A coexistence of two different magnetic
phases, which can be triggered effectively by external electric fields. More in
detail, the volume fraction of the collinear magnetic phase is found to
strongly increase at the expense of helically ordered regions when an electric
field is applied. These results confirm that the collinear magnetic phase is
responsible for the ferroelectric polarization of NdFe$_3$(BO$_3$)$_4$ and,
more importantly, demonstrate that magnetic phase coexistence provides an
alternative route towards materials with a strong magnetoelectric response.
