  Detailed investigation of the incommensurate magnetic ordering in a single
crystal of multiferroic NdMn2O5 has been performed using both non-polarized and
polarized neutron diffraction techniques. Below TN = 30.5 K magnetic Bragg
reflections corresponding to the non-chiral type magnetic structure with
propagation vector k1 = (0.5 0 kz1) occurs. Below about 27 K a new distorted
magnetic modulation with a similar vector kz2 occurs, which is attributed to
the magnetization of the Nd3+ ions by the Mn-sub-lattice. Strong temperature
hysteresis in the occurrence of the incommensurate magnetic phases in NdMn2O5
was observed depending on the cooling or heating history of the sample. Below
about 20 K the magnetic structure became of a chiral type. From spherical
neutron polarimetry measurements, the resulting low-temperature magnetic
structure kz3 was approximated by the general elliptic helix. The parameters of
the magnetic helix-like ellipticity and helical plane orientation in regard to
the crystal structure were determined. A reorientation of the helix occurs at
an intermediate temperature between 4 K and 18 K. A difference between the
population of right- and left-handed chiral domains of about 0.2 was observed
in the as-grown crystal when cooling without an external electric field. The
magnetic chiral ratio can be changed by the application of an external electric
field of a few kV/cm, revealing strong magnetoelectric coupling. A linear
dependence of the magnetic chirality on the applied electric field in NdMn2O5
was found. The results are discussed within the frame of the antisymmetric
super-exchange model for Dzyaloshinsky-Moria interaction.
