  Hexagonal YMnO_3 with space group P63cm is one of the rare examples of
multiferroic materials that exhibit co-dependence of a robust ferroelectric
phase along with antiferromagnetic ordering. This is primarily ascribed to
tilting of the MnO_5 polyhedra driven by magnetoelectric coupling. We report on
the effect of magnetic field at the atomic level in YMnO_3 by utilizing
magnetic field and temperature dependent neutron diffraction measurements. We
show that near the N\'eel temperature the lattice parameters, effective
magnetic moment per site, Mn-O bond lengths and O-Mn-O bond angles show large
variations and these are further modified in the presence of magnetic field. We
observe distinct changes with the application of magnetic field in the
paramagnetic state both in atomic positions and in the bulk dielectric
constant. Our results provide unambiguous confirmation of the role played by
exchange-striction over and above the much studied magnetoelectric coupling in
this frustrated antiferromagnet.
