Deuterium NMR spectral patterns observed at 61.4 MHz in a magneto-aligned ferroelectric phase of a partially deuterated smectogen are quantitatively interpreted based on Landau theory to understand the soliton-like distortion of the helicoidal structure. The distortion is induced gradually by rotating the sample in the NMR magnet using a goniometer probe. At each temperature, the simulation of angular dependent spectra enables us to determine the critical field for unwinding the helix. The study maps out a part of the (H, T) phase diagram. (c) 2005 Elsevier B.V. All rights reserved.