  We use resonant soft X-ray diffraction to track the photo-induced dynamics of
the antiferromagnetic structure in a NdNiO$_{3}$ thin film. Femtosecond laser
pulses with a photon energy of 0.61 eV, resonant with electron transfer between
long-bond and short-bond nickel sites, are used to excite the material and
drive an ultrafast insulator-metal transition. Polarization sensitive soft
X-ray diffraction, resonant to the nickel L$_{3}$-edge, then probes the
evolution of the underlying magnetic spiral as a function of time delay with 80
picosecond time resolution. By modelling the azimuthal dependence of the
scattered intensity for different linear X-ray polarizations, we benchmark the
changes of the local magnetic moments and the spin alignment. The measured
changes are consistent with a reduction of the long-bond site magnetic moments
and an alignment of the spins towards a more collinear structure at early time
delays.
