Abstract
Single-crystal neutron diffraction shows that, at 23.2+or-0.1K, NiBr2 undergoes a transition from the collinear easy-plane metamagnetic structure, stable up to TN=52K, to a structure in which the spin orientation within the ab plane is modulated with a periodicity which varies smoothly with temperature, becoming 0.037+or-0.003 a0* at 4.2K.