  In a recent paper, cond-mat/0404041, J. Vanacken et al. reported experimental
studies of crystals of Mn12-ac molecular nanomagnets in pulsed magnetic fields
with sweep rates up to 4000 T/s. Steps in the magnetization curve were
observed. The data were explained by collective dipolar relaxation. We give
here an alternative explanation that is based on thermal avalanches triggered
by defect molecules (faster relaxing species). These species are always present
in Mn12-ac molecular nanomagnets. We propose a simple method to test this
interpretation. Note that we do not question the possibility of collective
effects that are bassed on spin--spin interactions.
