  We report muon spin relaxation and magnetometry studies of bulk
Mn$_{1.4}$Pt$_{0.9}$Pd$_{0.1}$Sn and MnNiGa, two materials which have recently
been proposed to host topological magnetic states in thin lamella
(antiskyrmions for Mn$_{1.4}$Pt$_{0.9}$Pd$_{0.1}$Sn and biskyrmions for
MnNiGa), and show spin reorientation transitions in bulk. These measurements
shed light on the magnetic dynamics surounding the two magnetic phase
transitions in each material. In particular, we demonstrate that the behaviour
approaching the higher temperature transition in both samples is best
understood by considering a slow decrease in the frequency of dynamics with
temperature, rather than the sharp critical slowing down typical of second
order transitions. Furthermore, at low temperatures the two samples both show
spin dynamics over a broad range of frequencies that persist below the spin
reorienation transition. The dynamic behavior we identify gives new insight
into the bulk magnetism of these materials that may help underpin the
stabilization of the topologically non-trivial phases that are seen in thin
lamellae.
