  The stacking of intrinsically magnetic van der Waals materials provides a
fertile platform to explore tunable transport effects of magnons, presenting
significant prospects for spintronic applications. The possibility of having
topologically nontrivial magnons in these systems can further expand the scope
of exploration. In this work, we consider a bilayer system with intralayer
ferromagnetic exchange and a weak interlayer antiferromagnetic exchange, and
study the topological magnon-polaron excitations induced by magnetoelastic
couplings. Under an applied magnetic field, the system features a metamagnetic
transition, where the magnetic ground state changes from antiparallel layers to
parallel. We show that the metamagnetic transition is accompanied by a
transition of the topological structure of the magnon polarons, which results
in discernible changes in the topology induced transport effects. The
magnetic-field dependence of the thermal Hall conductivity and spin Nernst
coefficient is analyzed with linear response theories.
