  The growing family of two-dimensional (2D) materials that are now available
can be used to assemble van der Waals heterostructures with a wide range of
properties. Of particular interest are tunnelling heterostructures, which have
been used to study the electronic states both in the tunnelling barrier and in
the emitter and collector contacts. Recently, 2D ferromagnets have been studied
theoretically and experimentally. Here we investigate electron tunnelling
through a thin (2-6 layers) ferromagnetic CrBr3 barrier. For devices with
non-magnetic barriers, conservation of momentum can be relaxed by
phonon-assisted tunnelling or by tunnelling through localised states. In the
case of our ferromagnetic barrier the dominant tunnelling mechanisms are the
emission of magnons at low temperatures or scattering of electrons on localised
magnetic excitations above the Curie temperature. Tunnelling with magnon
emission offers the possibility of injecting spin into the collector electrode.
