  The magnon current holds substantial importance in facilitating the transfer
of angular momentum in spin-based electronics. However, the magnon current in
three-dimensional magnetic materials remains orders of magnitude too small for
applications. In contrast, magnon numbers in two-dimensional systems exhibit
significant enhancement and are markedly influenced by external magnetic
fields. Here, we investigate the magnon current in a two-dimensional easy-axis
ferromagnet and find a colossal magnon magnetoresistance (CMMR) effect, wherein
the change of the magnon conductance can reach as high as a thousand percent in
a moderate magnetic field. Moreover, the magnitude of the CMMR exhibits
significant dependence on the orientation of the magnetic field due to the
interplay between magnon- conserving and non-conserving scattering. We propose
a non-local magnon-mediated electrical drag experiment for the possible
experimental observation of the predicted effect. With the CMMR effect and a
much larger magnon number, magnon current in 2D materials shows promise as a
primary source for spin transport in spintronics devices.9
