  Altermagnets are a new class of magnetic materials with zero net
magnetization (like antiferromagnets) but spin-split electronic bands (like
ferromagnets) over a fraction of reciprocal space. As in antiferromagnets,
magnons in altermagnets come in two flavours, that either add one or remove one
unit of spin to the $S=0$ ground state. However, in altermagnets these two
magnon modes are non-degenerate along some directions in reciprocal space. Here
we show that the lifetime of altermagnetic magnons has a very strong dependence
on both flavour and direction. Strikingly, coupling to Stoner modes leads to a
complete suppression of magnon propagation along selected spatial directions.
This giant anisotropy will impact electronic, spin, and energy transport
properties and may be exploited in spintronic applications.
