  We show that the interaction between spin-polarized current and magnetization
dynamics can be used to implement black-hole and white-hole horizons for
magnons - the quanta of oscillations in the magnetization direction in magnets.
We consider three different systems: easy-plane ferromagnetic metals, isotropic
antiferromagnetic metals, and easy-plane magnetic insulators. Based on
available experimental data, we estimate that the Hawking temperature can be as
large as 1 K. We comment on the implications of magnonic horizons for spin-wave
scattering and transport experiments, and for magnon entanglement.
