  Magnons are the quanta of collective spin excitations in magnetically-ordered
systems and manipulation of magnons for computing and information processing
has witnessed the development of ``magnonics". A magnon corresponds to an
excitation of the magnetic system from its ground state and the creation of a
magnon thus increases the total energy of the system. In this perspective, we
introduce the antiparticle of a magnon, dubbed the antimagnon, as an excitation
that lowers the magnetic energy. We investigate the stability and thermal
occupation of antimagnons and verify our theory by micromagnetic simulations.
Furthermore, we show how the concept of antimagnons yields a unified picture to
understand the magnonic analog of the Klein effect, magnonic black-hole
horizons, and magnonic black-hole lasing. Our work may stimulate fundamental
interest in antimagnons, as well as their applications to spintronic devices.
