  Combining the technologies of quantum optics and magnonics, we find that the
circularly polarized laser can dynamically realize the quasiequilibrium magnon
Bose-Einstein condensates (BEC). The Zeeman coupling between the laser and
spins generates the optical Barnett field, and its direction is controllable by
switching the laser chirality. We show that the optical Barnett field develops
the total magnetization in insulating ferrimagnets with reversing the local
magnetization, which leads to the quasiequilibrium magnon BEC. This
laser-induced magnon BEC transition through optical Barnett effect, dubbed the
optomagnonic Barnett effect, provides an access to coherent magnons in the high
frequency regime of the order of terahertz. We also propose a realistic
experimental setup to observe the optomagnonic Barnett effect using current
device and measurement technologies as well as the laser chirping. The
optomagnonic Barnett effect is a key ingredient for the application to
ultrafast spin transport.
