The appearance of spontaneous coherence is a fundamental feature of a Bose-Einstein condensate and an essential requirement for possible applications of the condensates for data processing and quantum computing. In the case of a magnon condensate in a magnetic crystal, such computing can be performed even at room temperature. We study the process of the formation of a coherent magnon condensate by direct detection of microwave radiation emitted by magnons in a parametrically driven yttrium iron garnet bulk sample of a special shape. We show that after switching off the parametric pumping, the overpopulated magnon gas evolves to a state, whose coherence is only limited by the natural magnon relaxation into the crystal lattice.

• Received 22 January 2021
• Revised 24 August 2021
• Accepted 25 August 2021

DOI:https://doi.org/10.1103/PhysRevB.104.L100410