  We study the formation of a room temperature magnon Bose-Einstein condensate
(BEC) in nanoscopic systems and demonstrate that its lifetime is influenced by
the spatial confinement. We predict how dipolar interactions and nonlinear
magnon scattering assist in the generation of a metastable magnon BEC in
energy-quantized nanoscopic devices. We verify our prediction by a full
numerical simulation of the Landau-Lifshitz-Gilbert equation and demonstrate
the generation of magnon BEC in confined insulating magnets of yttrium iron
garnet. We directly map out the nonlinear magnon scattering processes behind
this phase transition to show how fast quantized thermalization channels allow
the BEC formation in confined structures. Based on our results, we discuss a
new mechanism to manipulate the BEC lifetime in nanoscaled systems. Our study
greatly extends the freedom to study the dynamics of magnon BEC in realistic
systems and to design integrated circuits for BEC-based applications at room
temperature.
