  Modern high-performance permanent magnets are made from alloys of rare earth
and transition metal elements, and large magnetization is achieved in the
alloys with high concentration of transition metals. We applied evolutionary
search scheme based on first-principles calculations to the Y-Co-B system and
predicted 37 cobalt-rich compounds with high probability of being stable.
Focusing on remarkably cobalt-rich compounds, YCo$_{16}$ and YCo$_{20}$, we
found that, although they are metastable phases, the phase stability is
increased with increase of temperature due to the contribution of vibrational
entropy. The magnetization and Curie temperature are higher by 0.22 T and 204 K
in YCo$_{16}$ and by 0.29 T and 204 K in YCo$_{20}$ than those of
Y$_{2}$Co$_{17}$ which has been well studied as strong magnetic compounds.
