  Previous observations of metastable magnetic skyrmions have shown that close
to the equilibrium pocket the metastable state has a short lifetime, and
therefore rapid cooling is required to generate a significant skyrmion
population at low temperatures. Here, we report that the lifetime of metastable
skyrmions in Cu$_2$OSeO$_3$ is extended by a factor of 50 with the introduction
of only 2.5% zinc doping, allowing over 50% of the population to survive when
field-cooling at a rate of just 1 K/min. Our systematic study suggests that the
lifetime enhancement is due to the removal of spins by the non-magnetic dopant,
which entropically limits the number of skyrmion decay pathways. We expect that
doping can be exploited to control the lifetime of the metastable SkL state in
other chiral magnets, offering a method of engineering skyrmion materials
towards application in future devices.
