  Compared with the traditional magnetic bubble, skyrmion has smaller size,
better stability therefore is considered as a very promising candidate for
future memory devices. When skyrmions are manipulated, erased and created, the
density of skyrmions can be varied, however the relationship between the radii
and the densities of skyrmions needs more exploration. In this paper, we study
this problem both theoretically and by using the lattice simulation. The
average radius of skyrmions as a function of material parameters, the strength
of external magnetic field and the density of skyrmions is obtained and
verified. With this explicit function, the skyrmion radius can be easily
predicted, which is helpful for the future study of skyrmion memory devices.
