Magnetic Properties of the Skyrmion Gas in Two Dimensions

The classical ferromagnet is analyzed in two dimensions via a mean-field treatment of the CP$^1$ model valid in the limit of low temperature and of weak magnetic field. At high skyrmion densities, we find that the magnetization begins to decrease linearly with temperature, but it then crosses over to a Curie-law at higher temperature. The comparison of these results with recent Knight-shift experiments on the quantum Hall state is fair.