  Exchange anisotropy provides a direction dependent mechanism for the
stability of the skyrmion lattice phase in noncentrosymmetric bulk chiral
magnets. Based on the Fourier representation of the skyrmion lattice, we
explain the direction dependence of the temperature-magnetic field phase
diagram for bulk MnSi through a phenomenological mean-field model incorporating
exchange anisotropy. Through quantitative comparison with experimental results,
we clarify that the stability of the skyrmion lattice phase in bulk MnSi is
determined by a combined effect of negative exchange anisotropy and thermal
fluctuation. The effect of exchange anisotropy and the order of Fourier
representation on the equilibrium properties of the skyrmion lattice is
discussed in detail.
