  The ordering of the classical Heisenberg model on the pyrochlore lattice with
the antiferromagnetic nearest-neighbor interaction J1 and the ferromagnetic
next-nearest-neighbour interaction J2 is investigated by means of a mean-field
analysis and a Monte Carlo simulation. For a moderate J2/J1-value, the model
exhibits a first-order transition into an incommensurate multiple-q ordered
state where multiple Bragg peaks coexist in the spin structure factor. We show
that there are two types of metastable multiple-q states, a cubic symmetric
sextuple-q state and a non-cubic symmetric quadruple-q state. Based on a Monte
Carlo simulation, we find that the cubic sextuple-q state appears just below
the first-order transition temperature, while another transition from the cubic
sextuple-q state to the non-cubic quadruple-q state occurs at a lower
temperature.
