  Effect of biquadratic exchange on phase transitions of a planar classical
Heisenberg (or XY) ferromagnet on a stacked triangular lattice is investigated
by Standard Monte Carlo and Histogram Monte Carlo simulations in the region of
a bilinear to biquadratic exchange interaction ratio $J_{1}/J_{2} \leq 1$. The
biquadratic exchange is found to cause separate second-order phase transitions
in a strong biquadratic exchange limit, followed by simultaneous dipole and
quadrupole ordering, which is of first order for an intermediate range of the
exchange ratio and changes to a second-order one again as $J_{1}/J_{2}$ is
further increased. Thus, a phase diagram featuring both triple and tricritical
points is obtained. Furthermore, a finite-size scaling analysis is used to
calculate the critical indices for both dipole and quadrupole kinds of
ordering.
