Effect of biquadratic exchange on the axial Heisenberg model: Application to the magnetic phase transitions in UNi sub 2 Si sub 2

The influence of biquadratic exchange on the Heisenberg model with strong axial anisotropy is examined for the case of spins on a body-centered tetragonal lattice with competing bilinear exchange interactions along the {ital c} axis. Contrary to the usual axial next-nearest-neighbor Ising model with third-neighbor exchange also included, a low-temperature spin-wave calculation reveals that biquadratic exchange can stabilize the sequence of phases period 3{r arrow}period 2. Mean-field analysis, with support from Monte Carlo simulations, suggests a higher-temperature transition to an incommensurate state. This sequence of phase transitions has recently been observed in UNi{sub 2}Si{sub 2}. The importance of fluctuations is emphasized.