  We extend the nonlinear spin wave theory (NLSWT) for the spin 1/2
antiferromagnetic Heisenberg model on a triangular lattice (TAFHM). This novel
NLSWT considers the corrections one order higher in 1/S than the linear spin
wave theory (LSWT). It also distinguishes in which circumstance the negative
energy excitation, the sign of the breakdown of LSWT, shall be renormalized to
be positive both by a boson normal ordering and a self-consistent iteration. We
draw a phase diagram by testing the stability of various magnetic orders for
different parameters. In particular, the incommensurate configuration is found
unstable by our study. The new phase transition point (PTP) of the collinear
configuration agrees well with various previous studies.
