  We investigate the effects of nonlinear couplings and external magnetic field
on the thermal entanglement in a two-spin-qutrit system by applying the concept
of negativity. It is found that the nonlinear couplings favor the thermal
entanglement creating. Only when the nonlinear couplings $|K|$ are larger than
a certain critical value does the entanglement exist. The dependence of the
thermal entanglement in this system on the magnetic field and temperature is
also presented. The critical magnetic field increases with the increasing
nonlinear couplings constant $|K|$. And for a fixed nonlinear couplings
constant, the critical temperature is independent of the magnetic field $B$.
