  Zero temperature phase diagram of XXZ spin chain in external magnetic field
is investigated at low energies using path integral approach.It has been shown
by spin wave analysis and then by nonlinear sigma model transformation that
below some critical field the system undergoes quantum Kosterlitz-Thouless
phase transition.Above that critical field for low anisotropies the system is
saturated while at strong anisotropy the system has Ising antiferromagnetic
order.
