  A new variational technique for investigation of the ground state and
correlation functions in 1D quantum magnets is proposed. A spin Hamiltonian is
reduced to a fermionic representation by the Jordan-Wigner transformation. The
ground state is described by a new non-local trial wave function, and the total
energy is calculated in an analytic form as a function of two variational
parameters. This approach is demonstrated with an example of the XXZ-chain of
spin-1/2 under a staggered magnetic field. Generalizations and applications of
the variational technique for low-dimensional magnetic systems are discussed.
