  The two dimensional Heisenberg antiferromagnet on the square lattice with
nearest (J1) and next-nearest (J2) neighbor couplings is investigated in the
strong frustration regime (J2/J1>1/2). A new effective field theory describing
the long wavelength physics of the model is derived from the quantum
hamiltonian. The structure of the resulting non linear sigma model allows to
recover the known spin wave results in the collinear regime, supports the
presence of an Ising phase transition at finite temperature and suggests the
possible occurrence of a non-magnetic ground state breaking rotational
symmetry. By means of Lanczos diagonalizations we investigate the spin system
at T=0, focusing our attention on the region where the collinear order
parameter is strongly suppressed by quantum fluctuations and a transition to a
non-magnetic state occurs. Correlation functions display a remarkable size
independence and allow to identify the transition between the magnetic and
non-magnetic region of the phase diagram. The numerical results support the
presence of a non-magnetic phase with orientational ordering.
