The two-layer square lattice quantum antiferromagnet with spins 1/2 shows a
zero-field magnetic order-disorder transition at a critical ratio of the
inter-plane to intra-plane couplings. Adding a uniform magnetic field tunes the
system to canted antiferromagnetism and eventually to a fully polarized state;
similar behavior occurs for ferromagnetic intra-plane coupling. Based on a bond
operator spin representation, we propose an approximate ground state
wavefunction which covers all phases by means of a unitary transformation. The
excitations can be efficiently described as independent bosons; in the
antiferromagnetic phase these reduce to the well-known spin waves, whereas they
describe gapped spin-1 excitations in the singlet phase. We compute the spectra
of these excitations as well as the magnetizations throughout the whole phase
diagram.Comment: 12 pages, 9 figs; added references; final version as publishe
