Spin waves and the critical behaviour of the magnetization in

Inelastic neutron scattering from a single crystal of the quasi-two-dimensional antiferromagnet has been used to measure the spin wave dispersion curve at 4 K. The exchange integrals were subsequently calculated from linear spin wave theory. The values meV, meV, meV and meV are within stability conditions calculated from mean-field theory. In addition, the critical behaviour of the gap in the spin wave energy at the Brillouin zone centre has been measured, and compared to the critical behaviour of the magnetization from neutron scattering data of the magnetic (020) Bragg peak. The gap varies with magnetization for , and with the square of the magnetization for . Two possible explanations are proposed: a competition between single-ion and dipolar anisotropies; or a crossover to XY-like excitations.