Role of anisotropy in the spin-dimer compound BaCuSi2O6

We present results of magnetization and electron paramagnetic resonance experiments on the spin-dimer system BaCuSi2O6. Evidence indicates that the origin of anisotropic terms in the spin Hamiltonian lies in magnetic dipolar interactions. Axial symmetry breaking is on a very small energy scale of <= 11 mK, confirming Bose-Einstein condensation critical scaling over an extended temperature range in the vicinity of the quantum critical point.