Magnetic properties of the S=12 quasisquare lattice antiferromagnet CuF ₂(H ₂O) ₂(pyz) (pyz = pyrazine) investigated by neutron scattering

We have performed elastic and inelastic neutron scattering experiments on single crystal samples of the coordination polymer compound CuF ₂(H ₂O) ₂(pyz) (pyz=pyrazine) to study the magnetic structure and excitations. The elastic neutron diffraction measurements indicate a collinear antiferromagnetic structure with moments oriented along the [0.7 0 1] real-space direction and an ordered moment of 0.60 ± 0.03 μ _B/Cu. This value is significantly smaller than the single-ion magnetic moment, reflecting the presence of strong quantum fluctuations. The spin wave dispersion from magnetic zone center to the zone boundary points (0.5 1.5 0) and (0.5 0 1.5) can be described by a two-dimensional Heisenberg model with a nearest-neighbor magnetic exchange constant J _2D=0.934 ± 0.0025 meV. The interlayer interaction J _perp in this compound is less than 1.5% of J _2D. The spin excitation energy at the (0.5 0.5 0.5) zone boundary point is reduced when compared to the (0.5 1 0.5) zone boundary point by ∼10.3% ± 1.4%. This zone boundary dispersion is consistent with quantum Monte Carlo and series expansion calculations for the S=12 Heisenberg square lattice antiferromagnet, which include corrections for quantum fluctuations to linear spin wave theory.