Magnetic properties of the quasi-two-dimensional S=1/2 Heisenberg antiferromagnet [Cu(pyz)2(HF2)]PF6

We report on high-field magnetization, specific heat, and electron-spin-resonance (ESR) studies of the quasi-two-dimensional spin-1/2 Heisenberg antiferromagnet [Cu(pyz)(2)(HF2)]PF6. The frequency-field diagram of ESR modes below T-N=4.38 K is described in the frame of the mean-field theory, confirming a collinear magnetic structure with an easy-plane anisotropy. The obtained results allowed us to determine the anisotropy/exchange interaction ratio, A/J=0.003, and the upper limit for the interplane/intraplane exchange interaction ratio, J'/J=1/16. It is argued that despite the onset of three-dimensional long-range magnetic ordering the magnetic properties of this material (including high-magnetic-field magnetization and nonmonotonic field dependence of the Neel temperature) are strongly affected by two-dimensional spin correlations.