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 Néel temperature) are strongly affected by two-dimensional spin correlations.