Magnetic Properties of the quasi-2D S=1/2 Heisenberg antiferromagnet [Cu(pyz)$_2$(HF$_2$)]PF$_6$

We report on electron spin resonance, high-field magnetization, and specific-heat studies of [Cu(pyz)$_2$(HF$_2$)]PF$_6$ single crystals, identified as a quasi-two-dimensional spin-1/2 Heisenberg antiferromagnet. Our measurements revealed $J_{inter}/J_{intra}\leq $ 0.063 and $A/J\sim$ 0.003, where $J_{inter}$, $J_{intra}$, $J$ are the interplane, intraplane and mean exchange interactions, respectively, and $A$ is the anisotropy constant. It is argued that the magnetic properties of this material (including high-magnetic-field magnetization and the temperature-field phase diagram) are strongly affected by two-dimensional spin fluctuations, despite of onset of 3D long-range magnetic ordering at $T_N\approx 4.4$~K. The ESR magnetic excitation spectrum in the 3D ordered phase is studied in detail.