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

We report on ESR, magnetization, and specific-heat studies of [Cu(pyz)₂(HF₂)]PF₆ single crystals, identified as a quasi-twodimensional spin-1/2 Heisenberg antiferromagnet. Our measurements revealed J_inter/J_intrainter,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 are strongly affected by two-dimensional spin fluctuations, despite of the onset of 3D long-range magnetic ordering at T_N ≈ 4.4 K. The temperature-field phase diagram and ESR magnetic excitation spectrum in the 3D ordered phase of [Cu(pyz)₂(HF₂)]PF₆ will be discussed in detail.