  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 TN = 4.38 K is described in the frame of the meanfield 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 inter/intra-plane exchange-interaction
ratio, J'/J = 1/16. It is argued that despite the onset of 3D long-range
magnetic ordering the magnetic properties of this material (including
high-magnetic-field magnetization and non-monotonic field dependence of the
Neel temperature) are strongly affected by two-dimensional spin correlations.
