Specific heat of the highly anisotropic antiferromagnet [Cu(pyz)₂(HF₂)]PF₆

The metal-organic compound [Cu(pyz)₂(HF₂)]X with X = PF₆ exhibits a quasi-cubic lattice of copper ions (S = ½), but the magnetic properties show a predominantly two-dimensional (2D) nature due to a large anisotropy in the exchange couplings. The magnetic entropy and the antiferromagnetic ordering, eventually occurring at about 4 K, were investigated by specific-heat measurements. For this we established a continuous relaxation-time technique, using a single relaxation process to get specific-heat data over a wide temperature range. The calorimetric investigations, performed between 2 and 100 K and in magnetic fields up to 14 T, have revealed a non-monotonic field dependence of the ordering temperature. The results are as expected from the model for an S = ½ two-dimensional square-lattice quantum Heisenberg antiferromagnet with an additional weak interlayer exchange (via Cu-F-H-F-Cu bonds). In comparison to the X = BF₄ compound, the antiferromagnetically ordered phase extends into much higher temperatures. In a more detailed analysis, we can extract all exchange interactions with an interlayer coupling ten times higher than in X = BF₄. Thus, the 2D character is significantly reduced in X = PF₆.