Magnetic Order and Magnetic Exchange Interactions in the Quasi-Two-Dimensional Magnets [Cu(pyz)₂(HF₂)]X with X = BF₄ and PF₆

We report on combined experimental and theoretical efforts to elucidate the magnetic properties of the organic magnets [Cu(HF₂)(pyz)₂]X with X = BF₄ and PF₆. By use of high-resolution specific-heat measurements on high-quality single crystals we were able to resolve small lambda-type anomalies indicative for a phase transition into a long-range ordered state. First-principles density-functional calculations allow to estimate the exchange couplings and confirm a three-dimensional antiferromagnetic order between ad-jacent Cu centers with a stronger coupling within the layers of pyz-bridged Cu and a much weaker cou-pling across the HF₂ linker units. Specific-heat data in magnetic field, B,reveal a slightly anisotropic nonmonotonic dependence of the Néel temperature T_N with B. The increase of T_N at low fields can be un-derstood by the reduction of phase fluctuations that are prominent in these highly anisotropic quasi-two-dimensional magnets. Towards higher fields, B suppresses the amplitude of the antiferromag-netic order parameter, and correspondingly T_N, by aligning the spins parallel to B. This nonmonotonic be-havior of T_N, as well as the magnetic part of the specific heat, cannot be described by a mean field theory that neglects the crucial role played by phase fluctuations. In contrast, we show that both properties are very well reproduced by Quantum Monte Carlo simulations. Pulsed-field magnetization data corroborate the strong anisotropy of the exchange interactions. The ratio of the inter- to intralayer exchange interac-tion is estimated as 0.0025 (for X = BF₄) and 0.009 (X = PF₆). High-field electron spin resonance meas-urements reveal a magnetic easy-plane anisotropy.