Nonmonotonic field dependence of the Neel temperature in the quasi-two-dimensional magnet [Cu(HF₂)(pyz)₂]BF₄

The measured thermodynamic phase diagram of the quasi-two-dimensional magnet [Cu(HF₂)(pyz)₂]BF₄ (pyz=pyrazine=N₂C₄H₄) exhibits an unusual nonmonotonic dependence of the Neel temperature T_N as a function of magnetic field H. The nonmonotonic behavior of T_N(H) results from two competing effects induced by the field: while H suppresses the amplitude of the order parameter by polarizing the spins along a given direction, it also reduces the phase fluctuations by changing the order parameter space from the sphere S² to the circle S¹. The latter effect dominates at low fields only if the system is close enough to its lower critical dimension (d_c = 2), i.e., when fluctuations become important. Our theoretical results reproduce the measured phase diagram and demonstrate that this unusual effect is realized in [Cu(HF₂)(pyz)₂]BF₄.