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

The measured thermodynamic phase diagram of the quasi-two-dimensional magnet [Cu(HF{sub 2})(pyz){sub 2}]BF{sub 4}(pyz=pyrazine=N{sub 2}C{sub 4}H{sub 4}) exhibits an unusual nonmonotonic dependence of the Neel temperature T{sub N} as a function of magnetic field H. The nonmonotonic behavior of T{sub 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 S{sup 2} to the circle S{sup 1}. The latter effect dominates at low fields only if the system is close enough to its lower critical dimension (d{sub 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(HF{sub 2})(pyz){sub 2}]BF{sub 4}.