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

Abstract

The measured thermodynamic phase diagram of the quasi-two-dimensional magnet $[\text{Cu}({\text{HF}}_{2}){(\text{pyz})}_{2}]{\text{BF}}_{4}(\text{pyz}=\text{pyrazine}={\text{N}}_{2}{\text{C}}_{4}{\text{H}}_{4})$ exhibits an unusual nonmonotonic dependence of the N\'eel 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 ${S}^{2}$ to the circle ${S}^{1}$. 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 $[\text{Cu}({\text{HF}}_{2}){(\text{pyz})}_{2}]{\text{BF}}_{4}$.