Interplay of frustration and magnetic field in the two-dimensional quantum antiferromagnet Cu(tn)Cl₂

Specific heat and ac magnetic susceptibility measurements, spanning low temperatures (T≥40 mK) and high-magnetic fields (B≤14 T), have been performed on a two-dimensional (2D) antiferromagnet Cu(tn)Cl₂ (tn=1,3-diaminopropane=C₃H₁₀N₂). The compound represents a S=1/2 spatially anisotropic triangular antiferromagnet realized by a square lattice with nearest-neighbor (J/k_B=3 K), frustrating next-nearest-neighbor (0<J^′/J<0.6), and interlayer (|J^″/J|≈10⁻³) interactions. The absence of long-range magnetic order down to T=60 mK in B=0 and the T² behavior of the specific heat for T≤0.4 K and B≥0 are considered evidence of a high degree of 2D magnetic order. In fields lower than the saturation field, B_sat=6.6 T, a specific heat anomaly, appearing near 0.8 K, is ascribed to bound vortex-antivortex pairs stabilized by the applied magnetic field. The resulting magnetic phase diagram is remarkably consistent with the one predicted for a square lattice without a frustrating interaction, expect that B_sat is shifted to values lower than expected. Potential explanations for this observation, as well as the possibility of a Berezinski-Kosterlitz-Thouless (BKT) phase transition in a spatially anisotropic triangular magnet with the collinear Néel ground state, are discussed.