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

Abstract

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 = C3H10N2). The compound represents an S = 1/2 spatially anisotropic triangular magnet realized by a square lattice with nearest-neighbor (J/kB = 3 K), frustrating nextnearest-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 T 2 behavior of the specific heat for T ≤ 0.4 K and B ≥ 0 are considered evidence of high degree of 2D magnetic order. In fields lower than the saturation field, Bsat = 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 the ideal square lattice, except that Bsat 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 Néel ground state, are discussed. PACS numbers: 75.40.-s, 75.10.JmRecently, Cu(tn)Cl 2 has been identified as a potential model system for the realization of the spatially anisotropic triangular lattice from the collinear Néel phase (J ′ /J < 0.6). 15 For Cu(tn)Cl 2 studied in B = 0, no evidence for long-range magnetic order was observed down to 60 mK, and the data suggested intralayer inter-action strengths of J/k B = 3 K and 0 < J ′ /J < 0.6, while the interlayer coupling is |J ′′ /J| ≈ 10 −3 . These interactions are described by the Hamiltonianwhere i, j label intralayer spins and k labels the interlayer ones. J J a b c FIG. 1: Realization of Heisenberg model of a spatially anisotropic triangular lattice within a single bc layer in Cu(tn)Cl2. The layers are stacked along the a direction. The full circles denote Cu 2+ ions.

Magnetic properties of the quasi-two-dimensionalS=12Heisenberg antiferromagnet[Cu<

“…Such a behavior has recently been observed in the quasi-2D square-lattice Heisenberg AF ͓Cu͑C 4 H 4 N 2 ͒ 2 ͑HF 2 ͔͒BF 4 ͑Refs. 8 and 9͒ and in the spatially anisotropic triangular magnet Cu͑tn͒Cl 2 , 10 where tn is 1,3-diaminopropane ͑C 3 H 10 N 2 ͒.…”

Field-inducedXYand Ising ground states in a quasi-two-dimensionalS=12Heisenberg antiferromagnet

“…3 They also predicted that this effective spin anisotropy, Δ in Eq.1, scales quadratically with H as Δ~0.1h 2 for small fields (h<2), where h is the normalized magnetic field h≡gμ B H/(JS). In spite of the intense research in this area, [9][10][11][12][13] hitherto the quadratic field dependence has never been confirmed, likely due to the non-negligible value of interplane exchange interaction J' in real systems. By contrast, the application of H in the EP in the 2D-XYAFM (Δ>0, and z=a,b) restricts the rotation in the ab-plane and induces an Ising ground state (see Fig.2(a)).…”

Experimental Study of the Magnetocaloric Effect in the Two-Dimensional Quantum System Cu(tn)Cl_2

“…[3], applied magnetic eld induced easy-plane anisotropy in Cu(tn)Cl 2 , resulting in the BKT transition, which causes reduction of the entropy in non zero magnetic eld. Correspondingly, the rst crossover from the normal to the inverse MCE, occurring at about 0.3 K (Fig.…”

“…The data analysis suggested intralayer antiferromagnetic nearest-neighbor (J/k B = 3 K), frustrating next-nearest-neighbor (0 < J /J < 0.6), and interlayer (|J /J| ≈ 10 −3 ) interactions. Thermodynamic studies identied the response of Cu(tn)Cl 2 in nonzero magnetic eld as a eld-induced Berezinskii-Kosterlitz-Thouless (BKT) phase transition below 1 K [3].…”

“…The magnetocaloric eect is a useful and sensitive technique for mapping magnetic phase transitions as well as for understanding the nature of magnetic transitions [4]. The MCE study was used to trace the phase transition in Cu(tn)Cl 2 expected at 0.8 K [3]. * corresponding author; e-mail: lucia.baranova@tuke.sk…”