Heat Capacities of the Two-Dimensional Triangular Heisenberg Antiferromagnets VX₂ (X; Cl, Br, and I)

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Heat capacity has been measured for the two-dimensional triangular Heisenberg antiferromagnets VX₂ (X; Cl, Br and I) below 60 K. A distinct λ-type peak has been observed at the respective transition temperatures; 35.4±0.5 K for VCl₂, 28.8±0.4 K for VBr₂ and 14.7±0.06 K for VI₂. The entropy change consumed in the ordering process has been revealed to be quite small suggesting the frustrated character of spin correlations. In VI₂, another strong peak suggesting the transition of the first kind has been found at 13.50±0.06 K. The heat capacity C below 5 K is reproduced by C⁄R=hT⁻²+bT³. From the value of h, the hyperfine field H_h at V⁵¹ nucleus is evaluated to be H_h=82±8 kOe for VCl₂ and VBr₂. Most of the bT³ is attributed to the lattice contribution.