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

Abstract

Heat capacity has been measured for the two-dimensional triangular Heisenberg antiferromagnets VX 2 (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 VCl 2 , 28.8±0.4 K for VBr 2 and 14.7±0.06 K for VI 2 . The entropy change consumed in the ordering process has been revealed to be quite small suggesting the frustrated character of spin correlations. In VI 2 , 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 = h T -2 + b T 3 . From the value of h , the hyperfine field H h at V 51 nucleus is evaluated to be H h = 82 ±8 kOe for VCl 2 and VBr 2 . Most of the b T 3 is attributed to the lattice contribution.