Critical Amplitude of Specific Heat Capacity in the Antiferromagnets of K₂NiF₄ Family Belonging to Two-Dimensional Ising Universality Class

Abstract

Anomalous specific heat capacities of K₂NiF₄ family were measured with a high resolution. They were found to exhibit a symmetric logarithmic divergence as expected in two-dimensional Ising system into which the present two-dimensional anisotropic Heisenberg antiferromagnets were classified in terms of the Hamiltonian (Remark: Graphics omitted.). The critical amplitude, \ ildeA⁄R was found to vary in proportion to Δ^1⁄2. In connection with this fact, the coefficient κ₀a of the inverse correlation length was found to be proportional to Δ^1⁄4. These dependences on Δ can be understood in the frame work of anisotropic cross-over scaling hypothesis in a strong scaling case. The results predict that the crossing-over from the two-dimensional Ising to Heisenberg behavior occurs at a nonzero temperature and that the anisotropic cross-over exponent φ lies at about 4.