A study of magnetic phase transition in a two-dimensional lattice by heat capacity measurements

Abstract

The heat capacity of cobalt formate dihydrate was measured above 1.8 K. The results showed a sharp peak at 5.12 K. The entropy gain above 2 K was estimated to be 0.33 R which is about one half of the total entropy R 1n 2 for S = 1 2 . The result is consistent with recent heat-capacity measurements at lower temperatures. Adding the entropy gain below 2 K which is associated with a broad Schottky-type anomaly to the above-mentioned value, the total entropy amounts to 0.67 R and agrees well with R 1n 2 within the experimental error. From these results the heat capacity above 2 K is considered to be due to the Co 2+ ions in the (100) plane (A plane) while the remaining one half of Co 2+ ions in the (200) plane (B plane) behave as a free-spin system above 2 K. Therefore we conclude that the phase transition at T N takes place only in the A plane and is caused by the strong anisotropic exchange interaction. The intra-layer exchange interaction in the A plane J AA/ k was estimated to be 4.3 K. The value kT N/ J AA is 1.2, which is compared with that of the two-dimensional Ising model, 1.13. From the comparison of the heat capacity of anisotropic cobalt formate with that of isotropic manganese formate, the influence of the anisotropy of the exchange interaction on the phase transition was discussed.