Heat capacity and magnetic behaviour of cobalt and manganese formate dihydrate

Abstract

The low-temperature heat capacities of magnetically two-dimensional materials, Mn(HCOO) 2.2H 2O and Co(HCOO) 2.2H 2O were measured. The results are consistent with previous heat-capacity measurements at higher temperatures. Schottky-like anomalies were observed at temperatures far below the antiferromagnetic transition temperatures T N of the magnetic lattices in the (100) planes. The entropy changes for both salts in that temperature range ( T< T N/3) were about one half of the total entropies. Adding the entropy changes in the higher temperature ranges, which have earlier been estimated, to the present results, we obtain good agreement with the theoretical predictions. These results suggest that the spin systems in the (200) planes remain paramagnetic below T N where the spin systems in the (100) planes exhibit long-range order. We conclude that the (200) systems order gradually with decreasing temperature, like normal paramagnetic saturation, in the exchange field caused by the spin systems in the (100) planes. The interlayer exchange interactions J AB were estimated as | J AB| k | = 0.025 K for the manganese and | J AB| k | = 0.6 K for the cobalt salt.