Magnetism and Phase Transition in Two-Dimensional Lattices; M(HCOO)2·2H2O (M; Mn, Fe, Ni, Co)

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Abstract
The heat capacities of Fe 2+ and Ni 2+ formate dihydrates were measured. Fe 2+ salt showed a sharp peak at T N =3.75±0.05 K with total magnetic entropy N k /2(ln 5+ln 2). Ni 2+ salt exhibited a distinct peak at T N =15.5±0.2 K and a Schottky type maximum around 3.1 K. It was discussed that M 2+ ions in (200) layer in this series of compounds were magnetically free and that M 2+ ions in (100) layer behaved as magnetically two-dimensional spin system which ordered at T N . It was concluded theoretically, using R. P. A. method, that the broad shoulder generally found in the specific heat curve just above T N was characteristic to the low dimensional spin system. This tendency was remarkable in the system with isotropic coupling such as Mn 2+ ions and not so distinct in the case of anisotropic Co 2+ ions. The present cases of Fe 2+ and Ni 2+ ions gave the examples of those with intermediate anisotropy.