Experiments on Two-Dimensional Heisenberg Systems

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Abstract
The crystal structure of the group of compounds with the general formula M2+M2+(Hal)4, where the magnetic ion may be Mn2+, Ni2+, or Cu2+ for instance, strongly suggests the possibility of studying the thermal and magnetic properties of almost isolated layers of magnetic atoms. The susceptibility vs temperature curves have been obtained for K2NiF4, K2MnF4, and Rb2MnF4 single crystals for which the interaction within a layer has the antiferromagnetic sign. Below TN both the curve for the parallel susceptibility and the heat capacity are in agreement with spin‐wave predictions for a two‐dimensional layer taking into account the anisotropy field as derived from the observed field for spin flopping. The values for kTN/J in K2MnF4, Rb2MnF4, and K2NiF4 agree with predictions from partition function series expansions. In Cu(CH3NH3)2Cl4 and Cu(C2H5NH5)2Cl4 the nearest‐neighbor interaction has the ferromagnetic sign. Two transition temperatures are found in the heat capacity, the higher ones probably corresponding to the Curie temperature of the isolated layer, the lower one the transition to long‐range order in three dimensions. The values for kTC(2)/J observed, are about 1.1.