Phase Transition of a Nearly Heisenberg Layer Antiferromagnet Cu(HCOO)_2・2(NH_2)_2CO・2H_2O

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Magnetic susceptibility and heat capacity of a nearly Heisenberg layer antiferromagnet Cu(HCOO)_2・2(NH_2)_2CO・2H_2O were measured between 1.2K and 90K. A broad maximum of the susceptibility associated with the development of short range order was observed near 60K. A remarkable peak was also found at 15.5±0.1K, which certainly shows an onset of phase transition at the temperature. Magnetic susceptibility is high temperature region agreed with the calculated one by using high temperature series expansion with J/K=-33K. A very small peak of heat capacity was found at 15.45±0.05K by carefull examination. The extra entropy associated with it was only about 0.006% of the total magnetic entropy R1n2. Comparing these results with those of nearly one and two dimensional Heisenberg antiferromagnets CuCl_2・2NC_5H_5 and Cu(HCOO)_2・4H_2O, we suggest that nearly Heisenberg spin system in two dimensional lattice sets into long range order at a finite temperature T_N with a negligibly small peak of heat capacity.
社団法人日本物理学会の論文
1976-05-15