Magnetic Phase Transition in the Two-Dimensional Heisenberg Antiferromagnet Mn(HCOO)2·2(NH2)2CO

Abstract

The development of the short-range ordering in Mn(HCOO) 2 ·2(NH 2 ) 2 CO has been observed by the measurements of magnetic heat capacity and susceptibility. The experimental results are well reproduced by the high-temperature series expansion for the two-dimensional Heisenberg antiferromagnet with the exchange constant J / k B =-0.34±0.02 K in the temperature region T >1.2 T SK ( T SK ; the Stanley-Kaplan temperature). The critical temperature T N =3.77±0.02 K≃1.1 T SK nearly coincides with that of Mn(HCOO) 2 ·2H 2 O. It is suggested that the long-range ordering is responsible for the small Ising-like anisotropy in the exchange interaction. A two-dimensional spin structure with the canceled Weiss field from the adjacent layers, as in K 2 NiF 4 , is conjectured.