Paramagnetic Anisotropy and Two-Dimensional Antiferromagnetism in Copper Formate Tetrahydrate

The paramagnetic anisotropies of the single crystals of copper formate tetrahydrate have been measured between 300 and about 80°K. The distinct layer structure parallel to the (001) plane make this crystal a good example of a two-dimensional antiferromagnetic lattice. While the average moment is lower than the normal value of 1.9μB, the crystal shows in this temperature range a molecular anisotropy which is typical of normal copper compounds. Although the molecular moment in the plane of the layer is only about 1.6μB compared to an almost normal value of 1.87μB normal to it, the temperature variation of the principal molecular susceptibilities show that both K‖ and K⊥ obey a Curie–Weiss law with almost the same Weiss constant, θ‖ = −160°K and θ⊥ = −170°K. This result is in agreement with the theory of a two-dimensional antiferromagnetic lattice, but disproves the earlier belief that the temperature variation of K‖ and K⊥ should be quite different, in this case K‖ obeying a Curie law and K⊥ obeying a Curie–Weiss law with a large Weiss constant. The temperature variation of K‖ and K⊥ within this temperature range is quantitatively explained using the high temperature series expansion solution for a two-dimensional Heisenberg antiferromagnet.