Evidence for a truly 2D Ising antiferromagnet

Trevor J Hicks

School of Physics and Materials Engineering
Monash University
Melbourne
Australia

Magnetic order in two dimensions is rather more marginal than in three dimensions. However it is clear that many materials which have exchange interactions predominantly in a plane achieve three dimensional order by residual interactions between the planes. For Ising antiferromagnets, order in two dimensions, with a finite critical temperature, is possible without inter-planar interactions, and because all materials have some degree of anisotropy, two-dimensional order can be achieved even with only magnetic dipole interactions1.

The MPS3 compounds (M=Mn,Fe,Ni) are planar honeycomb lattice antiferromagnets. MnPS3 is ordered with the help of dipole anisotropy and very small interplanar exchange interactions2. FePS3 was thought to be a relatively simple three dimensional antiferromagnet with strong Ising tendencies3. However, the previously determined magnetic structure is wrong, and, in further investigation of the structure, it has been found that FePS3 only has long range order in the two planar dimensions. The magnetic structure of FePS3 will be discussed and the static or dynamic nature of the short range order in the third dimension will be the subject of speculation.

1.    C. Pich, F. Schwabl, J. Magn. Magn. Mater. 148 (1995), 30.
2.   A.R. Wildes, B. Roessli, B. Lebech, K.W. Godfrey, J. Phys. C 10 (1998), 6417.
3.   K. Kurosawa, S. Saito, Y. Yamaguchi, J. Phys. Soc. Japan 52 (1983), 3919

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