Abstract
The quasi-elastic magnetic scattering from the planar antiferromagnets Ni, Mn, Fe has been studied over a wide range of temperatures both above and below the phase transition. In all three compounds the diffuse scattering above the phase transition takes the form of a ridge rather than a peak, thus giving the first concrete evidence for the two-dimensional nature of the magnetism. At (97.1, 38.4, 56.3°K, respectively), the crystals undergo sharp phase transitions to long-range order (LRO) in three dimensions. For , the sublattice magnetizations in Ni and Mn follow a law with , respectively. Mn is found to have two distinct magnetic phases, both with identical ordering within the planes but with different stacking arrangements of the spins between planes; both phases are found to have identical and to within the experimental accuracy of 0.1°K. The sublattice magnetization in Fe has a rather more complicated behavior, apparently due to magnetostrictive effects. Finally, in the ordered phase in each compound, the three-dimensional magnetic Bragg peaks are accompanied by "diffuse" scattering which is completely two-dimensional in form. These results are discussed in terms of a model in which the phase transition is viewed as being essentially two-dimensional in character, the three-dimensional ordering simply following as a necessary consequence of the onset of LRO with the planes. The systems therefore should have distinct two- and three-dimensional critical regions. The three-dimensional region apparently was not experimentally accessible with 0.1°K temperature control in Ni and Mn, indicating that in these compounds .
- Received 9 October 1969
DOI:https://doi.org/10.1103/PhysRevB.1.2211
©1970 American Physical Society