Neutron Scattering fromK2NiF4: A Two-Dimensional Heisenberg Antiferromagnet

The quasielastic magnetic scattering from ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$ over the temperature range from 97.2°K to at least 200°K is found to correspond to reciprocal lattice rods rather than points thus giving the first concrete evidence of the two-dimensional character of ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$. At 97.1°K the crystal undergoes an extremely sharp phase transition to long-range order in three dimensions. From 97.0°K to 5°K the sublattice magnetization follows a ${\mathrm{}(97.1-T)\mathrm{}}^{\beta }$ law with $\beta =0.15\mathrm{}$.