Neutron-Diffraction Determination of the Antiferromagnetic Structure of KCuF3

We have investigated the nuclear and magnetic diffraction of neutrons from powder and single-crystal samples of KCu${\mathrm{F}}_3$ at temperatures between 298 and 4.4°K, in order to establish the magnetic structure, and, in particular, to try to determine whether there exist any long-range one-dimensional correlations in this material. Previous work has shown that KCu${\mathrm{F}}_3$ has two stable polytype tetragonal structures which may coexist over a large range of temperatures. The samples used in this work indicated the simultaneous presence of both these structures, and gave diffraction peaks indicative of $D_{4h}^{}{}_{}{}^{18}$ and $D_{4h}^{}{}_{}{}^5$ space groups. Both structures are found to have type-$A$ magnetic ordering with the spins lying in the $a-b$ plane. Our results suggest that the $D_{4h}^{}{}_{}{}^{18}$ structure orders at 38±1°K, while the $D_{4h}^{}{}_{}{}^5$ may order at 22±4°K. The moment of the ${\mathrm{Cu}}^{2+}$ ion is determined to be $0.49\pm 0.07{\mu }_B$. No direct evidence for one-dimensional order above 38°K nor for long-range one-dimensional correlations has been found, but their presence has not been unambiguously ruled out.