Triangular-spin, kagome plane in jarosites

Abstract

Iron atoms in ${\mathrm{KFe}}_{3}$(OH${)}_{6}$(${\mathrm{SO}}_{4}$${)}_{2}$ and related jarosite-type materials occur in kagome planes separated by about 5.7 A\r{}. A planar magnetic interaction is indicated from magnetic-susceptibility and neutron diffraction measurements. Triangular-spin antiferromagnetic ordering (${T}_{N}$=46 and 65 K) is observed in ${\mathrm{KFe}}_{3}$(OH${)}_{6}$(${\mathrm{SO}}_{4}$${)}_{2}$, and ${\mathrm{KFe}}_{3}$(OH${)}_{6}$(${\mathrm{CrO}}_{4}$${)}_{2}$, respectively, from powder neutron diffraction, M\"ossbauer, and magnetic-susceptibility data. The magnetic cell of ${\mathrm{KFe}}_{3}$(OH${)}_{6}$(${\mathrm{SO}}_{4}$${)}_{2}$ is double that of the crystallographic cell along the c-axis direction, while in ${\mathrm{KFe}}_{3}$(OH${)}_{6}$(${\mathrm{CrO}}_{4}$${)}_{2}$ the two cells are identical. A weak moment of 0.03${\ensuremath{\mu}}_{B}$/(Fe atom) occurs at temperatures below 65 K, the antiferromagnetic ordering temperature, in ${\mathrm{KFe}}_{3}$(OH${)}_{6}$(${\mathrm{CrO}}_{4}$${)}_{2}$. The susceptibilities of ${\mathrm{KCr}}_{3}$(OH${)}_{6}$(${\mathrm{SO}}_{4}$${)}_{2}$, a new material, follow a Curie-Weiss law above 20 K. Below this temperature the susceptibilities suggest ordering at 1--2 K. Triangular-spin configurations follow by the application of Bertaut's matrix method to the kagome lattice. Superexchange (134\ifmmode^\circ\else\textdegree\fi{}) is stronger by more than an order of magnitude in ${\mathrm{KFe}}_{3}$(OH${)}_{6}$(${\mathrm{SO}}_{4}$${)}_{2}$ and ${\mathrm{KFe}}_{3}$(OH${)}_{6}$(${\mathrm{CrO}}_{4}$${)}_{2}$ than in ${\mathrm{KCr}}_{3}$(OH${)}_{6}$(${\mathrm{SO}}_{4}$${)}_{2}$. Comparison is made with ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ and ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$.