Magnetic ordering and spin dynamics in potassium jarosite; A Heisenberg kagom lattice antiferromagnet

Nishiyama, Masahide*; Maegawa, Satoru*; Inami, Toshiya; Oka, Yoshio*

The spin dynamics of the Heisenberg kagom lattice antiferromagnet, potassium jarosite KFe(OH)(SO), have been investigated by means of NMR. The spin-lattice relaxation rate, 1/, in the ordered phase decreases sharply with lowering temperature. The experimental rate is well explained by the two-magnon process of the spin waves having an energy gap of 15 K. The temperature dependence of the sublattice magnetization also supports the existence of the spin wave. These are the first experimental evidence that the low-energy excitation in the frustrated classical kagom lattice antiferromagnet is described by the spin wave. We calculate the spin wave in the q = 0 type 120 spin structure with weak single-ion-type anisotropy, and discuss the characteristics of the spin fluctuations in the Heisenberg kagom lattice antiferromagnet.