Magnetic structures of the triangular lattice magnets AFe(SO4)(2) (A = K, Rb, Cs)

In the crystal structures of CsFe(SO4)(2), RbFe(SO4)(2), and KFe(SO4)(2), t he magnetic Fe3+ ions form a triangular array in well separated layers. CsF e(SO4)(2) and RbFe(SO4)(2) may be regarded as realizations of the highly fr ustrated triangular lattice antiferromagnet, whereas KFe(SO4)(2) is a suspe cted realization of the row model. The latter model is characterized by two couplings J' and J, and for J'/J>0.5 forms a helical spin structure with a n incommensurate repeat distance. The regular triangular lattice magnet may be described by the row model with J'=J, and its "120 degrees" spin Struct ure may be regarded as a special case of this helical structure. We have de termined the low temperature (1.3 K) magnetic structures adopted by CsFe(SO 4)(2), RbFe(SO4)(2), and KFe(SO4)(2) by powder neutron diffraction. CsFe(SO 4)(2) and RbFe(SO4)(2) adopt the expected 120 degrees helical spin structur e of the triangular lattice magnet, but KFe(SO4)(2) does not adopt the expe cted incommensurate helical structure of the row model. Rather, it adopts a sine wave modulated structure. Possible reasons for this behavior are disc ussed. (C) 1998 American Institute of Physics. [S0021-8979(98)50511-7].