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

Abstract

In the crystal structures of CsFe(SO4)2, RbFe(SO4)2, and KFe(SO4)2, the magnetic Fe3+ ions form a triangular array in well separated layers. CsFe(SO4)2 and RbFe(SO4)2 may be regarded as realizations of the highly frustrated triangular lattice antiferromagnet, whereas KFe(SO4)2 is a suspected 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 an incommensurate repeat distance. The regular triangular lattice magnet may be described by the row model with J′=J, and its “120°” spin structure may be regarded as a special case of this helical structure. We have determined the low temperature (1.3 K) magnetic structures adopted by CsFe(SO4)2, RbFe(SO4)2, and KFe(SO4)2 by powder neutron diffraction. CsFe(SO4)2 and RbFe(SO4)2 adopt the expected 120° helical spin structure of the triangular lattice magnet, but KFe(SO4)2 does not adopt the expected incommensurate helical structure of the row model. Rather, it adopts a sine wave modulated structure. Possible reasons for this behavior are discussed.