On the conflicting pictures of magnetism for the frustrated triangular lattice antiferromagnet CuFeO2

The magnetic structures of the triangular lattice antiferromagnet CuFeO 2 below 14 K are described by an Ising model despite the fact that its high-spin Fe 3+ (d 5 ) ions (S = 5/2, L = 0) cannot have a uniaxial magnetic moment. To resolve this puzzling picture of magnetism, we estimated the relative strengths of various spin-exchange interactions of CuFeO 2 by performing a spin dimer analysis and then determined the relative stabilities of a number of ordered spin states of CuFeO 2 . Our calculations show that, in terms of a Heisenberg model, the noncollinear 120° spin arrangement predicted for a triangular lattice antiferromagnet is more stable than the collinear four-sublattice antiferromagnetic structure observed for CuFeO 2 below 11 K. To find a probable cause for stabilizing the collinear spin alignment along the c axis below 14 K, we considered the defect ions Fe 2+ and Cu 2+ of the CuFeO 2 lattice created by oxygen deficiency and oxygen excess, respectively. Our electronic structure analysis suggests that these defect ions generate uniaxial magnetic moments along the c axis and hence induce the surrounding Fe 3+ ions to orient their moments along the c axis.