On the Conflicting Pictures of Magnetism for the Frustrated Triangular Lattice Antiferromagnet CuFeO₂

Abstract

The magnetic structures of the triangular lattice antiferromagnet CuFeO2 below 14 K are described by an Ising model despite the fact that its high-spin Fe3+ (d5) 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 CuFeO2 by performing a spin dimer analysis and then determined the relative stabilities of a number of ordered spin states of CuFeO2. 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 CuFeO2 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 Fe2+ and Cu2+ of the CuFeO2 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 Fe3+ ions to orient their moments along the c axis.