Spin dimer and classical spin analyses of the ordered magnetic structures of alkali iron pyrophosphates NaFeP2O7 and LiFeP2O7

Abstract

The magnetic oxides NaFeP₂O₇ and LiFeP₂O₇, made up of FeO₆ octahedra containing high-spin Fe³⁺ (d⁵) ions, undergo a three-dimensional antiferromagnetic ordering at low temperatures. The strengths of various Fe–O⋯O–Fe super-superexchange interactions of NaFeP₂O₇ and LiFeP₂O₇ were estimated on the basis of spin dimer analysis to probe the nature of their ordered magnetic structures. It is found that the critical factor governing the strength of a Fe–O⋯O–Fe super-superexchange interaction is not the Fe⋯Fe distance but the O⋯O distance. Using the spin exchange parameters thus obtained, the total spin exchange interaction energies were calculated for various ordered spin arrangements of NaFeP₂O₇ and LiFeP₂O₇ on the basis of classical spin analysis to confirm that the observed magnetic structures are the magnetic ground states.