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

The magnetic oxides NaFeP2O7 and LiFeP2O7, made up of FeO6 octahedra containing high-spin Fe3+ (d(5)) 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.