Collective Superexchange and Exchange Coupling Constants in the Hydrogenated Iron Oxide Particle Fe₈O₁₂H₈

Motivated by the fact that Fe₂O₃ nanoparticles are used in the treatment of cancer, we have examined the role of ligands on the magnetic properties of these particles by focusing on (Fe₂O₃)₄ as a prototype system with H as ligands. Using the Broken-Symmetry Density Functional Theory, we observed a strong collective superexchange in the hydrogenated Fe₈O₁₂H₈ cluster. The average antiferromagnetic exchange coupling constant between the four iron-iron oxo-bridged pairs was found to be -178 cm^-1, whereas coupling constants between hydroxo-bridged pairs were much smaller. We found that despite the apparent symmetry of the iron atom framework, it is not reasonable to assume this symmetry when fitting the exchange coupling constants. We also analyzed the geometrical and magnetic properties of Fe₈O₁₂H _n for n = 0-12 and found that hydrogenating oxo-bridges would generally inhibit the Fe-O-Fe antiferromagnetic superexchange interactions. Antiferromagnetic lowest total energy states become favorable only when specific distributions of hydrogen atoms are realized. The (HO)₄-Fe₄(all spin-up)-O₄-Fe₄(all spin-down)-(OH)₄ configuration in Fe₈O₁₂H₈ presents such an example. This symmetric configuration can be considered a superdiatomic system.