Non-Heisenberg covalent magnetism in iron oxide clusters

Abstract

The sensitivity of material properties to the atomic and nanoscale morphology is most clearly demonstrated in small gas-phase clusters. In particular, magnetism serves as an extremely sensitive probe of the smallest modifications of atomic environment. This Rapid Communication demonstrates the drastic changes in both the exchange mechanism and the atomic moments in iron oxide clusters as compared to the bulk. In particular, the exchange is essentially non-Heisenberg and the exchange interactions are increased by an order of magnitude compared to bulk hematite. In addition, very large atomic magnetic moments are observed on oxygen sites due to strong spin polarization. As ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}^{+}$ and ${\mathrm{Fe}}_{5}{\mathrm{O}}_{7}^{+}$ are pure trivalent, double exchange is excluded and super exchange is the dominant exchange mechanism in these iron oxide clusters. Therefore, the non-Heisenberg behavior is attributed to covalent magnetism as the hybridization between $\mathrm{Fe}\phantom{\rule{4pt}{0ex}}3d$ and $\mathrm{O}\phantom{\rule{4pt}{0ex}}2p$ orbitals for clusters is strongly enhanced compared to bulk hematite and becomes magnetic state dependent.