Images of magnetic excitations measured on the Maps spectrometer above and below Tv compared to computer calculations. Note the anomalous behaviour of excitations in the circled region
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Magnetite (Fe3O4) is one of the most ancient and technologically important magnetic materials
Beyond its magnetic properties, magnetite is also the prototypical example of a metal-insulator transition called a Verwey transition (below TV = 122 K) which is argued to arise from the charge ordering of 2+ and 3+ iron valences. Since its discovery nearly 70 years ago, the driving forces behind the Verwey transition are still not completely understood. In the metallic phase, valence fluctuations modify the magnetic coupling between spins. The measurement of magnetic excitations with neutron scattering provides a window to observe the complicated electronic behaviour near the transition. Using the MAPS instrument, we observe anomalous magnetic excitations whose behaviour is modified above TV due to the occurrence additional ferromagnetic coupling, called double exchange. We find that the double exchange interaction affects only certain special modes, not all modes, indicating that valence fluctuations are constrained by intersite electronic correlations above TV. Our results support some of the early ideas about magnetite from PW Anderson and NF Mott.
RJ McQueeney, (Iowa State University / Ames Laboratory, USA), M Yethiraj (Bragg Institute, ANSTO, Australia), S Chang (Ames Laboratory, USA), W Montfrooij (Missouri University, USA), TG Perring (ISIS), J Honig, P Metcalf (Purdue University, USA)
Research date: December 2007
RJ McQueeney et al, Phys. Rev. Lett. 99 (2007) 246401.
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