Shedding a new “light” on the puzzles of iron oxides

Hebatalla Elnaggar
Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC)

The role of electron correlations in solid-state materials is captured by ‘The whole is greater than the sum of its parts’ – Aristotle. Correlated materials cannot be described by a sum of non-interacting electrons; on the contrary, the interaction of the charge and spin of the electrons with each of the surrounding electrons is decisive for the electronic and magnetic behavior. These interactions give rise to intriguing phenomena ranging from magnetic order to Mott insulation and superconductivity.

A manifestation of the intricate interplay of the various degrees of freedom can be seen when Fe₃O₄ is cooled below TV = 125 K leading to a metal to insulator transition. This transition is accompanied by a long-range charge and orbital ordering in addition to a structural change^1,2. Despite decades of research since the discovery of the transition in 1939, its mechanism remained elusive. In the first part of my talk, I will show how novel resonant inelastic x-ray scattering (RIXS) magnetic dichroism experiments and theoretical computation³ reveal the existence of noncollinear orbital magnetic ordering in the high temperature phase of Fe₃O₄^4,5. This noncollinear orbital ordering is coupled to the X3 phonon mode creating a polaronic precursor for the metal to insulator transition⁴. Furthermore, I will discuss a new charge reordering that occurs in the metallic-like phase of Fe₃O₄ and acts as a descriptor of correlations⁶.

Finally, I will discuss the capability of RIXS to measure original higher-order magnetic excitations^7,8. Conventional wisdom suggests that one photon that carries one unit of angular momentum (1ℏ) can change the spin angular momentum of a single magnetic site with one unit (ΔM_S=±1ℏ) at most following the selection rules in the case of strong 2p spin-orbit coupling. This implies that a two-photon process such as 2p3d RIXS can change the spin angular momentum of the system with a maximum of two units (ΔM_S=±2ℏ). I will address the fundamental question:

Can we manipulate more than two spins in magnetic systems using a two-photon process as RIXS?

This brings me to my future research plans and developments with a focus on higher-order magnons and the new scientific opportunities that well controlled thin-films such as perovskites will enable. This includes studying multi-magnon dispersion, dimensionality-dependence and dynamics using pump-probe 2p3d RIXS. Such information can be used to design new materials where the higher-order magnons are enhanced, opening the realms of possibilities for using these novel quasiparticles.

1. E. Verwey, Nature, 144(3642):327–328, 1939.
2. Senn et. al., Nature, 481, 173 (2012).
3. Elnaggar et. al., book chapter in “Magnetism and Accelerator-Based Light Sources” ISBN 978-3-030-64623-3 (2021)
4. Elnaggar et. al., Phys. Rev. Lett., 123, 207201, (2019). — Editors’ suggestion, Phys.org, Spotlight on Science articles @ESRF
5. Elnaggar et. al., Phys. Rev. B, 101, 085107 (2020)
6. Elnaggar et. al., Phys. Rev. Lett., 127, 186402, (2021).
7. A. Nag, et. al., Phys.  Rev.  Lett. 124, 067202 (2020).
8. H. Elnaggar, et. al., Nat. Comm. 14, 2749 (2023).