Hebatalla Elnaggar

---

The role of electronic correlations in materials can be captured by the idea that "the whole is
greater than the sum of its parts". Correlated materials defy description by a sum of non-
interacting electrons; instead, the interactions both among the electrons and with their
surrounding environment critically determine their electronic and magnetic behaviour. These
interactions give rise to intriguing phenomena such as magnetic order, Mott (metal-insulator)
transitions, and superconductivity. In this talk, I will present two striking examples of this intricate
interplay in iron oxides.
In the first part of my talk, I will focus on Fe3O4 which undergoes a metal-to-insulator transition
when it is cooled below 125 K[1]. This transition is accompanied by complex long-range charge
and orbital ordering, as well as a structural distortion[2]. Despite extensive research since the
discovery of this transition in 1939, its mechanism has remained elusive. Here, I will present
Resonant Inelastic X-ray Scattering (RIXS) magnetic dichroism experiments and theoretical
computations that reveal the existence of short-range noncollinear orbital magnetic ordering in
the high-temperature phase of Fe3O4[3,4]. Furthermore, I will discuss a novel charge reordering
occurring in this phase, which serves as a descriptor of electronic correlations[5]. These results
provide a new perspective on the metal to insulator transition of Fe3O.
In the second part, I will explore the capability of RIXS to measure higher-order THz magnetic
excitations, known as multi-magnons. Conventional wisdom suggests that a photon carrying one
unit of angular momentum can change the spin angular momentum of a magnetic system by
one unit. This implies that a two-photon scattering process such as RIXS can change
the spin angular momentum of the magnetic system by two units. Here, I will
demonstrate how, despite this expectation, RIXS can excite multi-magnons in Fe2O4 with
changes in the spin angular momentum beyond ### due to spin non-conserving
interactions[6].
Finally, I will briefly present my future research plans, which aim to understand the role of spin
non- conserving interactions in enhancing the efficiency of exciting THz multi-magnons using
light. I will conclude by outlining my long-term vision to translate the understanding of THz multi-
magnons, gained through x-ray spectroscopy, into novel functionalities for future ultrafast spin-
based technology.
References:
[1] E. Verwey, Nature 144, 327 (1939).
[2] M. Senn et al., Nature 481, 173 (2012).
[3] H. Elnaggar et al., ACS Appl. Mater. Interfaces 11, 36213 (2019).
[4] H. Elnaggar et al., Phys. Rev. Lett. 123, 207201 (2019).
[5] H. Elnaggar et al., Phys. Rev. Lett. 127, 186402 (2021).
[6] H. Elnaggar et al., Nat. Commun. 14, 2749 (2023).

---

IMPMC, Sorbonne Université - CNRS
Seminar Room of the Institute of Physics II
Contact: Erwann/Matteo