Séminaire - Shedding a new light onto the puzzles of magnetite - Heba Elnaggar

Lundi 13 décembre à 10 h 30

Heba Elnaggar - Postdoc à l'IMPMC

IMPMC - Sorbonne Université - 4 place Jussieu - 75005 Parios - Tour 23, 4e étage, couloir 22-23, salle 401

Shedding a new light onto the puzzles of magnetite

Abstract

Magnetite (Fe₃O₄) is one of the most abundant Fe bearing minerals on Earth and it finds many applications in areas such as paleomagnetism, medicine, data recording, and engineering. It is believed to have governed the early Martian climate and favoured biologically relevant chemical reactions. Fe₃O₄ is known to man as the first compass and an intriguing half metallic oxide of interest to spintronics applications. It also undergoes a metal to insulator transition at T_V ∼125K which is referred to as the Verwey transition. Ever since the work of Verwey, Fe₃O₄ was considered as an archetype of strongly correlated oxide systems. This gave rise to considerable efforts devoted to understanding the origin of the Verwey transition. It has however remained a daunting task to determine the driving force of the transition due to its complexity: Fe₃O₄ is a mixed valence, strongly correlated system where many interactions such as Jahn-Teller (dynamical and static), spin-orbit and phonons are very close in energetics. In this talk, I will discuss a series of X-ray spectroscopic measurements that shed new light on some of the open questions. 

• I investigated whether Fe ions in Fe₃O₄ possess an orbital magnetic moment and how to quantify it given the fact that it can be either collinear or noncollinear to the spin magnetic moment. This work led me to discover that the orbital magnetic moment plays a significant role as a trigger for the Verwey transition. 

• I addressed whether the trimeron network (plotted in green in the right panel of Figure 1) is destroyed just above the Verwey transition, or do short range trimeron (or other type of short-range order) persist? 

• I studied the effect of temperature on the short-range structural correlations up to the Curie temperature (T_C= 840 K) which revealed that a new charge ordering takes place at high temperature.