PHOTON SCIENCE

An international team at DESY and the synchrotron radiation source SOLEIL in France has uncovered the special way in which iron diffuses in the near-surface region of magnetite. By using specially designed thin-films of this iron-oxide, containing the isotope ⁵⁷Fe, hopping of iron atoms through the crystal lattice was studied by nuclear forward scattering carried out at the PETRA III beamline P01. Surprisingly, the results show that most of the iron hops only through octahedral sites in the crystal lattice. Despite the uncovered low energy barrier, the diffusion process is very slow. The outcome of this study provides new insights addressing the stability of magnetite when used in various applications such as magnetic nanoparticles.
 
As iron-containing minerals are abundant in the Earth’s crust, iron oxides have permeated many different aspects of the world around us. Magnetite (Fe₃O₄) is a common iron ore and the oldest magnetic material known to mankind. Its magnetic properties have probably been used in compass-like instruments since the Middle Ages. At present, magnetite nanoparticles have emerged as a very promising material in medicine, either for drug delivery, imaging or cancer therapy by hyperthermia. Each of these applications makes use of magnetite’s magnetic properties: External magnetic fields can be used for steering drug-containing nano-vehicles for contrast-enhancements in magnetic resonance imaging and to destroy cancerous tumours by heating through the Joule effect.

Modern nano-fabrications techniques used to synthesise iron oxides are faced with the problem of their relative stability. It is difficult to control the formation of single-phase magnetite (Fe₃O₄) versus maghemite (γ-Fe₂O₃) and hematite (α-Fe₂O₃), as the structure and oxidation states of iron atoms of these iron oxides are slightly different. In fact, the barriers for transformation of one phase into the other are so low, that even at room-temperature over time unwanted phase transformations can take place. These can have dramatic consequences for the magnetic properties in a particular application. Even less is known about how the phase stability is affected by the surrounding atmosphere which, depending on the application, can be airy or even watery. The fundamental process for these phase transformations is the diffusion of iron within the solid-state material. To study near-surface cation diffusion, an isotopically labelled thin-film was prepared on a magnetite single crystal. The structure of similar thin-films was studied at SOLEIL beamline SixS to be able to correlate structural defects and cation diffusion. Among the different oxides, iron is directly surrounded by either four or eight oxygen atoms which are called tetrahedral and octahedral co-ordination, respectively. Being both, isotope and site selective, nuclear forward scattering at PETRA III beamline P01 showed that not only the temperature-induced diffusion is very slow, but also that it predominantly takes place in the slightly bigger octahedral sites.

Until now, iron diffusion in the different oxides was studied mostly in the bulk of the materials at elevated temperatures. Now, for the first time, the diffusion mechanism has been elucidated in the near-surface region at relatively low temperatures. These aspects are very important for our understanding of the stability of magnetite when used as nano-particles for various magnetic applications when size plays a role.


Reference:
Steffen Tober, Jan-Christian Schober, Marcus Creutzburg, Esko Erik Beck, Guilherme Dalla Lana Semione, Simon Chung, Leon Jacobse, Björn Arndt, Alina Vlad, René Steinbrügge, Hans-Christian Wille, Ilya Sergueev, Heshmat Noei, Kai Schlage, Olaf Leupold, Vedran Vonk, and Andreas Stierle, Site-resolved near-surface cation diffusion in magnetite, Phys. Rev. Lett. (2025), DOI: 10.1103/PhysRevLett.134.236203