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Key to the control of phase transitions?

16 April 2018

What are the mutual influences of electronic properties, crystallographic structures and magnetism? Moreover, how do they affect phase transitions? To shed light on these subjects, Dr. Carolin Schmitz-Antoniak from PGI-6, together with other research partners, has investigated surface modified magnetite nanoparticles using element-specific synchrotron radiation spectroscopy at the BESSY II synchrotron radiation source in Berlin. What they found could be the key to controlling certain phase transitions. Their work has been featured on the front cover of the March issue of “Annalen der Physik”, a journal renowned in the field of physics for more than 225 years.

Annalen-der-PhysikThe cover image represents one of the magnetite core nanoparticles with its amorphous shell. Red atoms at the surface correspond to ions with localized electrons trapped at interfacial defects, responsible for the suppression of the Verwey transition. Synchrotron radiation spectroscopy has been used to investigate the electronic structure and magnetism of the core-shell particles and is illustrated by the small, bright bluish light source in the background.
Copyright: WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, from: Ann. Phys. (Berlin) 2018, 530, 1700363

The phase transition monitored in this work is called the Verwey transition (named after the chemist E.J.W. Verwey). It is a low-temperature, metal-to-insulator phase transition connected to a change in crystal structure and magnetic anisotropy at slightly higher temperatures. “Thus, magnetite can be understood as a prototype system to investigate the impact of electronic, crystallographic and magnetic modifications on the phase transition”, explains Schmitz-Antoniak.
In nanoparticles with a diameter of 6nm, about 20% of all atoms are at the surface, giving the opportunity to directly tailor the properties of a considerable number of atoms by surface modifications. The authors capped the nanoparticles with amorphous silica and showed that this silica shell had a strong impact on the electronic structure and magnetism of the iron ions in the magnetite core while maintaining the crystal structure of bare magnetite nanoparticles.

Although the researchers had already expected the electrons to be influenced to some degree since it is well-known that silica can trap electrons at defects, they were surprised to find out that this trapping of electrons at the core-shell interface completely suppressed the phase transition in the whole magnetite core, i.e. a formation of the metallic high-temperature phase of magnetite was not observed. “This result clearly reveals the importance of the electronic structure for the understanding and the control of phase transitions”, emphasizes Schmitz-Antoniak. In addition, this work offers a possible explanation for contradictions in recent literature about the Verwey transition at the nanoscale by pointing out the importance of the surface.

Original publication:

Suppression of the Verwey Transition by Charge Trapping;
Carolin Schmitz-Antoniak, Detlef Schmitz, Anne Warland, Masih Darbandi, Soumyajyoti Haldar, Sumanta Bhandary, Biplab Sanyal, Olle Eriksson, Heiko Wende;

Ann. Phys. (Berlin) 2018, 530, 1700363;
DOI: 10.1002/andp.201700363