Multiferroicity and multiple valence effects in V-doped HfO2

Datum

23.10.2025

Zeit

13:00 - 15:00

Sprecher

Vincenzo Fiorentini

Zugehörigkeit

Chair of Materials Science and Nanotechnology

Serie

TUD nanoSeminar

Sprache

en

Hauptthema

Physik

Andere Themen

Physik

Host

Arezoo Dianat

Beschreibung

Multiferroics (materials enjoying multiple states of order) are rare. Ferroelectric ferromagnets, in particular, are even more rare. We explore [1] the unconventional route of doping the ferroelectric HfO2 with vanadium, which can also be viewed as a solid solution with ferromagnetic VO2.
 
 In a nutshell, polarization is conserved and magnetization increases linearly over the whole range considered (0 to 37% VO2). The solution should be stable up to about 15% VO2 for growth or annealing at 1000 K. The electronic structure evolves from large-gap HfO2 towards that VO2 with the appearance, with progressively increasing weight, of V occupied states at the upper edge of the valence band, as well as higher-lying V empty gap states. The results are in good agreement with very recent experiments.
 
 The fact that V has multiple valence, and XAS experiments, suggest that oxygen deficiency may play a role in this system [2]. We therefore study V-oxygen vacancy (X) complexes VnX and VnX2, finding that V enables a sizable concentration of vacancies, which are otherwise quite rare. Large clusters form, the largest concentration being that of V8X and V8X2, and declining for larger V content.
 
 Multiple valence in this context materializes in the presence of additional low-lying V gap states that can accept electrons from vacancies (which are double donors), so that a V atom can capture an electron acquiring a local magnetization of 2 instead of 1, and effectively becoming “3+” rather than “4+” (with rare instances of unmagnetized “5+” cases in large clusters).
 
 The clusters have indeed large magnetizations, increased by electrons provided by the vacancies, e.g. up to 13 magnetons for V9X2. Core levels shift according to the nominal valence, as seen in XAS. A thus far unresolved issue is that the observed 3+ to 4+ XAS signal is about 2, whereas it is calculated to be 1/2.
 
 [1] V. Fiorentini, P. Alippi, and G. Cuniberti, Phys. Rev. Materials, in print
 [2] to be published

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Letztmalig verändert: 19.10.2025, 07:35:07