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
Professor Vincenzo Fiorentini will be hosted by the Chair for Nanotechnology in the winter semester 2024-25 as a senior Dresden Fellow. Vincenzo is an associate professor of condensed matter physics at Cagliari University, Italy, and also worked at Trieste University and the International Center for Theoretical Physics, the Fraunhofer Institute for Applied Solid Physics in Freiburg, the Fritz-Haber Institut of the MPG in Berlin, the Walter Schottky Institute in Munich (as a Humboldt scholar), and IMEC Leuven. In the 2020-2024 quadrennium, he was attaché of the Italian Embassy in Berlin. Vincenzo is married and has three children (28, 25, 17), currently lives in Berlin, and in his free time reads and plays jazz.
Vincenzo works in computational materials physics and related methods (~150 scientific articles, ~16500 citations, h-index=51). His main contributions are in the theory of, among others, III-V nitrides, and high-k, wide-gap, and ferroic oxides, plus a couple of important methodology developments. As a professor for just short of 30 years, Vincenzo taught classes on many topics in condensed matter and general physics, supervised about 30 Master's and a dozen Ph.D. theses, as well as of the order of a dozen post-docs. In keeping with his expertise, research plans for his stay concern magnetoelectricity and magnetoresistivity in hafnia and its possible applications to memristors.
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
Professor Vincenzo Fiorentini will be hosted by the Chair for Nanotechnology in the winter semester 2024-25 as a senior Dresden Fellow. Vincenzo is an associate professor of condensed matter physics at Cagliari University, Italy, and also worked at Trieste University and the International Center for Theoretical Physics, the Fraunhofer Institute for Applied Solid Physics in Freiburg, the Fritz-Haber Institut of the MPG in Berlin, the Walter Schottky Institute in Munich (as a Humboldt scholar), and IMEC Leuven. In the 2020-2024 quadrennium, he was attaché of the Italian Embassy in Berlin. Vincenzo is married and has three children (28, 25, 17), currently lives in Berlin, and in his free time reads and plays jazz.
Vincenzo works in computational materials physics and related methods (~150 scientific articles, ~16500 citations, h-index=51). His main contributions are in the theory of, among others, III-V nitrides, and high-k, wide-gap, and ferroic oxides, plus a couple of important methodology developments. As a professor for just short of 30 years, Vincenzo taught classes on many topics in condensed matter and general physics, supervised about 30 Master's and a dozen Ph.D. theses, as well as of the order of a dozen post-docs. In keeping with his expertise, research plans for his stay concern magnetoelectricity and magnetoresistivity in hafnia and its possible applications to memristors.
