Final Report Summary - FUNDMS (Functionalisation of Diluted Magnetic Semiconductors)
Across human history and until today, technology advances have been fuelled by materials developments. Also in the current silicon-based technology, silicon and its oxides are increasingly replaced by newly developed compounds with superior performance. In a long run, particularly enthralling are multifunctional materials as well composite media, assembled in a bottom-up way with nanoscale precision.
Among the multifunctional materials especially attractive are ferromagnetic semiconductors which combine the resources of semiconductors (in the heart of chips, light sources, solar batteries, …) and ferromagnets (employed in hard discs, tapes, electric engines, …). Moreover, these systems show entirely new functionalities, as e.g. the manipulation of magnetism by the electric field. While various families of ferromagnetic semiconductors have already been studied, particular attention is being paid to diluted magnetic semiconductors (DMSs), consisting of technologically important semiconductors doped with magnetic transition metal (TM) impurities, for example (Ga,Fe)N or (Zn,Co)O. Indeed, GaN and its derivatives like (Al,Ga)N are strategic semiconductor materials, commonly employed in light emitting diodes and high power electronics, so that the incorporation of magnetism-related functionalities to these materials is particularly appealing. However, despite the apparent chemical simplicity of DMSs, the field of their studies has developed into one of the most controversial in condensed matter physics [see, T. Dietl, Nature Mater. 9, 965-974 (2010)].
The FunDMS project was built on the conviction, which has been substantiated by its execution, that DMSs invalidate the paradigm of semiconductor and magnetism physics about a uniform distribution of magnetic ions over the crystal volume in these material systems. We argued and then demonstrated [see, A. Bonanni and T. Dietl, Chem. Soc. Rev. 39, 528-539 (2010)] that any meaningful experimental and theoretical studies of DMS samples should be preceded by state-of-the-art element specific and spin sensitive nanocharacterization. Only knowing accurately how particular elements are positioned in the lattice, we may attempt to understand the macroscopic properties.
By implementing this methodology, particularly by exploiting synchrotron facilities for FunDMS objectives, we have been able to reveal, control, understand theoretically, and then learn how to exploit the nanoscale heterogeneity in the distribution of TM ions and/or carriers. One of the methods of controlling the aggregation of TM ions was patented prior to publication [A. Navarro-Quezada et al., Appl. Phys. Lett. 101, 081911 (2012)]. Similarly, the demonstration of strong room temperature luminescence, a precursor of the laser action, activated by impurity aggregation was patented and then published [T. Devillers et al., Sci. Rep. 2, 722 (2012)].
Furthermore, we have experimentally examined and theoretically described many aspects of the mechanisms accounting for ferromagnetism in DMSs with a uniform Mn distribution. In the case of (Ga,Mn)N films we discovered the presence of ferromagnetic superexchange. At the same time, we have put forward new and powerful arguments demonstrating that the strong spin-spin interaction specific to (Ga,Mn)As-type DMSs is mediated by holes in the valence band. One challenging question we have addressed concerns the influence of quantum hole localization upon hole-mediated ferromagnetism. We have shown that critical fluctuations in the carrier density specific to Anderson-Mott localization leads to the coexistence ferromagnetic and superparamagnetic regions, the latter taking gradually over on decreasing the hole concentration.
The results of our work within the FunDMS project have been described in 74 publications, presented during 99 invited talks, and broadly included in the review: T. Dietl, H. Ohno, Rev. Mod. Phys. (April 2014), arXiv:1307.3429 another review being under preparation.
Among the multifunctional materials especially attractive are ferromagnetic semiconductors which combine the resources of semiconductors (in the heart of chips, light sources, solar batteries, …) and ferromagnets (employed in hard discs, tapes, electric engines, …). Moreover, these systems show entirely new functionalities, as e.g. the manipulation of magnetism by the electric field. While various families of ferromagnetic semiconductors have already been studied, particular attention is being paid to diluted magnetic semiconductors (DMSs), consisting of technologically important semiconductors doped with magnetic transition metal (TM) impurities, for example (Ga,Fe)N or (Zn,Co)O. Indeed, GaN and its derivatives like (Al,Ga)N are strategic semiconductor materials, commonly employed in light emitting diodes and high power electronics, so that the incorporation of magnetism-related functionalities to these materials is particularly appealing. However, despite the apparent chemical simplicity of DMSs, the field of their studies has developed into one of the most controversial in condensed matter physics [see, T. Dietl, Nature Mater. 9, 965-974 (2010)].
The FunDMS project was built on the conviction, which has been substantiated by its execution, that DMSs invalidate the paradigm of semiconductor and magnetism physics about a uniform distribution of magnetic ions over the crystal volume in these material systems. We argued and then demonstrated [see, A. Bonanni and T. Dietl, Chem. Soc. Rev. 39, 528-539 (2010)] that any meaningful experimental and theoretical studies of DMS samples should be preceded by state-of-the-art element specific and spin sensitive nanocharacterization. Only knowing accurately how particular elements are positioned in the lattice, we may attempt to understand the macroscopic properties.
By implementing this methodology, particularly by exploiting synchrotron facilities for FunDMS objectives, we have been able to reveal, control, understand theoretically, and then learn how to exploit the nanoscale heterogeneity in the distribution of TM ions and/or carriers. One of the methods of controlling the aggregation of TM ions was patented prior to publication [A. Navarro-Quezada et al., Appl. Phys. Lett. 101, 081911 (2012)]. Similarly, the demonstration of strong room temperature luminescence, a precursor of the laser action, activated by impurity aggregation was patented and then published [T. Devillers et al., Sci. Rep. 2, 722 (2012)].
Furthermore, we have experimentally examined and theoretically described many aspects of the mechanisms accounting for ferromagnetism in DMSs with a uniform Mn distribution. In the case of (Ga,Mn)N films we discovered the presence of ferromagnetic superexchange. At the same time, we have put forward new and powerful arguments demonstrating that the strong spin-spin interaction specific to (Ga,Mn)As-type DMSs is mediated by holes in the valence band. One challenging question we have addressed concerns the influence of quantum hole localization upon hole-mediated ferromagnetism. We have shown that critical fluctuations in the carrier density specific to Anderson-Mott localization leads to the coexistence ferromagnetic and superparamagnetic regions, the latter taking gradually over on decreasing the hole concentration.
The results of our work within the FunDMS project have been described in 74 publications, presented during 99 invited talks, and broadly included in the review: T. Dietl, H. Ohno, Rev. Mod. Phys. (April 2014), arXiv:1307.3429 another review being under preparation.