Symposium : N
Inhomogeneous and hybrid magnetic semiconductor systems
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| 12:20 | Lunch break | |
| 13:50 | Opening | |
| 14:00 | Properties and functionalities of MnAs-nanoparticle / III-V-semiconductor hybrid structures Authors : M. Tanaka, P. N. Hai, and S. Ohya Department of Electrical Engineering and Information Systems, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Resume : We review recent our studies on the fabrication, structure, properties, and functionalities of MnAs / III-V semiconductor hybrid structures. By annealing (GaMn)As alloy at 480 – 700oC, MnAs nanoscale particles are precipitated in a GaAs matrix, forming GaAs:MnAs granular structure. This composite materials are shown to be compatible with III-V semiconductor heterostructures [1]. In properly designed III-V heterostructures containing GaAs:MnAs, we observe a variety of strong spin-dependent phenomena, such as large magnetooptical effects [2], spin dependent tunneling [3], huge magnetoresistance [4][5], electromotive force [6], and long spin-relaxation time [7].
References
[1] M. Tanaka, M. Yokoyama, P-N. Hai, and S. Ohya, \"Properties and functionalities of MnAs/III-V hybrid and composite structures\", in Spintronics, Semiconductors and Semimetals, Vol. 82, edited by T. Dietl, D. D. Awschalom, M. Kaminska, and H. Ohno (Academic Press, New York, November 2008).
[2] M. Tanaka, J. Cryst. Growth 227/228, 839 (2001); M. Yokoyama, H. Yamaguchi, T. Ogawa, and M. Tanaka, J. Appl. Phys. 97, 10D317 (2005).
[3] P. N. Hai et al, Appl. Phys. Lett. 89, 242106 (2006); P. N. Hai et al, Phys. Rev. B77, 214435 (2008).
[4] M. Yokoyama, T. Ogawa, A.M. Nazmul, and M. Tanaka, J. Appl. Phys. 99, 08D502 (2006).
[5] P. N. Hai, S. Ohya, and M. Tanaka, S. E. Barnes, S. Maekawa, Nature 458, 489 (2009).
[6] P. N. Hai, S. Ohya, and M. Tanaka, Nature Nanotechnology, in press (2010). | 1 1 |
| 14:30 | Spinodal decomposition and magnetic properties of dilute magnetic semiconductors Authors : Kazunori Sato, Graduate School of Engineering Science, Osaka Univerisity Resume : Recently, spinodal decomposition phenomena attract much attention in the fabrication of dilute magnetic semioconcutors (DMS), since many experimental results indicate that the magnetic properties of DMS are strongly affected by the occurrence of spinodal decomposition [1]. In this talk, I will propose computational materials design of DMS by tuning the spinodal decomposition.
The electronic structure of DMS system is calculated by using the Korringa-Kohn-Rostoker coherent potential approximation method. Calculated free energy indicates that the DMS has a solubility gap in its phase diagram. By using the Monte Carlo method, I will show that the DMS systems become super-paramagnetic due to the phase decomposition and the super-paramagnetic blocking temperature can be controlled by tuning the spinodal decomposition [2].
Next, I will propose ‘co-doping’ method to suppress spinodal decomposition in DMS. From the total energy calculations, it is shown that the solubility of magnetic impurities is strongly enhanced under the existence of interstitial donors [2]. Based on the Monte Carlo simulations, we will propose low temperature annealing method to remove interstitial co-dopants for recovering the ferromagnetism after the crystal growth.
This work is based on the collaboration with H. Fujii, L. Bergqvist, P. H. Dederichs and H. Katayama-Yoshida.
[1] A. Bonanni, Semicond. Sci. Technol. 22 (2007) R41. [2] K. Sato et al., Rev. Mod. Phys. Phys. in printing. | 1 2 |
| 15:00 | Electron microscopy of nano-inclusion formation in (Ga,Mn)As magnetic semiconductors Authors : A. Kovács1, T.Kasama1, J. Sadowski2,3, J. Domagala3, R. Mathieu4, R. E. Dunin-Borkowski1 and T. Dietl3,5 1 Center for Electron Nanoscopy, Technical University of Denmark, Kgs. Lyngby 2800, Denmark 2 MAX-Lab, Lund University, P.O. Box 118, 221 00 Lund, Sweden 3 Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warszawa, Poland 4 Department of Engineering Sciences, Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden 5 Institute of Theoretical Physics, University of Warsaw, PL-00-681 Warszawa, Poland Resume : The origin of ferromagnetism in transition metal (TM) doped semiconductors is still poorly understood. Recent studies [1] have underlined the need to study the incorporation of magnetic ions into the host lattice at the nanoscale, in order to establish whether the material is truly a diluted magnetic semiconductor, or whether nanocrystals that contain secondary magnetic phases are present.
Here, we study the transition from a diluted (Ga,Mn)As magnetic semiconductor (0.5 at.% Mn) to a phase-segregated microstructure upon annealing to 630°C. We use a range of TEM techniques to study the local structural, chemical and magnetic properties of epitaxial (Ga,Mn)As layers grown on GaAs by molecular beam epitaxy. Aberration corrected TEM images and nano-beam diffraction patterns from individual precipitates are used to show that both cubic and hexagonal crystals form with sizes of 8 to 16 nm. STEM images and EELS are used to show that local density fluctuations are present adjacent to the Mn(Ga)As nanoparticles. We propose that these regions are voids.
[1] A. Bonanni and T. Dietl, Chem. Soc. Rev. 39, 528 (2010) | 1 3 |
| 15:15 | Clustering of magnetic ions in GaN Authors : N. Gonzalez Szwacki(1), J. A. Majewski(1), and T. Dietl(1,2) (1) Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, ul. Hoża 69, 00-681 Warsaw, Poland (2) Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, PL- 02-668 Warszawa, Poland Resume : We present results of ab initio studies concerning clustering of magnetic Cr, Mn, and Fe ions in GaN. These results address hot topic of ferromagnetism in inhomogeneous diluted magnetic semiconductors and provide new insight in the physical mechanisms governing the formation of anti-ferromagnetic and ferromagnetic clusters of magnetic ions.
We have performed systematic theoretical investigations, in the framework of density functional theory, on the formation and magnetic ordering of small (up to four magnetic ions) magnetic clusters in zinc blende and wurtzite GaN, and for comparison also of free standing magnetic clusters. We employ GGA and LDA+U approximations for the exchange-correlation functional (also spin-polarized) and study the energetics and magnetic order of the systems. The investigation of total energies clearly shows that Cr, Mn, or Fe ions prefer to be close to each other, occupying neighboring Ga sites. It turns out that the formation of neighboring metal pairs is energetically less favorable in the case of doping with Cr atoms than in the case of Mn or Fe doping. The ferromagnetic ordering is favorable in the case of Cr and Mn doping, whereas the anti-ferromagnetic ordering is favorable in the case of Fe doping. For Cr and Mn, we have also obtained that the preference for the FM ordering tends to be weaker for larger clusters. Those and other results will be compared and discussed in the light of recent experiments and theoretical predictions. | 1 4 |
| 15:30 | Coffee break | |
| 15:50 | Ge:Mn nanostructures Authors : M. Jamet1, A. Barski1, I.-S. Yu1, T. Devillers1, A. Jain1, E. Prestat1, P. Bayle-Guillemaud1, C. Beigné1, V. Baltz1, E. Arras1, P. Pochet1, F. Lançon1, S. Tardif2, S. Cherifi2, V. Favre-Nicolin1, F. d’Acapito3, J. Cibert2 1 INAC, CEA-UJF, 17 rue des Martyrs, 38054 Grenoble, France 2 Institut Néel, CNRS-UJF, 25 rue des Martyrs, 38042 Grenoble, France 3 CNR-INFM-OGG GILDA CRG, ESRF, 6 rue Jules Horowitz, 38043 Grenoble, France Resume : The field of ferromagnetic semiconductors evolves very fast nowadays for their potential use in spintronic devices. Up to now, efforts have mainly focused on Diluted Magnetic Semiconductors but Curie temperatures in these materials still remain modest. One possible route to increase at least locally transition temperatures is to use spinodal decomposition leading to transition metal-rich high TC nanostructures. We focus here on (Ge,Mn) considered as a model system for spinodal decomposition and compatible with Si-based microelectronics. The growth of Ge1-xMnx (0.01 | 2 1 |
| 16:20 | GaAs-MnAs nanowires Authors : J. Sadowski - MAX-Lab, Lund University (Sweden) and Institute of Physics PAS, Warsaw (Poland) Resume : Nanowires combining the ferromagnetic semiconductors and/or ferromagnetic metals with semiconducting nanostructures can bring interesting applications for example in nonvolatile memory devices with magnetic information storage. The possibility of obtaining such nanoscale structures by self assembled growth methods has some advantage in comparison to the methods involving nanolithography techniques, typically used in the semiconductor technology. We have combined the self assembled GaAs NWs grown by molecular beam epitaxy on Si substrates, with ferromagnetic materials such as GaMnAs and MnAs. The use of Si substrates for the MBE growth of GaAs, at proper conditions leads to the growth of NWs catalyzed by Ga droplets spontaneously formed at SiO2/Si surface. The Ga droplets are quite uniform and have diameters in the range of 150 – 200 nm, which also determines the diameter of GaAs NWs. The GaAs nanowires are then used as the templates for deposition of GaMnAs and/or MnAs with use of the same MBE system, additionally equipped with the Mn source. We will show examples of core shell structures consisting of high-temperature MBE grown GaAs cores and ferromagnetic shells consisting of low temperature grown GaMnAs and MnAs. The structural, transport and magnetic properties of this kind of nanostructures will be discussed.
This work was partially supported by the Swedish Research Council (VR) and by the EC Network SemiSpinNet (PITN-GA-2008-215368). | 2 2 |
| 16:50 | Ferromagnetism in Mn-doped III - V nanowires Authors : M. Galicka, M. Bukała, R. Buczko, P. Kacman Institute of Physics PAS, al. Lotnikow 32/46, 02-668 Warsaw, Poland Resume : Mn-doped III-V semiconductor nanowires (NWs) attract lot of interest as candidates for one-dimensional ferromagnetic structures, thus for the prospects of integrating two important research domains, i.e., the semiconductor spintronics with self-assembled semiconductor nanostructures. Using ab initio methods based on the density functional theory we study the stability of the Mn-doped III-V NWs. Since it is possible to grow III-V NWs of two different crystal structures, we check whether the crystal structure of the wire has an impact on the magnetic and electronic properties of Mn doped GaAs and InAs NWs. Only very thin NWs (with diameters up to 5nm) were studied. Our calculations were performed for zero pressure and zero temperature. We have also assumed that the surface atoms are not saturated by foreign, e.g. hydrogen atoms.
The calculations show that the distribution of Mn ions as well as their mutual spin alignment depends crucially on the crystallographic structure of the wire. Moreover, the strength of the magnetic coupling depends on the diameter of the wire. The ferromagnetism was obtained in wires of wurtzite structure.
The research leading to these results has received funding from the European Community\'s Seventh Framework Programme ([FP7/2007-2013] under grant agreement n°215368. | 2 3 |
| 17:05 | Dynamics of PbTe quantum dot formation in CdTe studied in situ by TEM Authors : H. Groiss1, F. Schäffler1, G. Hesser2, N. Zakharov3, P. Werner3, K. Koike4, M. Yano4 1Institute of Semiconductor and Solid State Physics, and 2ZONA, both Johannes Kepler University, Linz, Austria 3Max Planck Institut für Mikrostrukturphysik, Halle, Germany 4Osaka Institute of Technology, Osaka, Japan Resume : Recently, we have demonstrated the synthesis of coherent quantum dots (QD) based on phase separation between the immiscible components of an epitaxially grown PbTe/CdTe heterostructure. Upon thermal annealing at 220 to 360°C, two-dimensional PbTe epilayers disintegrate into highly symmetric QD. By employing a heatable TEM sample holder, we could monitor in situ the complete transformation process. {002} dark field imaging exploits the strong diffraction contrast between the rock salt lattice of PbTe and the zinc blende lattice of CdTe. Time resolved measurements reveal the kinetics of the disintegration process, which starts with a capillary instability at the two interfaces followed by lateral dendrite formation that finally break up into coherently embedded PbTe QD. Time spans for the complete transformation range from minutes to hours in the studied temperature range. We can distinguish processes governed by fast interface diffusion along the PbTe/CdTe interface and slower bulk interdiffusion. Interface diffusion dominates, but at higher temperatures also Ostwalt ripening via bulk diffusion was observed, in which QD below a critical radius dissolve.
Work supported by FWF via project SFB 025. | 2 4 |
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