Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D31: Focus Session: Spin-Dependent Phenomena in Semiconductors: Magnetic Interactions in Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Olaf van 't Erve, Naval Research Laboratory Room: 207A |
Monday, March 2, 2015 2:30PM - 3:06PM |
D31.00001: GMAG Dissertation Award Talk: Zero-moment Half-Metallic Ferrimagnetic Semiconductors Invited Speaker: Michelle E. Jamer Low- and zero-moment half-metallic ferrimagnetic semiconductors [1,2] have been proposed for advanced applications, such as nonvolatile RAM memory and quantum computing. These inverse-Heusler materials could be used to generate spin-polarized electron or hole currents without the associated harmful fringing magnetic fields. Such materials are expected to exhibit low to zero magnetic moment at room temperature, which makes them well-positioned for future spin-based devices. However, these compounds have been shown to suffer from disorder [3]. This work focuses on the synthesis of these compounds and the investigation of their structural, magnetic, and transport properties. Cr$_{2}$CoGa and Mn$_{3}$Al thin films were synthesized by molecular beam epitaxy, and V$_{3}$Al and Cr$_{2}$CoAl were synthesized via arc-melting. Rietveld analysis was used to determine the degree of ordering in the sublattices as a function of annealing. The atomic moments were measured by X-ray magnetic circular and linear dichroism confirmed antiferromagnetic alignment of sublattices and the desired near-zero moment in several compounds.\\[4pt] In collaboration with George E. Sterbinsky, Photon Sciences Directorate, Brookhaven National Laboratory; Dario Arena Photon Sciences Directorate, Brookhaven National Laboratory; Laura H. Lewis, Chemical Engineering, Northeastern University; and Don Heiman, Physics, Northeastern University. \\[4pt] [1] H. van Leuken, R.A. de Groot, Phys. Rev. Lett. 74, 1171 (1995).\\[0pt] [2] S. Skaftouros, K. \"{O}zdo\u{g}an, E. \c{S}a\c{s}\i o\u{g}lu and I. Galanakis, Phys. Rev. B 87, 024420 (2013).\\[0pt] [3] M.E. Jamer, B.A. Assaf, T. Devakul and D. Heiman, Appl. Phys. Lett. 103, 142403 (2013). [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D31.00002: A Designed Room Temperature Multilayered Magnetic Semiconductor Dinah Simone Bouma, Michalis Charilaou, Catherine Bordel, Ryan Duchin, Alexander Barriga, Adam Farmer, Frances Hellman A room temperature magnetic semiconductor has been designed and fabricated by using an epitaxial antiferromagnet (NiO) grown in the (111) orientation, which gives surface uncompensated magnetism for an odd number of planes, layered with the lightly doped semiconductor Al-doped ZnO (AZO). Magnetization and Hall effect measurements of multilayers of NiO and AZO are presented for varying thickness of each. The magnetic properties vary as a function of the number of Ni planes in each NiO layer; an odd number of Ni planes yields on each NiO layer an uncompensated moment which is RKKY-coupled to the moments on adjacent NiO layers via the carriers in the AZO. This RKKY coupling oscillates with the AZO layer thickness, and it disappears entirely in samples where the AZO is replaced with undoped ZnO. The anomalous Hall effect data indicate that the carriers in the AZO are spin-polarized according to the direction of the applied field at both low temperature and room temperature. NiO/AZO multilayers are therefore a promising candidate for spintronic applications demanding a room-temperature semiconductor. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D31.00003: Magnetic Exchange Interactions in Long Range Ordered Diluted Organometallic Semiconductors Naveen Rawat, Lane Manning, Madalina Furis Exchange Interactions in diluted organometallic crystalline thin films of Phthalocyanines made of a mixture of organo-soluble derivatives of metal-free (H$_2$Pc) molecule and MnPc is investigated. The tuning of optical and magnetic properties in organometallics is driven by their emergence in optoelectronic applications involving flexible electronics. Thin films with metal to metal-free Pc ratios ranging from 1: 1 to 1:10 were fabricated using solution processing that produces macroscopic grains. In the case of Mn-Pc, our previos measurements showed enhanced hybridization of ligand $\pi$-electronic states with the Mn d-orbitals as well as indirect exchange interaction similar to that of RKKY type exchange. The evolution of Zeeman splitting of specific MCD-active states resulted in enhanced effective $\pi$-electrons g-factors, analogous to diluted magnetic semiconductors (DMS) systems. Recent Variable temperature Magnetic Circular Dichroism (VTVH-MCD) measurements has now revealed that the exchange interaction is Antiferromagnetic. Recent MCD data for mixed derivatives will be presented along with their temperature dependance that further probes this exchange interaction. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D31.00004: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 3:42PM - 3:54PM |
D31.00005: Strong Spin-Exchange Interactions in Magnetically Doped Colloidal Nanocrystals William Rice, Wenyong Liu, Thomas Baker, Gen Chen, Jeffrey Pietryga, Victor Klimov, Scott Crooker Using magnetic circular dichroism, magneto-photoluminescence, and time-resolved Faraday rotation measurements, we study the spin-exchange interactions between excitons and embedded magnetic ions in Mn-doped colloidal nanocrystals. In contrast to undoped nanocrystals, the Mn-doped nanocrystals show giant Zeeman splittings of the nanocrystal conduction and valence bands and a very rapid dephasing of the optically created excitons (two orders of magnitude faster than undoped CdSe). Although the exciton spin coherence is short (less than 10 ps), the $sp-d$ spin exchange between the exciton and Mn moments induces a long-lived precession of the Mn$^{2+}$ paramagnetic moments that persists out to nanosecond timescales. We study this induced Mn precession as a function of nanocrystal size, Mn doping density, temperature, and magnetic field. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D31.00006: Driven magnetic patterns in quantum dots Alex Matos-Abiague, James Pientka, Jong E. Han, Igor Zutic We theoretically investigate the response of magnetic impurities in a quantum dot driven by a dc current. A bias voltage applied between the leads attached to the quantum dot drives the current. In addition, an external gate is used to tune the energy levels of the dot. The steady state magnetic configuration and current are self-consistently determined by using the non-equilibrium Green function formalism. The results reveal the emergence of different magnetization patterns in dependence on the bias and gate voltages for various sets of system parameters (number of magnetic impurities, tunneling coupling between dot and leads, and size of the dot). Under certain conditions and as the result of correlations between current and local magnetization, signatures of magnetic patterns formation can be observed in the I-V characteristics of the quantum dot. This allows for the design and characterization of magnetic patterns in quantum dots by electrical means. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D31.00007: Ground State Properties of Magnetic Quantum Dots with Multiple Occupancies James Pientka, Igor Zutic, Jong Han Semiconductor quantum dots doped with magnetic impurities provide an intriguing opportunity to explore the interplay of confinement, Coulomb and exchange interactions [1,2]. Using exact diagonalization we study the ground state properties of a magnetic quantum dot with multiple occupancies. We show that the ground state not only depends on the orientation of the carrier and impurity spins, but is also very sensitive to the position of the magnetic impurities in the quantum dot. Our results reveal magnetic frustration and strongly correlated states, qualitatively different from the Fermi liquid behavior. [1] J. M. Pientka R. Oszwaldowski, A. G. Petukhov, J. E. Han, and I. Zutic, Phys. Rev. B 86, 161403(R) (2012).[2] R. Oszwaldowski, P. Stano, A. G. Petukhov, and I. Zutic, Phys. Rev. B 86, 201408(R)(2012). [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D31.00008: Hole States and Magnetic Anisotropy of a Quantum Dot Dan Rederth, Hari Chapagain, Rafal Oszwaldowski, A.G. Petukhov In the era of spin-based advanced semiconductor materials [1], spin can be used for the control of quantum devices based on quantum dots (QDs) [2]. To facilitate the control of the electronic and magnetic properties, magnetic ions can be incorporated in the QDs. We study the properties of such a magnetic II-VI QD charged with one hole. To account for the complex structure of valence band, we propose a method based on the Luttinger-Kohn Hamiltonian. With a robust numerical algorithm suitable for any QD geometry, we study the interplay of quantum confinement and magnetic anisotropy of a flat QD. We go beyond the virtual crystal approximation [3]; our model also allows for position-dependent direction of magnetization. We discuss the differences between our and previous results, as well as the effects of temperature (mean-field approximation), and of the spin-orbit split-off band. We also discuss possible fluctuations of magnetization in QDs. Supported by DOE DE-SC00004890. [1] T. Dietl and H. Ohno, Rev. Mod. Phys. \textbf{86}, 187 (2013). [2] R. Hanson et al., Rev. Mod. Phys. \textbf{79}, 1217 (2007). [3] K. Vyborny, J. Han, R. Oszwaldowski, I. Zutic, and A. Petukhov, Phys. Rev. B \textbf{85}, 155312 (2012); F. V. Kyrychenko and J. Kossut, Phys. Rev. B \textbf{70}, 205317 (2004). [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D31.00009: Nanostructured lithium oxide-hematite magnetic oxide semiconductors Monica Sorescu, Vasilii Bushunow, Lucian Diamandescu, Felicia Tolea, Mihaela Valeanu, Tianhong Xu The study aims at exploring the formation of magnetic oxide semiconductors at the nanoscale, which is of crucial importance for catalysis, sensing and electrochemical applications. xLi2O-(1-x)alpha-Fe2O3(x $=$ 0.1, 0.3, 0.5, and 0.7) nanoparticle systems were successfully synthesized by mechanochemical activation of Li2O and alpha-Fe2O3 mixtures for 0-12 hours of ball milling time. X-ray powder diffraction (XRD), Mossbauer spectroscopy and magnetic measurements were used to study the phase evolution. Rietveld refinement of the XRD patterns yielded the values of the particle size as function of composition and milling times. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for hematite to sextets and a doublet upon duration of the milling process with lithium oxide. Magnetic measurements recorded at 5 K to room temperature (RT) in an applied magnetic field of 50,000 Oe showed that the magnetization of the milled samples is larger at low temperatures than at RT and increases with decreasing particle size. Zero field cooling measurements made possible the determination of the blocking temperatures of the specimens as function of ball milling time and evidenced the occurrence of superparamagnetism in the studied samples. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D31.00010: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 4:54PM - 5:06PM |
D31.00011: Onset of spin polarization degeneracy in quantum wires Alfredo Sanchez, Jean-Pierre Leburton We report on the emergence of complex spin-polarized regimes arising from electron-electron interactions in quantum wires by using a three-dimensional unrestricted Hartree-Fock approach. We predict three distinct spin-polarized configurations in the presence of a finite magnetic field, two of which appear only above a concentration threshold. In the limit of vanishing magnetic fields, the electron system evolves into a pair of symmetric and degenerate regimes, indicating spontaneous spin polarization. The value of the concentration threshold and the energy of the polarized states all depend on the transverse dimensions of the wire. In particular, spontaneous spin polarization is suppressed if the wire cross section is highly asymmetric. Our investigations on spin-related effects are of significant relevance for spintronics applications and for the study of the 0.7 conductance anomaly in semiconductor quantum point contacts. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D31.00012: $\pi - $d Electron Coupling in Excited State in Organic-Magnetic Nanocomposites Mingxing Li, Min Wang, Lei He, Yu-Che Hsiao, Qing Liu, Hengxing Xu, Long Y. Chiang, Loon-Seng Tan, Augustine Urbas, Bin Hu The coupling between $\pi $ electron in organic semiconducting materials and d electron in ferromagnetic materials presents an important mechanism, namely $\pi $-d electron coupling, to develop magneto-optical and magneto-electronic properties. The $\pi $-d electron coupling has been heavily studied in ground state. This presentation reports the $\pi $-d electron coupling in excited state by combining intramolecular charge-transfer dipoles in semiconducting $\pi $ electron system with spin dipoles from surface-modified magnetic nanoparticles based on organic-magnetic nanocomposites. Our magneto-dielectric studies show that the excited state has a much stronger $\pi $-d electron coupling, as compared to ground state, under photoexcitation. We further study the coupling mechanism by analyzing the line-shape of magneto-dielectric response. We find that increasing the Coulomb interactions between electrical dipoles and spin dipoles causes a line-shape narrowing. On contrast, increasing the spin interactions between them leads to a line-shape broadening. As a result, we conclude that the long-range Coulomb interactions and short-range spin interactions are responsible for realizing strong $\pi $-d electron coupling in excited state in organic-magnetic nanocomposites. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D31.00013: First-Principles study of tris(8-hydroxyquinoline)iron(III) molecules: A promising spin filter material Wei Jiang, Miao Zhou, Zheng Liu, Dali Sun, Z. Valy Vardeny, Feng Liu Using first-principles calculations, we have systematically investigated the structural, electronic, and magnetic properties of facial and meridional tris (8-hydroxyquinoline)iron(III) (Feq$_{3})$ molecules, solvent-free Feq$_{3}$ crystals and thin films. Our calculation results show that both Feq$_{3}$ isomers have a high spin state of 5 $\mu_{B}$ as the ground state when an on-site Hubbard-U term is included to treat the highly localized Fe 3$d$ electrons, in agreement with experiment, while the standard DFT calculations produce a low spin Fe state of 1 $\mu_{B}$. Furthermore, the freestanding Feq$_{3}$ films are found to be paramagnetic, but become Ferromagnetic (FM) within each layer when deposited on a NiFe substrate. This is induced by a strong anti-ferromagnetic (AFM) coupling between the first molecular layer and FM substrate. Also, an AFM coupling is found between the molecular layers. These findings suggest that Feq$_{3}$ molecular films may serve as a promising spin filter material in spintronic devices. [Preview Abstract] |
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