Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Magnetic Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Jairo Sinova, Texas Agricultural and Mechanical University Room: 320 |
Thursday, March 21, 2013 8:00AM - 8:36AM |
T18.00001: Spin transistor action via tunable Landau-Zener transitions in magnetic semiconductor quantum wells Invited Speaker: Dieter Weiss Spin-transistors, employing spin-orbit interaction like Datta-Das prototypes [1], principally suffer from low signal levels due to limitations in spin injection efficiency, fast spin relaxation and dephasing processes. Here we present an alternative concept to implement spin transistor action where efficiency is improved by keeping spin transport adiabatic [2]. To this end a helical stray field B, generated by ferromagnetic Dysprosium stripes, is superimposed upon a two-dimensional electron system in (Cd,Mn)Te, containing Mn ions with spin 5/2. Due to the giant spin splitting, occurring at low temperatures and small B in (Cd,Mn)Te quantum wells, the B-helix translates into a spin-helix and the electron spins follow adiabatically the imposed spin texture. Within this approach the transmission of spin-polarized electrons between two contacts is regulated by changing the degree of adiabaticity, i.e. an electron's ability to follow the spin helix. This is done by means of a small applied homogeneous magnetic field while the degree of adiabaticity is monitored by the channel resistance. Our scheme allows spin information to propagate efficiently over typical device distances and provides an alternative route to realize spintronics applications. We note that our concept is not restricted to a particular choice of materials, temperature, methods of spin injection, manipulation as well as detection. \\[4pt] Work done in cooperation with Christian Betthausen, Institute of Experimental and Applied Physics, University of Regensburg, D-93040 Regensburg, Germany; Tobias Dollinger, Henri Saarikosi, Institute of Theoretical Physics, University of Regensburg, D-93040 Regensburg, Germany; Valeri Kolkovsky, Grzegorz Karczewski, Tomasz Wojtowicz, Institute of Physics, Polish Academy of Sciences, PL-02668 Warsaw, Poland; and Klaus Richter, Institute of Theoretical Physics, University of Regensburg. \\[4pt] [1] H. C. Koo et al., Control of spin precession in a spin-injected field effect transistor. Science 325, 1515 (2009). \\[0pt] [2] C. Betthausen et al., Spin-Transistor Action via Tunable Landau-Zener Transitions. Science 337, 324 (2012). [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T18.00002: Disorder in Mn doped InSb studied at the atomic scale by cross-sectional STM Paul Koenraad, Samuel Mauger, Juanita Bocquel, Caitlin Feeser, Nidhi Parashar, Bruce Wessels We present an atomically resolved study of MOVPE grown Mn doped InSb. Both topographic and spectroscopic measurements have been performed by X-STM. The measurements show a perfect crystal structure and reveal that Mn acts as a shallow acceptor. The Mn concentration obtained from the cross-sectional STM data compares well with the intended doping concentration. While the pair correlation function of the Mn atoms showed that their local distribution is uncorrelated beyond the STM resolution for observing individual dopants, disorder in the Mn ion location is noted. This inhomogeneous distribution is proposed to play an important role in the magnetic behavior. [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T18.00003: Giant magnetoresistance in InMnAs/InAs heterojunctions and its composition and temperature dependence John Peters, Christopher Garcia, Bruce Wessels The transport properties of magnetic semiconductors play a central role in spintronics as they provide an effective insight into spin related phenomena. Motivated by predictions of large magnetoresistance effects in dilute magnetic semiconductor heterojunctions, the electronic and magnetotransport properties of narrow gap heterojunction diodes have been demonstrated. We report here on the positive magnetoresistance of $p-$In$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As/$n-$InAs magnetic semiconductor heterojunctions and its dependence on Mn concentration and temperature. The junction magneto-conductance is well described by an analytical expression for the total conductance $G_{tot}$ of two spin-split bands. From the junction magneto-conductance an effective g-factor due to a giant Zeeman effect was determined for varying Mn concentration. The effective g-factor increased with increasing Mn concentration from 98 to 131 for x$_{\mathrm{Mn}}=$0.01 to x$_{\mathrm{Mn}}=$0.06. /newline /newline Use of the Center for Nanoscale Materials at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T18.00004: Magnetism of IV-VI compound based DMS Hitoshi Fujii, Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida The electronic structure and the magnetic properties of Mn doped GeTe, which is IV-VI compound semiconductor based dilute magnetic semiconductors (DMS), are calculated from first principles. Although the ferromagnetism was discovered in GeMnTe before III-V compound DMS systems [1], IV-VI DMS have not been so popular in DMS community due to the low Curie temperature and the incompatibility with present electronic materials. However, the carrier concentration and hence the magnetic properties can be controlled easily by forming Ge vacancies. In this work, in order to discuss potentiality of IV-VI DMS systems as semiconductor spintronics materials, the electronic structure are calculated based on the local density approximation and we use the Korringa-Kohn-Rostoker coherent potential approximation method [2]. The magnetic exchange interactions between Mn impurities are calculated by using the Lichitenstein's method [3]. Based on the calculation results, we will also discuss the Curie temperature by using Monte Carlo simulations.\\[4pt] [1] R. Cochrane, M. Rlishke, J. Toin-Olsen, Phys. Rev. B 9, 3013 (1974).\\[0pt] [2] MACHIKANEYAMA2002 developed by Akai, http://kkr.sci.osaka-u.ac.jp\\[0pt] [3] A. I. Liechtenstein, M. I. Katsnelson, V. P. Antropov, and V. A. Gubanov: J. Magn. Magn. Mater. 67 (1987) 65. [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:48AM |
T18.00005: All-optical ultrafast control of the four-state memory of ferromagnetic semiconductors by using coherent trains of femtosecond optical pulses Invited Speaker: Ilias Perakis We present a many-body theoretical framework based on density matrix equations of motion for investigating ultrafast all-optical manipulation of ferromagnetism in magnetic semiconductors. We develop a theory of collective spin dynamics triggered by femtosecond photoexcitation and demonstrate non-thermal control of magnetization switchings between the four metastable magnetic states of (Ga,Mn)As by using sequences of linearly-polarized optical pulses. We study the influence of such pre-designed coherent pulse trains on the four-state magnetic memory and demonstrate its full ultrafast control by tuning of relative phase, intensity, and frequency. We show the development of a light-induced magnetization tilt governed by suitable quantum-mechanical superpositions of conduction and valence band states created during the optical pulse. This femtosecond magnetization dynamics is followed by a distinct picosecond temporal regime governed by the magnetic anisotropy of thermal holes. We address the fundamental question of how spins couple to transient optical coherences during time intervals shorter than the photo-excitation and elucidate the role of the competition between magnetic exchange and spin-orbit interactions. Our results indicate the possibility of reading/writing magnetic states at THz speed and propose protocols for multiple switchings between the four metastable states.\\[4pt] [1] All-optical four-state magnetization reversal in (Ga,Mn)As ferromagnetic semiconductors, M. D. Kapetanakis, P. C. Lingos, C. Piermarocchi, J. Wang, and I. E. Perakis, Appl. Phys. Lett. 99, 091111 (2011).\\[0pt] [2] Femtosecond Coherent Control of Spins in (Ga, Mn)As Ferromagnetic Semiconductors Using Light, M. D. Kapetanakis, I. E. Perakis, K. J. Wickey, C. Piermarocchi, and J. Wang, Phys. Rev. Lett. 103, 047404 (2009).\\[0pt] [3] Ultrafast light-induced magnetization dynamics of ferromagnetic semiconductors, J. Chovan, E. G. Kavousanaki, and I. E. Perakis, Phys. Rev. Lett. 96, 057402 (2006). [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T18.00006: Calculated x-ray linear dichroism spectra for Gd-doped GaN Tawinan Cheiwchanchamnangij, Walter Lambrecht Gd doped GaN has been claimed to be a dilute magnetic semiconductor with colossal magnetic moments. However, the origin of huge magnetic moments is still controversial. The x-ray linear dichroism (XLD) spectrum of the Gd L3 edge and the multiple scattering calculations from Ney et al. (J. Magn. Magn. Mater. 322, 1162 (2010)) suggested that about 15\% of Gd atoms should be on antisites. In contrast, our first principle calculations indicate that once the Gd is put on the N site, it will move to the interstitial site and cause large structure relaxation. The formation energy of the system is, therefore, in the order of 10 eV per Gd atom which is extremely large. We show that XLD spectra for L-edges can be analyzed in terms of suitable linear combinations of the partial densities of states of the Gd d-electrons. Core-hole effects are also included. The XLD spectra extracted from our calculations of Gd on the Ga site is shown to fit the experimental spectrum and no Gd on the N site is needed. [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T18.00007: Achieving Room-temperature Ferromagnetism in N-doped ZnO with Inhomogeneity Vivian Tran, Masayoshi Seike, Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida Wide-gap semiconductors, such as ZnO, are attractive host materials for dilute magnetic semiconductors (DMS) due to potential applications in optoelectronic and magneto-optical devices. Recent experiments on N-doped ZnO DMS have reported room-temperature ferromagnetism (RTFM) under a homogeneous distribution of N-dopants. However, analogies to the previously studied transition-metal-doped ZnO systems suggest that RTFM originates from inhomogeneity in the system. Through first-principles calculations, we show that the N-dopants tend to cluster and that RTFM in N-doped ZnO DMS can be achieved by controlling the inhomogeneity in the system. That is, Monte Carlo simulations indicate that self-organized N-rich nanostructures form under layer-by-layer growth conditions. Furthermore, our calculations show that these nanostructures have strong ferromagnetic coupling between N-atoms within each nanostructure in addition to high blocking temperature, assuming a homogeneous distribution of dopants within each nanocluster. These self-organized nanostructures have potential applications to high-density magnetic memory. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T18.00008: Electronic bulk and surface transport in n- and p-InAs films on GaAs substrates Yao Zhang, V. Soghomonian, J.J. Heremans, L.J. Guido We experimentally studied magnetotransport of bulk carriers and surface electrons in InAs MOCVD-grown on GaAs, as well as the spin interaction between surface carriers and transition metal ions. Hall and Shubnikov-de Haas data show the existence of 3 carrier types: interface carriers at the GaAs/InAs interface, bulk carriers and surface state carriers. In n-type samples total density $\textit{n}$ and total mobility $\mu$ increase with increasing n-doping. At a threshold doping level, transport in the system changes from multi-carrier to single-carrier. In p-type InAs, $\textit{n}$ and $\mu$ show a strong temperature dependence, partly due to carrier freeze-out. The p-type InAs also shows GaAs/InAs interface carriers. At low temperatures and low magnetic fields, weak antilocalization (AL) is observed due to spin-orbit interaction, mostly from electrons with Rashba spin-orbit interaction in the surface accumulation layer. Due to its sensitivity to spin phenomena AL can be used as a sensitive probe of interactions between the surface electrons and local magnetic moments. The magnetic species modify the surface electron spin-flip scattering and spin-orbit scattering. Spin-orbit scattering is seen to be increased by Co$^{2+}$ and Ni$^{2+}$, while suppressed by Fe$^{3+}$. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T18.00009: Coexistent Ferromagnetic and Semiconducting behavior in CoO/ZnO Multilayer Films Frances Hellman, Hyeon-Jun Lee, Catherine Bordel, Michalis Charilaou, Julie Karel Ferromagnetic semiconductor behavior up to just below 300 K is shown in CoO/Al-doped ZnO (AZO) multilayers, shown by magnetic measurements and anomalous and ordinary Hall effect. The magnetism oscillates with odd versus even number of Co layers in the insulating antiferromagnetic CoO and (separately) with the thickness of the doped semiconducting AZO layers, and vanishes if AZO is replaced by undoped insulating ZnO. Magnetization is attributed to uncompensated (111) ferromagnetic planes of insulating CoO for odd numbers of atomic planes per layer which are coupled together via RKKY exchange mediated by electron carriers in the non-magnetic AZO layers. The period of the oscillation with AZO thickness qualitatively matches the Fermi wavevector calculated from the carrier concentration measured by ordinary Hall effect. Magnetic polarization of the AZO carriers is confirmed via anomalous Hall effect which is proportional to the magnetization. X-ray magnetic circular dichroism confirm magnetic properties. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T18.00010: Interaction of Mn with Ge-quantum dot surfaces and its impact on quantum dot growth and morphology Petra Reinke, Christopher Nolph, Joseph Kassim, Jerrold Floro The magnetic doping of Ge-quantum dots (QD) and Ge thin film materials has garnered considerable interest due their anticipated use in nanoscale spintronics device structures. In this study we probe with scanning tunneling microscopy the interaction of Mn with the growth surfaces in strain-driven synthesis of Ge-QDs on Si(100)-(2x1). The growth surfaces are the Ge-QD\textbraceleft 105\textbraceright facet and the Ge(100) surface of the wetting layer (WL). Mn interactions with the QD\textbraceleft 105\textbraceright facet is particularly interesting, and shows the formation of Mn-islands with a geometry bounded by the surface reconstruction, and a backbonding of Mn-d electrons into the surface states of the rebonded Ge\textbraceleft 105\textbraceright facet. Annealing introduces (\textless 570 K) dramatic changes in bonding, and initiates intermixing of Ge and Mn. Further increase in the temperature drives the Mn-surface diffusion and leads to the formation of germanide clusters. In the co-deposition of Mn and Ge with 2-23 at{\%} of Mn, the morphology of the Ge QDs is gradually modulated, QDs are significantly smaller for high Mn concentrations, with a concurrent thickening of the WL. We will discuss the co-deposition process in the framework of surface processes in the Mn-Ge-QD system. [Preview Abstract] |
Thursday, March 21, 2013 10:48AM - 11:00AM |
T18.00011: Spin-gating an antiferromagnetic semiconductor conductivity Xavier Marti, Ignasi Fina, Di Yi, Jian Liu, Claudy Rayan-Serrao, Jiun-Haw Chu, Siriyara Jagannatha Suresha, Jakub Zelezny, Jan Masek, Tomas Jungwirth, Ramamoorthy Ramesh Magnetic semiconductors entwine two of the most successful concepts in both fundamental physics and industrial applications where ferromagnetic materials have played an undismissable role. Recently antiferromagnets have been proposed as alternative material systems [1,2]. Antiferromagnetic spintronics have been demonstrated by the fabrication of tunnel devices [3,4], atomic-size proof-of concepts [5], even devices without auxiliary ferromagnetic layers [6]. Here we present the control of the electrical conductivity of an antiferromagnetic semiconductor by manipulating the magnetic state of a contiguous ferromagnetic layer acting as a spin-based gate. We present an oxide-based fully epitaxial heterostructure, its structural characterization and the electrical measurements showing a direct link between state of the ferromagnetic gate and ohmic resistance of the semiconductor, even displaying distinct remnant resistance states. [1] S. Shick et al., Phys. Rev. B 81, 212409 (2010) [2] T. Jungwirth et al., Phys. Rev. B 83, 035321 (2011) [3] B.G. Park et al., Nature Materials 10, 347--351 (2011) [4] X. Marti et al., Phys. Rev. Lett. 108, 017201 (2012) [5] S. Loth et al., Science 335, 6065 (2012) [6] D. Petti et al., submitted [Preview Abstract] |
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