Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B22: Focus Session: Theory of Spin-based Semiconductor Devices |
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Sponsoring Units: GMAG DMP FIAP Chair: Lukasz Cywinski, University of Maryland Room: 324 |
Monday, March 16, 2009 11:15AM - 11:51AM |
B22.00001: Role of motive forces for the spin torque transfer for nano-structures Invited Speaker: Despite an announced imminent commercial realization of spin transfer random access memory (SPRAM) the current theory evolved from that of Slonczewski [1,2] does not conserve energy. Barnes and Maekawa [3] have shown, in order correct this defect, forces which originate from the spin rather than the charge of an electron must be accounted for, this leading to the concept of spin-motive-forces (smf) which must appear in Faraday's law and which significantly modifies the theory for spin-valves and domain wall devices [4]. A multi-channel theory in which these smf's redirect the spin currents will be described. In nano-structures it is now well known that the Kondo effect is reflected by conductance peaks. In essence, the spin degrees of freedom are used to enhance conduction. In a system with nano-magnets and a Coulomb blockade [5] the similar spin channels can be the only means of effective conduction. This results in a smf which lasts for minutes and an enormous magneto-resistance [5]. This implies the possibility of ``single electron memory'' in which the magnetic state is switched by a single electron. \\[4pt] [1] J. C. Slonczewski, {\bf Current-Driven Excitation of Magnetic Multilayers} J. Magn. Magn. Mater. 159, L1 (1996). \\[0pt] [2] Y. Tserkovnyak, A. Brataas, G. E. W. Bauer, and B. I. Halperin, {\bf Nonlocal magnetization dynamics in ferromagnetic heterostructures}, Rev. Mod. Phys. 77, 1375 (2005). \\[0pt] [3] S. E. Barnes and S. Maekawa, {\bf Generalization of Faraday's Law to Include Nonconservative Spin Forces} Phys. Rev. Lett. 98, 246601 (2007); S. E. Barnes and S. Maekawa, {\bf Currents induced by domain wall motion in thin ferromagnetic wires.} arXiv:cond-mat/ 0410021v1 (2004). \\[0pt] [4] S. E., Barnes, {\bf Spin motive forces, measurement, and spin-valves.} J. Magn. Magn. Mat. 310, 2035-2037 (2007); S. E. Barnes, J. Ieda. J and S. Maekawa, {\bf Magnetic memory and current amplification devices using moving domain walls.} Appl. Phys. Lett. 89, 122507 (2006). \\[0pt] [5] Pham-Nam Hai, Byung-Ho Yu, Shinobu Ohya, Masaaki Tanaka, Stewart E. Barnes and Sadamichi Maekawa, {\bf Electromotive force and huge magnetoresistance in magnetic tunnel junctions.} Submitted Nature, August, (2008). [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B22.00002: Revisiting the ``Spin-Transistor'' Abu Naser Zainuddin, Lutfe Siddiqui, Supriyo Datta A ``spin-transistor'' in principle requires efficient injection (source), efficient detection (drain) and electrical manipulation (gate). For sometime now, electrical manipulation based on the Rashba effect has been well established and in recent years there has been significant progress in the design of injectors and detectors. Lateral spin-valve structures showing $\sim$50\% spin-polarization has been reported. In view of these advances it seems appropriate to evaluate various ``spin- transistor'' concepts. With this in mind, we have developed non- equilibrium Green's function (NEGF) based model and benchmarked against existing experiments. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B22.00003: Dynamical magnetoelectric feedback effects in magnetic resonant tunneling structures Christian Ertler, Jaroslav Fabian Heterostructures made of stacked layers of both magnetic and nonmagnetic semiconductors provide a lot of opportunities for controlling and tuning their spin-dependent transport properties. For instance, highly efficient spin valves,spin switching and spin filtering devices have been demonstrated by using magnetic resonant tunneling structures [1]. Here, we show that in a resonant tunneling double barrier structure, which comprises a ferromagnetic quantum well made of a dilute magnetic semiconductor material, interesting dynamical effects can occur [2]. In such systems the transport and magnetic properties become strongly coupled, since the ferromagnetic order in the quantum well is mediated by the itinerant carriers. Both the Coulomb interaction of the particles and the magnetic exchange field give rise to strong feedback effects on the tunneling current. Interestingly, for a broad voltage range self-sustained high-frequency oscillating currents associated with an oscillating well magnetization appear. The requirements for the occurrence of these dc-driven magnetoelectric oscillations are investigated and possible device setups, which should allow for an experimental observation, are discussed. [1] J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano and I. Zutic, Acta Phys. Slov. 57, 565 (2007). [2] C. Ertler and J. Fabian, Phys. Rev. Lett. 101, 077202 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B22.00004: Theory of Semiconductor Spin Lasers Christian Gothgen, Rafal Oszwaldowski, Igor Zutic Semiconductor lasers with spin-polarized carriers' injection have important advantages as compared to the conventional lasers in which the carriers are unpolarized. While such spin lasers have been successfully realized and shown to provide spin-polarization modulation and threshold current reduction [1-4], there remain important theoretical challenges in understanding their operation. We demonstrate that the maximum threshold reduction is larger than previously thought possible and, surprisingly, can be enhanced by ultrafast spin relaxation of holes [5]. By combining our analytical model [5] and numerical studies of spin lasers we explore the effects of quantum confinement in the gain region and identify different modes of operation. We thank A. Petrou for valuable discussions. This work is supported by US ONR and NSF-ECCS CARRER. [1] J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). [3] S. Hovel et al., Appl. Phys. Lett. 92, 041118 (2008). [4] D. Basu et al., Appl. Phys. Lett. 92, 09119 (2008). [5] C. Gothgen et al., Appl. Phys. Lett. 93, 042513 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B22.00005: Design Guidelines for Spin-Polarized Lasers Michael Holub, Igor Vurgaftman, Jerry Meyer, Berend Jonker Semiconductor lasers driven by a spin-polarized current are expected to provide a threshold current reduction and optical polarization control. The design of spin-polarized lasers is critical to the realization of these effects. Thus, we have investigated the effect of electron spin injection on semiconductor laser performance using a spin-dependent rate equation model.$^1$ The magnitude of the threshold reduction is shown to depend on intrinsic properties of the active region and laser cavity, and can approach a factor of 3.5 for fully spin-polarized electrons. The threshold reduction is found to be strongest in lasers with undoped active regions, recombination strongly dominated by Auger processes, and low threshold gain. Introduction of a ferromagnetic electrode in the vicinity of the active region for efficient spin injection generally results in higher internal loss and a requirement for greater material gain, which raises the laser's baseline threshold as well as lessens the projected threshold reduction. The placement of a ferromagnetic contact on spin-polarized laser performance will be discussed. \\ \\ $^1$ I. Vurgaftman, M. Holub, B. T. Jonker, and J. R. Meyer {\it Appl. Phys. Lett.} {\bf 93}, 031102 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B22.00006: Spin-orbit coupling effects in Fe/GaAs heterostructures: First principles calculations Martin Gmitra, Alex Matos-Abiague, Claudia Ambrosch-Draxl, Jaroslav Fabian The tunneling anisotropic magnetoresistance (TAMR) effect in semiconductor heterostructures containing a single ferromagnetic layer is potentially useful for spintronics devices. Important, TAMR has recently been observed in a metallic system, namely, in Fe/GaAs/Au junctions. Surprisingly, while all the bulk components of the system are cubic, the observed anisotropy is twofold, of the $C_{2v}$ class. This suggests that rather than coming from the bulk anisotropy of the density of states, the effect arises from the interface that indeed has a reduced symmetry. A phenomenological model reflecting this symmetry in the form of the Bychkov-Rashba and the Dresselhaus spin-orbit coupling was proposed, giving a quantitative fit to the experiment. Here we report on comprehensive first principle calculations of the spin-orbit effects stemming from the interface anisotropy, providing support to the phenomenological theory. In particular, we have performed FPLAPW density functional calculations of an Fe/GaAs slab to extract quantitative information about the proposed model that are Bychkov-Rashba and Dresselhaus parameters. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B22.00007: Magnetic phase transitions driven by non-equilibrium spins and their potential applications to magnetic cooling Lutfe Siddiqui, Abu Naser Zainuddin, Supriyo Datta It is well-known that the Curie temperature in diluted magnetic semiconductors (DMS) like GaMnAs can be controlled by changing the equilibrium density of holes in the material. In this letter we predict,that even with a constant hole density, large changes in the magnetization can be obtained with a relatively small imbalance in the spin population. We show, by coupling mean field theory of diluted magnetic semiconductor ferromagnetism with master equations governing the Mn spin-dynamics, that a splitting of the up-spin and down-spin quasi-Fermi level in the channel by 0.1meV have the same effect as an external magnetic field of 1 T. Experimentally, it has been shown that splitting of the quasi-Fermi level for the two spins by 0.1meV can be conveniently obtained in the channel region of a lateral spin-valve structure with anti-parallel contacts, which can be used to demonstrate the effect we are proposing. Such an effect could also form the basis for a novel class of cooling devices where the electrical input to a spin-valve leads to ferromagnetic ordering of Mn ions which then demagnetize by absorbing energy from the environment. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B22.00008: Magnetic memory and logic based on spin effects in graphene John Zavada, Yuriy Semenov, Ki Wook Kim We report on a novel approach to the problem of low-power-consuming non-volatile magnetic random access memory (MRAM) and logic design that is based on the unique properties of the graphene placed in interface between two magnetic dielectric layers. We find that by combining the electrical effect on the exchange bias field and a giant magneto-resistance effect of the graphene/ferromagnet hybrid structures, a new non-volatile MRAM device is possible. In such a device an electric bias realizes the low energy writing bits instead of an external magnetic field with high energy consumption. In particular, the structure under consideration consists of a three ferromagnetic dielectric layers, which are coupled through monolayer and bilayer graphene films. Interplay of two graphene layers can mediate the exchange bias fields applied to different sides of the free ferromagnets resulting in programmable logic operations. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B22.00009: Metal-insulator transition in a quantum wire with alternating Rashba interaction Henrik Johannesson, George I. Japaridze, Alvaro Ferraz We propose and analyze a device scheme by which an electrical current can be controlled via a gate-operated spin-orbit interaction. The device consists of a quasi-one-dimensional (1D) ballistic channel in a gated semiconductor heterostructure, contacted to a source and a drain and with the gates producing an alternating Rashba spin-orbit interaction. When the period of the Rashba modulation becomes commensurate with the 1D electron density, the spin-orbit interaction opens a charge gap, leading to a suppression of the current. Using bosonization and a perturbative RG approach we explore how electron-electron interactions influence the effect. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B22.00010: Gate control of single-electron spins through Berry Phase in a realistic asymmetric confining potentials in III-V semiconductor Quantum Dots Sanjay Prabhakar, James Raynolds Among recent proposals for next-generation, non-charge-based logic is the notion that a single electron can be trapped and its spin can be manipulated through the application of gate voltages (Rev. Mod. Phys.79, 1217 (2007)). In this talk we present numerical simulations of Berry Phase of electron spins in single electron devices for realistic asymmetric confining potentials in support of experimental work at the University at Albany, State University of New York aimed at the practical development of post-CMOS concepts and devices. We solve the Schr\"{o}dinger equation including spin-orbit effects using a numerical finite-element based technique. We will discuss the calculation of Berry Phase for electrons (Phys. Rev. B 73, 125330 (2006)) in electrostatically defined quantum dots including the Rashba and Dresselhaus spin-orbit interactions computed numerically from realistic asymmetric confining potentials. The new simulation results open the possibility of spin manipulation through the gate induced Berry phase. This work is supported through funding from the DARPA/NRI INDEX center. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B22.00011: Colossal Spincaloritronic Cooling by Adiabatic Spin-Entropy Expansion in Nanospintronics Hiroshi Katayama-Yoshida, Tetsuya Fukushima, Van An Dinh, Kazunori Sato The exchange interactions in DMS are short ranged and can not play an important role for realizing high-T$_C$ because the solubility of magnetic impurity is too low to achieve magnetic percolation [1]. We show that spinodal nano-decomposition under layer-by-layer crystal growth condition (2D) leads to characteristic quasi-one dimensional nano-structures (Konbu- Phase) with highly anisotropic shape and high T$_C$ ($\rangle$ 1000K) even for low concentrations in DMS [2]. We design a spin-currents- controlled 100 Tera bits/icnh$^2$, Tera Hz switching, and non- volatile MRAM without Si-CMOS based on Konbu-Phase [3]. In addition to the conventional Peltier effect, we propose a colossal spincaloritronic cooling based on the adiabatic spin- entropy expansion in a Konbu-Phase (Zn,Cr)Te with very high blocking temperature (T$_B$ $\rangle$ 1000 K) by spinodal nano- decomposition and by nano-column of Half-Heusler NiMnSi (T$_C$ = 1050 K) [4]. [1] K. Sato et al., Phys. Rev. B70, 201202 (2004). [2] H. Katayama-Yoshida et al., Phys. stat. sol. (a) 204 (2007) 15. [3] Japanese Patent: JP3571034, US Patent: US 7,164,180 B2, EU Patent: EP 1548832A1, Taiwan Patent:1262593, Korean Patent: 0557387. [4] H. Katayama-Yoshida et al., Jpn. J. Appl. Phys. 46 (2007) L777. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B22.00012: impurity-impurity interaction in graphene nanoribbons Jian-Ming Tang The high mobility and small spin-orbit interaction makes graphene a promising candidate material for building spin-based quantum devices. Embedded magnetic dopants or magnetized defects due to many-body interaction may act as single spin qubits in these devices. The long decay length for the impurity levels near the Dirac point suggest that the double exchange interaction can compete with the RKKY exchange interaction. The impurity level splitting for two impurities in bulk and in nanoribbons are studied using a tight-binding approach. In the case of nanoribbons, the modification to the interaction due to the presence of edge states will be discussed. [Preview Abstract] |
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