Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H3: Room Temperature Semiconductor Spintronics |
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Sponsoring Units: GMAG Chair: Berry Jonker, Naval Research Laboratory Room: Oregon Ballroom 203 |
Tuesday, March 16, 2010 8:00AM - 8:36AM |
H3.00001: Multiferroic spintronics Invited Speaker: Multiferroics are singular materials that can exhibit simultaneously electric and magnetic orders, with in some cases a magnetoelectric between the two. As such, these compounds bring new functionalities to spintronics [1] and new device possibilities, such as multinary memory elements or magnetic random access memories with electrical write operation. For practical purposes, the main problem of multiferroics is their scarcity. Notable examples in the perovskite family include the low temperature ferromagnetic ferroelectric material (La,Bi)MnO3, and the room temperature antiferromagnetic ferroelectric compound BiFeO. We will present experiments on 2 to 4 nm thick ferromagnetic (La,Bi)MnO3 layers used as tunnel barriers in junctions with a La2/3Sr1/3MnO3 bottom electrode and a Au top electrode. In these junctions, the tunnel current is modulated by the ferromagnetic order parameter of the barrier material via the spin-filter effect, giving rise to a TMR of up to 100{\%} [2,3]. Remarkably, 2 nm-thick (La,Bi)MnO3 films are also ferroelectric [4]. Accordingly, the concomitance of ferromagnetism and ferroelectricity in a unique tunnel barrier allows obtaining a four level resistance state from the combination of the spin-filter effect and the influence of ferroelectricity on tunneling [4]. The interest of the antiferromagnetic-ferroelectric BiFeO3 is for room-temperature control of magnetization by the application of an electric field. We will show that BiFeO3 films can be used to establish a robust exchange-bias effect [5]. Remarkably, the exchange field correlates with the multiferroic domain size, as expected from Malozemoff's model of exchange bias extended to multiferroics [6]. Perspectives for electric control of spintronics devices will be given. A way to overcome the scarcity of multiferroic materials at room temperature is to design a artificial multiferroics by combining ferroelectric and ferromagnetic materials. We will present experiments on heterostructures combining ferroelectric tunnel barriers of BaTiO3 and ferromagnetic electrodes (Fe or Co). This kind of heterostructures allows to generate, within a single device, not only a tunnel magnetoresistance (TMR) phenomena related to the relative orientation of the magnetization of the electrodes and a very large tunnel electroresistance (TER) -- up to 750000{\%} at 3nm- induced by the ferroelectric polarisation of the barrier [7]. They also give rise to a unusual modulation of the spin polarisation at the interface by the ferroelectricity [8] resulting in a large TEMR (Tunnel Electro MagnetoResistance) effect. [1] M. Bibes and A. Barth\'{e}l\'{e}my, IEEE Trans. Electron Dev. 54, 1003 (2007) [2] M. Gajek et al, J. Appl. Phys 97, 103909 (2005) [3] M. Gajek et al, Phys. Rev. B 72, 020406(R) (2005) [4] M. Gajek et al, Nature Materials 6, 296 (2007) [5] H. B\'{e}a et al, Appl. Phys. Lett. 89, 242114 (2006) [6] H. B\'{e}a et al, Phys. Rev. Lett 100, 017204 (2008) [7] V. Garcia et al.; Nature 460, 81 (2009) [8] C. G. Duan et al.~; Phys. Rev. Lett. 97, 047201 (2006) [Preview Abstract] |
Tuesday, March 16, 2010 8:36AM - 9:12AM |
H3.00002: Electrical creation of spin polarization in silicon at room temperature Invited Speaker: The integration of magnetism and mainstream semiconductor electronics could impact information technology in ways beyond imagination. A pivotal step is the implementation of spin-based electronic functionality in silicon devices. Much of the interest in silicon derives from its prevalence in semiconductor technology and from the robustness and longevity of spin as it is only weakly coupled to other degrees of freedom in the material. Recently it has become possible to induce and detect spin polarization in otherwise non-magnetic semiconductors (GaAs and Si) using all-electrical structures, but so far at temperatures below 150 K and only in n-type material. The main challenges are: (i) to design fully electrical silicon-based spintronic devices with large spin signals, (ii) to demonstrate device operation at room temperature, (iii) to do so for n-type and p-type material, and (iv) to find ways to manipulate spins and spin flow with a gate electric field. After a brief overview of the state of affairs, our recent advances in these areas are described. In particular, we demonstrate room-temperature electrical injection of spin polarization into n-type and p-type silicon from a ferromagnetic tunnel contact, spin manipulation using the Hanle effect, and the electrical detection of the induced spin accumulation. It is shown that a spin splitting as large as 2.9 meV can be created in Si at room temperature, corresponding to an electron spin polarization of 4.6{\%}. The results open the way to the implementation of spin functionality in complementary silicon devices and electronic circuits operating at ambient temperature, and to the exploration of their prospects as well as the fundamental rules that govern their behavior. \\[4pt] [1] S.P. Dash, S. Sharma, R.S. Patel, M.P. de Jong and R. Jansen, Nature \textbf{462}, 491 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:48AM |
H3.00003: Spin transport in graphene: Injection, relaxation, and electron-hole asymmetries Invited Speaker: Graphene is an attractive material for spintronics due to the low intrinsic spin-orbit and hyperfine coupling, which should lead to excellent spin transport properties. Experimentally, graphene spin valves are the first gate-tunable material to exhibit spin transport at room temperature. This fact alone makes it a strong candidate for future spin-based logic applications. Devices exhibit a spin diffusion length on the order of 1-3 microns at room temperature, and the non-local spin signals as high as 60 ohms has been achieved in our laboratory. These favorable properties could even be improved further by increasing the spin lifetimes (which are currently at typical values of 100 ps). by improving the material quality. Apart from the good performance characteristics, graphene also has unique properties which makes it an interesting system for studying spin-dependent phenomena. First the band structure has an electron-hole symmetry that typical semiconductors lack. This opens up some interesting possibilities regarding bipolar spintronic devices and the possible role of pn junctions on spin injection. Our studies in this area have led to the observation of a novel electron-hole asymmetry regarding the bias dependence of the spin injection. Second, the ultrathin nature of the graphene and its surface conduction allow for the modification of spin transport properties by controlled chemical doping. Our studies here have led to new insights on the origin of spin-relaxation in graphene spin valves. Third, the lack of large depletion regions allows one to control the spin injection and detection properties through atomic scale engineering of the ferromagnet/graphene interfaces. Our studies in this area investigate the role of engineered tunnel barriers on the efficiency of spin injection and detection. In this talk, I will discuss graphene spintronics in general, present some of our research results, and finally discuss the future prospects for the field. [Preview Abstract] |
Tuesday, March 16, 2010 9:48AM - 10:24AM |
H3.00004: Quantum science with spin impurities in diamond Invited Speaker: We will describe our recent experiments involving manipulating individual electronic and nuclear spins associated with Nitrogen Vacancy color centers in diamond. Current efforts towards realization of quantum computers operating at room temperature, quantum optical networks and nanoscale magnetic sensors will be discussed. [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 11:00AM |
H3.00005: Spin Routes in Organic Semiconductors Invited Speaker: Organic semiconductors are characterized by a very low spin-orbit interaction, which, together with their chemical flexibility and relatively low production costs, makes them an ideal materials system for spintronics applications. The first experiments on spin injection and transport occurred only a few years ago, and since then considerable progress has been made in improving performance as well as in understanding the mechanisms affecting spin-related phenomena. Nevertheless, several challenges remain in both device performance and fundamental understanding before organic semiconductors can compete with inorganic semiconductors or metals in the development of realistic spintronics applications. In this presentation I summarize the main experimental results and their connections with devices such as light-emitting diodes and electronic memory devices, and outline the scientific and technological issues that make organic spintronics a young but exciting field. [Preview Abstract] |
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