Gate Control of Spin Transport in Multilayer Graphene By H. Goto et al. Kun Xu

Advantages Advantages of spin over charge: Easily manipulatable with externally applied magnetic fields Long coherence/relaxation time GMR - Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and nonmagnetic layers. Already used in magnetic storage technologies. Once created it tends to stay that way for a long time – unlike charge states, which are easily destroyed by scattering or collision with defects, impurities or other charges. Seamless integration of electronic, optoelectronic and maetoelectronic multifunctionality on the single device.

GMR Giant magnetoresistance Sandwich structure FNF Spin valve (HDD read/write heads) The 2007 Nobel Prize in physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR GMR - Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and nonmagnetic layers. Already used in magnetic storage technologies. In the absence of an external magnetic field, the direction of magnetization of adjacent ferromagnetic layers is antiparallel due to a weak anti-ferromagnetic coupling between layers. The result is high-resistance magnetic scattering. When an external magnetic field is applied, the magnetization of the adjacent ferromagnetic layers is parallel. The result is lower magnetic scattering, and lower resistance. [1]

Disadvantages Existing spin devices do not amplify signals although they are successful switches or valves). Spin amplifier constructed from well known components. The spin current polarization is detected by a spin valve and transformed into a voltage. A conventional amplifier is used to generate a charge current proportional to the detected spin current. A spin current source transforms the charge current into the output spin current.

Datta-Das Device Current modulated by the degrees of precession in electron spin introduced by the gate field a structure made from indium-aluminum-arsenide and indium-gallium-arsenide Larmor precession. The emitter emits electrons with their spins oriented along the direction of the electrode’s magnetization, while the collector (with the same electrode magnetization) acts as a spin filter and accepts electrons with the same spin only. In the absence of any changes to the spins during transport, every emitted electron enters the collector. In this device, the gate electrode produces a field that forces the electron spins to precess, just like the precession of a spinning top under the force of gravity. The electron current is modulated by the degree of precession in electron spin introduced by the gate field: An electron passes through the collector if its spin is parallel, and does not if it is antiparallel, to the magnetization.

Spin-based quantum Computer Qubit – intrinsic binary units Quantum entanglement Single electron trapped in a quantum dot Gives quantum computer to operate in parallel Since spins inherently have long coherence length/immune to the long-range electrostatic coulomb interactions between charges. Factor large integers into prime numbers

Spin transport in graphite based devices Carbon nanotubes Graphene Multilayer graphene (MLG) Weak spin-orbit and hyperfine interaction Gate control of spin conduction Weak spin-orbit and hyperfine interaction ---- carbon: light element

Device Structure MLG Exfoliated from kish graphite 2.5nm thick, about 7 layers (by SEM/AFM) Doped Si/SiO2 substrate Chromium/gold – non magnetic Cobalt – ferromagnetic E-beam lithography/vacuum deposition of metals

Device Structure 50nm Co electrodes 200nm/330nm Separated by L=290nm

Device Structure Cr/Au nonmagnetic electrodes 5nm/100nm thick

Measurement Four terminal lock-in technique 4.2K Excitation current of 1.0 uA, 119Hz Back gate bias The great promise of spintronic technology is based upon the fundamental ability of electron spins in electronic materials to preserve coherence for relatively long times. A typical electron “remembers” its initial spin orientation for a nanosecond. This time scale is indeed long when compared with the typical times– femtoseconds–for electron momentum relaxation. Perhaps a more revealing quantity than spin lifetime (which is usually called spin relaxation time T1 or spin decoherence time T2, depending on the context of the experiment) is the spin diffusion length LS which measures how far electrons diffuse in a solid without losing spin coherence. Imersed in liquid Helium at 4.2k

Spin Signal: Rs Rs=Rp-Rap Proportional to R when FN interfaces are opaque Proportional to 1/R when FN interfaces are transparent

Spin Signal: Rs

Spin Signal: Rs Vn=1.5V

Spin relaxation length

Spin relaxation length MLG Graphene: 1.5-2 um at room temperaure, may stay the same at low temperature because the mean free path in graphene is almost independent of temperature

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