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

2. 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.

3. 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]

4. 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.

5. 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.

6. 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

7. 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

9. 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

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

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

12. 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

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

14. Spin Signal: Rs

15. Spin Signal: Rs
Vn=1.5V

16. Spin relaxation length

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

18. ?