SpinPolarized Tunneling Through SelfAssembled Molecular Monolayers

1
Spin-Polarized Tunneling Through Self-Assembled
Molecular Monolayers

• S.K. Slater, J.R. Petta, D.C. Ralph
• Cornell University
• March 24, 2004

This work supported by NSF (DMR-0244713), DARPA
(N00173-03-1-G011) and the ARO
(DAAD19-01-1-0541).
2
Motivation

• Determine how much spin coherence is possible
  tunneling through an organic monolayer
• Investigate the effects of defects in the
  monolayer and its interfaces
• Look for interesting microscopic physics due to
  tunneling through a small aperture in the
  monolayer

3
Device Geometry
Electron flow for positive bias
Holes are 2-15 nm in diameter, based on
resistances of controls without barriers.
Self-assembled monolayer (SAM) barriers are
octanethiol or benzenedithiol, made up of
approximately 400 molecules.
K. S. Ralls, R. A. Buhrman, and R. C. Tiberio,
Appl. Phys. Lett. 55, 2459 (1989) C. Zhou, M. R.
Deshpande, M. A. Reed, L. Jones II, and J. M.
Tour, Appl. Phys. Lett. 71, 611 (1997). J. R.
Petta, D. G. Salinas, D. C. Ralph, Appl. Phys.
Lett. 77, 4419 (2000).
4
Measurement of Spin Coherence Magnetoresistance
Coherent Tunneling
Electron Flow
1) Spins are polarized by first lead
3) Spins flip to align with second lead
e-
e-
e-
SH
Ni
Ni
2) Spin polarization is preserved in tunneling
When the leads are anti-aligned, this spin flip
causes the resistance to increase.
5
Example of Coherent Tunneling
The positive junction magnetoresistance (JMR) is
a measure of the increase in resistance when the
leads are anti-aligned.
6
Bias Dependence of JMR (measurement)
The amplitude of the JMR falls off quickly with
bias.
The sign of the JMR changes with the sign of the
bias.
7
Temperature Dependence of JMR (measurement)
JMR decreases with temperature. An anomalous
peak at negative bias shrinks rapidly and moves
toward zero bias as the temperature is raised.
8
Bias and Temperature Dependence of JMR (theory)
Two-step tunneling through localized states in
the barrier can result in a rapid decrease in
JMR with increasing bias or temperature and
negative JMR.
Higher bias or temperature ? more available trap
states ? more two-step tunneling ? smaller JMR.
Many of the trap states are likely located at the
interfaces, particularly at the second interface,
where metal is deposited on the formed SAM. This
can lead to asymmetry in the bias dependence.
E. Y. Tsymbal, O. N. Mryasov, and P. R. LeClair,
J. Phys.Condens. Matter 15, R109 (2003). W.
Wulfhekel, H. F. Ding, and J. Kirschner, J. Magn.
Magn. Mater. 242-245, 47 (2002). J. Zhang and R.
M. White, J. Appl. Phys. 83, 6512 (1998).
9
Telegraph Noise Single- and Few-Molecule
Fluctuations
The amplitude of the telegraph noise varies with
bias in discrete values.
There is a slight increase in amplitude when the
leads are aligned antiparallel.
10
Conclusions

• A high amount of spin coherence is indicated by
  large values of JMR (up to 12.7, as compared to
  21 for perfect spin coherence).
• Rapid decrease of JMR with bias and temperature
  are observed and attributed to 2-step tunneling
  through trap states. Junctions with fewer
  defects may therefore have high values of JMR
  over a larger range of bias and temperature.
• Discrete telegraph noise indicate fluctuations in
  small numbers of molecules.