Spin scattering by organic radicals
P. LeClair
Physics & Astronomy, MINT, University of Alabama
G.J. Szulczewski, Weihao Xu, Justin Kriel
Chemistry, MINT, University of Alabama
Dina Genkina, Abel Demisse
undergraduate interns
Mathijs van Schijndel
Visiting Grad (TU Eindhoven)
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Motivation
• characterize the electronic, magnetic and physical structure of
ferromagnetic metal/insulator/molecule interfaces.
• understand spin-polarized electron injection into organic
semiconductors.
Tunnel junctions
Spin-valves
Co
LiF
Co
1-10 nm
~100 nm
MgO
CoFe
Co
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Semiconductors
Organic
Inorganic
molecules held together by
van der Waals forces
covalent/ionic bonds
between atoms
charges hop between molecules
conduction bands
mobility ~ 10-6 - 10 cm2 V-1 s-1
> 103 cm2 V-1 s-1
soft/fragile
hard/brittle
Szulczewski et al. Nature Materials 8 (2009) 693
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
property trade-offs (as usual)
Universal Display Corp.
Phys. Rev. B 75 (2007) 113203
Nature Materials 8 (2009) 693
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
So what’s the problem?
• metal-organic interfaces, interface sensitivity of tunneling
• “fragile” molecules
• low “spin scattering” ?
• complicated
(compared to, say, MgO)
Nature Materials 8 (2009) 693
• a plus: significant work to date for OLEDs!
• start simpler: investigate molecular building blocks
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Magnetoresistance of LSMO/20 nm TPP/Co
NH
N
N
HN
sizable MR observed
us: up to 40% at 10K
several groups
US, Italy, Netherlands ...
Xu et al. Appl. Phys. Lett. 90, 072506 (2007)
superconducting electrodes
confirms tunneling
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
large spin coherence length in amorphous Rubrene ….
Measured P vs Rubrene thickness
• spin diffusion length
of 13nm estimated
• fairly well-characterized
samples
Al/Al2O3/Rubrene/Co
Shim, Raman, Moodera, PRL, 100 (2008)
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
again, what’s the problem?
• still to complicated
• there are many organics. we can’t just pick them out of a hat!
• need a simpler starting point
– structure-property correlation is a requirement
• so we go ‘back to the future’ … (again)
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Molecular Tunnel Junctions
• One layer of a simple, flexible molecule - benzoic acid derivatives
• Model system for more complex molecules - ‘scaffolding’
• Correlate transport and molecular structure
Structure
Photoemission (UPS,XPS)
Raman/IR
Inelastic electron tunneling
metal 2
Self-assembly yields wellordered monolayers
benzoic
Al2O3
Al
Transport
Conductance (V,H,T)
Spin-polarized tunneling
Al2O3
Al
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Inelastic Tunneling Spectroscopy (IETS)
• tunneling electrons excite vibrational modes
•
tells us
– the molecule is still there
– it participates in transport
• when eV ≥ hv
– stepped increase in dI/dV
“tunneling energy loss spectroscopy”
d2I/dV2
dI/dV
detail: tiny signals … challenging measurement
V
hv
V
hv
Poster on this, or stop by Bevill 180 and I’ll show you …
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Al/Al2O3/X-benzoic acid/Co, T = 2K
IETS: we really have transport through the molecule, largely intact
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Co
Co
I
I 3d
Al2O3
N 1s
I
I
I
Al2O3
Al/Al2O3/BA/(Co or Pb)
Co reacts with heavier halogens
Pb does not
halogen substitution leads to
organic radicals?
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Conductance Through Benzoic Acids
• controlled introduction of radicals
• when radical is present, strong ‘zero bias anomaly’
• size scales with degree of reaction
reaction, ZBA observed?
CN F
Cl
I
Co
✘
~
✔
✔
Pb
✘
✘
✘
✘
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
So what is the “anomaly?”
iodobenzoic/Co
usually “anomaly” = “we don’t know”
when we have a reaction …
… there is an unpaired electron
… which means an unpaired spin
… which gives a Kondo peak
strongly T, H, V dependent
as expected
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
spin-dependent scattering
• organic radicals = unpaired electron
• should also have ‘normal’ spin flip scattering
• revealed through application of H
consistent with aromatic radical
significant portion of conductance is due to spin-flip scattering!
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center