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1. 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

2. 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

3. Semiconductors OrganicInorganic molecules held together by covalent/ionic bonds van der Waals forces 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

4. 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

5. So what’s the problem? • metal-organic interfaces, interface sensitivity of tunneling • “fragile” molecules • low “spin scattering” ? • complicated • (compared to, say, MgO) • a plus: significant work to date for OLEDs! • start simpler: investigate molecular building blocks Nature Materials 8 (2009) 693 Center for Materials for Information Technology an NSF Materials Science and Engineering Center

6. Xu et al. Appl. Phys. Lett. 90, 072506 (2007) Magnetoresistance of LSMO/20 nm TPP/Co sizable MR observed us: up to 40% at 10K several groups US, Italy, Netherlands ... superconducting electrodes confirms tunneling Center for Materials for Information Technology an NSF Materials Science and Engineering Center

7. 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

8. 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

9. 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 Self-assembly yields well-ordered monolayers metal 2 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

10. dI/dV V hv 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” detail: tiny signals … challenging measurement d2I/dV2 V Poster on this, or stop by Bevill 180 and I’ll show you … hv Center for Materials for Information Technology an NSF Materials Science and Engineering Center

11. 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

12. Co Co I I I I I 3d Al2O3 Al2O3 N 1s 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

13. 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? Center for Materials for Information Technology an NSF Materials Science and Engineering Center

14. So what is the “anomaly?” usually “anomaly” = “we don’t know” iodobenzoic/Co • 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

15. 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