Center for Materials for Information Technology an NSF Materials Science and Engineering Center 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. Spin-valvesTunnel junctions Co LiF Co CoFe MgO 1-10 nm Co ~100 nm

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Semiconductors OrganicInorganic molecules held together by covalent/ionic bonds van der Waals forces between atoms charges hop between moleculesconduction bands mobility ~ cm 2 V -1 s -1 > 10 3 cm 2 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. Nature Materials 8 (2009) 693Phys. Rev. B 75 (2007)

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) 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 Magnetoresistance of LSMO/20 nm TPP/Co Xu et al. Appl. Phys. Lett. 90, (2007) 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 spin diffusion length of 13nm estimated fairly well-characterized samples Measured P vs Rubrene thickness Al/Al 2 O 3 /Rubrene/Co large spin coherence length in amorphous Rubrene …. 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 benzoic Al metal 2 Transport Conductance (V,H,T) Spin-polarized tunneling Structure Photoemission (UPS,XPS) Raman/IR Inelastic electron tunneling Self-assembly yields well- ordered monolayers Al 2 O 3 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” detail: tiny signals … challenging measurement dI/dV V hv d 2 I/dV 2 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 IETS: we really have transport through the molecule, largely intact Al/Al 2 O 3 /X-benzoic acid/Co, T = 2K

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Al 2 O 3 Co I 3d N 1sAl/Al 2 O 3 /BA/(Co or Pb) Co reacts with heavier halogens Pb does not halogen substitution leads to organic radicals? I I I I

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 CNFClI Co ✘ ~ ✔✔ Pb ✘✘✘✘ reaction, ZBA observed?

Center for Materials for Information Technology an NSF Materials Science and Engineering Center 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 iodobenzoic/Co So what is the “anomaly?” usually “anomaly” = “we don’t know”

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!