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THE UNIVERSITY OF ALABAMA

Transport facilities. Quantum Design PPMS T : 350mK -- 400K H : 0 -- 7T. custom electronics I(V,T) dI/dV(V,T) d 2 I/dV 2 (V,T) Z(ω,θ,H,T). magnetoresistance hall effect impedance spectroscopy tunneling spectroscopy: inelastic tunneling spectroscopy (IETS)

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THE UNIVERSITY OF ALABAMA

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  1. Transport facilities • Quantum Design PPMS • T: 350mK -- 400K • H: 0 -- 7T • custom electronics • I(V,T) • dI/dV(V,T) • d2I/dV2(V,T) • Z(ω,θ,H,T) • magnetoresistance • hall effect • impedance spectroscopy • tunneling spectroscopy: • inelastic tunneling spectroscopy (IETS) • spin polarization (Meservey-Tedrow) • Radiant ferroelectric tester THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

  2. Magnetoresistance I(V) dI/dV (V) Examples ... THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

  3. Meservey-Tedrow tunneling • thin film SC in a field (~2-3T) • Zeeman split quasiparticle DOS • dI/dV ~ Nsc Nfm • asymmetry = spin polarization • robust, microscopic theory • “directly” gives spin imbalance • of tunneling current THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

  4. Co / Al2O3 / Al Examples ... spin polarization of Co films onset of ferromagnetism: Ni on Bi THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

  5. dI/dV V hv d2I/dV2 Lambe & Jaklevic, Phys. Rev. 165, 821 (1968) V hv Inelastic Tunneling Spectroscopy (IETS) • tunneling electrons excite vibrational modes • see Raman & IR modes • no strong selection rules • tells us • the molecule is still there • it participates in transport • when eV ≥ hv • stepped increase in dI/dV • new set of final states available • example: studying model catalysts • CO on Rh/Al2O3 -- CO vibrations “tunneling energy loss spectroscopy” detail: tiny signals … challenging The University of Alabama

  6. Al/Al2O3/X-benzoic acid/Co, T = 2K IETS: we really have transport through the molecule, it is intact The University of Alabama

  7. ν: stretch β: in-plane bend γ: out-of-plane bend indexing - compare with ATR-FTIR aromatic is intact chemisorbed benzoate ion absence of C=O, presence of COO- The University of Alabama

  8. ATR-FTIR IETS Al/Al2O3/4-iodobenzoic/Co But … what is the state of the I substituent? aromatic-halogen modes insensitive probe with XPS The University of Alabama

  9. Impedance spectroscopy of thin films

  10. How? • apply ac voltage, measure current & phase • gives Z(ω) • model system with an equivalent circuit • each component physically meaningful • probing dielectric/spectral function • ac lock-in techniques or ‘impedance analyzer’ • complementary: dI/dV vs V at fixed freq

  11. physical realizations • lossy capacitor • tunnel junction • imperfect dielectric • cell walls, to an extent • electrochemistry ...

  12. impedance spectroscopy magneto-dielectric measurements

  13. Example: MgO-MTJ • high-frequency operation for MRAM, reader • MTJ is like a capacitor too ... • what is C doing? • significant magnetocapacitance .../CoFeB/MgO/CoFeB ... via Anelva

  14. Bi2NiMnO6–La2NiMnO6 superlattices • magnetodielectric materials • strongly T-dependent capacitance • ‘lossy’ near FM Curie temp • coupling of FM and dielectric order!

  15. near transition temperature, strong variation of dielectric constant with magnetic field • fluctuations in electric & magnetic dipole ordering • interface-induced stress • changes in dielectric constant correlate with magnetization

  16. VO2 metal-insulator transition • still poorly-explained metal-insulator transition • percolation of metallic regions? • impedance spectra should help ...

  17. capacitance vanishes at M-I transition resistance drops ‘leaky’ capacitor model percolation of metallic regions? ‘dielectric’ changes little below transition

  18. How to measure a derivative? • drive system with Vdc + δVacsin ωt • measure response at ω or 2ω frequency / phase sensitive technique … lock-in amplifiers … a lot of electronics The University of Alabama Center for Materials for Information Technology P. LeClair Toshali Sands, Puri, 8 Jan 2009

  19. Keithley 263 home-made summing amp Vdc 20, 40, 400, 4000 101-104 lo & hi pass EG&G 113 ÷ + x ÷ unity gain 1:1 add DC Vac LIA DUT V dV 1, 10, 102, 103, 104 Stanford 830 Stanford 830 HP 3478A current preamp 103-106 V/A Keithley 428 A x I DC 101-104 EG&G 113 + notch filter HP 3458A dI LIA Stanford 830 How to measure a derivative? The University of Alabama Center for Materials for Information Technology P. LeClair Toshali Sands, Puri, 8 Jan 2009

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