Transport facilities Quantum Design PPMS T 350 m

Transport facilities Quantum Design PPMS T: 350 m. K -- 400 K H: 0 -- 7 T custom electronics I(V, T) d. I/d. V(V, T) d 2 I/d. V 2(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

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

Meservey-Tedrow tunneling • thin film SC in a field (~2 -3 T) Zeeman split quasiparticle DOS • d. I/d. V ~ 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

Examples. . . spin polarization of Co films onset of ferromagnetism: Ni on Bi Co / Al 2 O 3 / Al THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

Inelastic Tunneling Spectroscopy (IETS) • tunneling electrons excite vibrational modes – see Raman & IR modes – no strong selection rules d. I/d. V • tells us – the molecule is still there – it participates in transport • when e. V ≥ hv – stepped increase in d. I/d. V – new set of final states available V hv d 2 I/d. V 2 • example: studying model catalysts – CO on Rh/Al 2 O 3 -- CO vibrations “tunneling energy loss spectroscopy” detail: tiny signals … challenging The University of Alabama V Lambe & Jaklevic, Phys. Rev. 165, 821 (1968) hv

Al/Al 2 O 3/X-benzoic acid/Co, T = 2 K IETS: we really have transport through the molecule, it is intact The University of Alabama

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 COOThe University of Alabama

ATR-FTIR IETS Al/Al 2 O 3/4 -iodobenzoic/Co But … what is the state of the I substituent? aromatic-halogen modes insensitive probe with XPS The University of Alabama

Impedance spectroscopy of thin films THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

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: d. I/d. V vs V at fixed freq THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

physical realizations • lossy capacitor • tunnel junction • imperfect dielectric • cell walls, to an extent • electrochemistry. . . THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

impedance spectroscopy magneto-dielectric measurements

Example: Mg. O-MTJ • high-frequency operation for MRAM, reader • MTJ is like a capacitor too. . . • what is C doing? • significant magnetocapacitance THE UNIVERSITY OF ALABAMA . . . /Co. Fe. B/Mg. O/Co. Fe. B. . . via Anelva CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

Bi 2 Ni. Mn. O 6–La 2 Ni. Mn. O 6 superlattices • magnetodielectric materials • strongly T-dependent capacitance • ‘lossy’ near FM Curie temp • coupling of FM and dielectric order! THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

• 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 THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

VO 2 metal-insulator transition • still poorly-explained metal-insulator transition • percolation of metallic regions? • impedance spectra should help. . . THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

capacitance vanishes at M-I transition resistance drops ‘leaky’ capacitor model percolation of metallic regions? ‘dielectric’ changes little below transition THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

How to measure a derivative? • drive system with Vdc + δ Vacsin 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. Le. Clair Toshali Sands, Puri, 8 Jan 2009

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