the university of alabama

THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn NSF Science and Engineering Center

Quantum Design PPMST: 350mK -- 400KH: 0 -- 7T

Transport facilities

magnetoresistance

hall effect

impedance spectroscopy

tunneling spectroscopy:

inelastic tunneling spectroscopy (IETS)

spin polarization (Meservey-Tedrow)

Radiant ferroelectric tester

custom electronicsI(V,T)dI/dV(V,T)d2I/dV2(V,T) Z(ω,θ,H,T)

THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn NSF Science and Engineering Center

Examples ...

MagnetoresistanceI(V)

dI/dV (V)

THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn NSF Science and Engineering Center

• thin film SC in a field (~2-3T) Zeeman split quasiparticle DOS

• dI/dV ~ Nsc Nfm

• asymmetry = spin polarizationrobust, microscopic theory

“directly” gives spin imbalance of tunneling current

Meservey-Tedrow tunneling

THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn NSF Science and Engineering Center

Co / Al2O

3 / Al

spin polarization of Co films

onset of ferromagnetism:Ni on Bi

Examples ...

The University of Alabama

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

d2I/dV2

dI/dV

Vhv

Vhv

Lambe & Jaklevic, Phys. Rev. 165, 821 (1968)

The University of Alabama

IETS: we really have transport through the molecule, it is intact

Al/Al2O3/X-benzoic acid/Co, T = 2K

The University of Alabama

ν: stretchβ: in-plane bendγ: out-of-plane bend

indexing - compare with ATR-FTIRaromatic is intactchemisorbed benzoate ionabsence of C=O, presence of COO-

The University of Alabama

Al/Al2O3/4-iodobenzoic/Co

IETS

ATR-FTIR

But … what is the state of the I substituent?aromatic-halogen modes insensitiveprobe with XPS

THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn NSF Science and Engineering Center

Impedance spectroscopy of thin films

THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn 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: dI/dV vs V at fixed freq

THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn NSF Science and Engineering Center

physical realizations•lossy capacitor

•tunnel junction

•imperfect dielectric

•cell walls, to an extent

•electrochemistry ...

impedance spectroscopy

magneto-dielectric measurements

THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn NSF Science and Engineering Center

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

THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn NSF Science and Engineering Center

Bi2NiMnO6–La2NiMnO6 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 TECHNOLOGYAn 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 TECHNOLOGYAn NSF Science and Engineering Center

VO2 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 TECHNOLOGYAn NSF Science and Engineering Center

capacitance vanishes at M-I transitionresistance drops‘leaky’ capacitor model

percolation of metallic regions?

‘dielectric’ changes little below transition

The University of Alabama Center for Materials for Information Technology P. LeClair Toshali Sands, Puri, 8 Jan 2009

How to measure a derivative?

• drive system with Vdc + δVacsin ωt• measure response at ωor ω

frequency / phase sensitive technique

… lock-in amplifiers… a lot of electronics

€

I(Vdc + δVac cosωt) = I(Vdc ) +dI

dVδVac cosωt( ) +

d2I

dV 2

δIac2

4cos 2ωt( )

⎛

⎝ ⎜

⎞

⎠ ⎟+K

The University of Alabama Center for Materials for Information Technology P. LeClair Toshali Sands, Puri, 8 Jan 2009

DU

T

DC

LIAdI

dV

DC

LIA

A

÷

÷

x

Vdc

Vac

20, 40, 400, 4000

1, 10, 102, 103, 104

unity gain1:1 add

101-104

lo & hi passEG&G 113

current preamp103-106 V/AKeithley 428

I

V

101-104

EG&G 113+ notch filter

home-made summing amp

Keithley 263

Stanford 830 Stanford 830

Stanford 830

HP 3458A

HP 3478A

x+

How to measure a derivative?