THE UNIVERSITY OF ALABAMA
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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
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ν: stretchβ: in-plane bendγ: out-of-plane bend
indexing - compare with ATR-FTIRaromatic is intactchemisorbed benzoate ionabsence of C=O, presence of COO-
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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?