the university of alabama center for materials for information technology an nsf science and...
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![Page 1: THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center Quantum Design PPMS T: 350mK -- 400K H:](https://reader038.vdocument.in/reader038/viewer/2022110322/56649d095503460f949db694/html5/thumbnails/1.jpg)
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)
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THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn NSF Science and Engineering Center
Examples ...
MagnetoresistanceI(V)
dI/dV (V)
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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
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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 ...
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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)
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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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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-
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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
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THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGYAn NSF Science and Engineering Center
Impedance spectroscopy of thin films
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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
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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 ...
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impedance spectroscopy
magneto-dielectric measurements
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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
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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!
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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
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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 ...
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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
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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
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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?