Download Karthik V. Raman Realizing Spintronic devices using organic

Realizing Spintronic devices using
organic molecules
Karthik V. Raman
Molecular orbital theory
Determining molecular electronic structure by treating electrons as moving under the influence
of the nuclei in the whole molecule
Organic semiconductors
π-conjugated systems
pz
Molecular property
MO levels
Optical bandgaps: 1.5 to 3 eV
Organic radical Spin-filters
Organic conductors
/ superconductors
C. Herrmann et. al. JACS, 2010
Molecular magnets
Manriquez et. al. Science 1991
Functional
Molecules
Molecular switches
IBM Zurich
Molecular Spin battery
Spin-crossover molecules Nature Commun. 2012
Y. Morita et. al. Nature Mater.2011
Transport in organic molecules
LUMO
Low Bandwidth
~ < 1 eV
HOMO
Courtesy: Bulovic
Courtesy: Bulovic
Why organic molecules in spintronics ?
Light elements, low Z
De-coherence

Weak spin-orbit interaction

Hyperfine interaction
α Z4
Long spin lifetimes ~10 - 100 s : Si ~ 10 - 100 ns
i
Bhf



Hopping transport !
Mobility <
s<
0.1cm2/V-s
30 nm
j
Bhf
Hyperfine fields
Magnetic pens on
OLED screens

Bhfi
k
Bhf
Francis et. al. New J. Phys. (2004)
Bobbert et. al. Phy. Rev. Lett. (2008)
Interface-assisted
molecular spintronics
Semiconductor
K. V. Raman, App. Phys.
Rev. 1, 031101 (2014)
Spintronics
Organic
Spintronics
Metal
Spintronics
Molecular
Spintronics
Spintronics
Spin
caloritronics
Multiferroics
Oxide
Spintronics
Opto
Spintronics
FM1
Spacer Layer
Tunnel barrier
Organic
semiconductor
FM1
FM2
P1
FM2
Semiconductor
or Metal
P2
Spin Valve
MTJ
Jullière’s Model
TMR 
2 P1P2
1  P1P2
Modified Jullière’s Model
2 P1 P2e  ( d d o ) / s
GMR 
1  P1 P2e ( d d o ) / s
Organic magnetic junctions
Organic Spin valves
Sol. Sta. Commun. 122, 181 (2002)
Nature 427, 821 (2004)
Nature Mater. 8, 707 (Review-2009)
Chapter 1, Organic Spintronics, Z.V. Vardeny (2010)
Xiong et al, Nature (2004)
FM1
FM2
300K
77K
4.2K
8
Alq3
TMR (%)
6
P1
Organic tunnel junctions
PRL 98, 016601 (2007)
PRL 100, 226603 (2008)
tOS
P2
APL 90, 072506 (2007)
4
tOS < 10nm
2
0
-100
-50
0
H (Oe)
50
100
OMTJ devices
Rubrene junction
Hybrid junction
2.2 nm
0.5 nm
2.2 nm
Layer formation observed by X-TEM
(2)
RT
5Å
~5
(4)
RT
Å
10
1
~
8Å
1
e
~
F
9Å
8
~
ne
e
r
3Å
2
b
~
Ru o
C
O3
Al 2
2x
x
SiO
OS layer depends on seed layer, growth temperature
Growth related interface effects
Profilometer studies
Pentacene growth
40
Nominal Rubrene
Thickness
20nm
15nm
Step Height (nm)
30
Hybrid
Hybrid
~27nm
~27nm
~21nm
~21nm
~23nm
~23nm
20
Rubrene
Rubrene
On the oxidized surface
~16nm
~16nm
10
Real Rubrene
Thickness
Real
Rub.
Thickness
0
~ 9 nm Al
9nm Al
-10
On the metal Cu surface
-20
0
2
4
6
Scan Length (m)
8
10
Surface electronic properties influence the growth of OS
Thayer et al. PRL (2005)
Inelastic tunneling spectroscopy

To study vibrational modes of the molecule

Deduce structural, chemical and electronic modifications
Selection rules
Strong electron phonon coupling in OS !
Orientation dependence !
Tunneling Spectroscopy, Plenum Press, P.K. Hansma
Inelastic tunneling spectroscopy
Principle of operation
T = 4.2 K
d2I/dV2
OS
Vbias>=ħω
>0 ph
M
M
M
Phonon
Excitation
ħωph
Elastic path
Using
-ħωph
ħωph
Lock in
Set-up
M Inelastic path (~1%)
2nd Harmonic signal
M-I-M structure
V
Inelastic tunneling spectroscopy measurements
Control Jn (CJ)
Al-O stretching
mode
Inelastic tunneling spectroscopy measurements
Hybrid Jn (HJ)
Inelastic tunneling spectroscopy measurements
Resonant states
Defects
Rubrene Jn (RJ)
Inelastic tunneling spectroscopy measurements
Computed Raman and IR
peaks for rubrene
molecule
R Weinberg-Wolf, et al, J. Phys.:
Condens. Matter (2007)
Raman et al. Phy. Rev. B, 2009
New Research avenues
o Important role of molecule/FM interfaces in spin injection
o Interface morphology and Interface chemistry is strongly correlated
FM
Sanvito, Nature 2010
Can we tune molecule morphology to control interface
chemistry and magnetism ?
Working with planar phenalenyl derivatives
Stronger interaction with FM surface: Charge transfer and hybridization
Zinc Methyl Phenalenyl
• Planar molecule like a Graphene
fragment
• Delocalized electron cloud
Unpaired spin
Magnetic moment
FM2
FM1
Magneto-resistance measurements with Co and Permalloy
(Ni:Fe) electrodes
4.2 K
50 mV
F1
AMR
F2
Raman et. al. Nature 493, 509 (2013)
Magneto-resistance measurements with Co and Permalloy
(Ni:Fe) electrodes
4.2 K
50 mV
AMR
Interface MR (IMR) effect !
Raman et. al. Nature 493, 509 (2013)
Confirmation of Interface response
Interface magnetoresistance (IMR) effect
Decoupling
layer
Absence of IMR effect from
bottom interface
12%
Only one FM
electrode !
IMR model
o MR is an interface effect
o A 䇺magnetically hard layer䇻 exist at the interface
R
ZMP
Pinned layer
Co
Py
H
Zn-C
Co
ZMP
Co
Understanding molecular adsorption on transition metal
surfaces
Physisorption
Chemisorption
pz
Energy
dz2
Energy
Molecular
p-states
Surface
d-states
+
dyz
dzx
Unhybridized
molecular
states
EF
Isolated
molecule
DOS
π-d antibonding
interface
states
π-d
bonding
interface
states
DOS
Spin-UP
Spin-DOWN
K. V. Raman, App. Phys. Rev. 1, 031101 (2014)
•
Introduces spin dependency in the electronic interface states
•
Emergence of new magnetic properties by molecular genome techniques
DFT: Main Results
ZMP molecule develops a net magnetic moment aligned antiparallel to the bottom magnetic surface.
Probing interface hybrid states using SP-STMs
Spin contrast spectroscopy measurements
Ha
CoPc
2 ML Fe
•
•
(University of Hamburg, Germany)
Observation of molecular states around the Fermi
level
ü
Matches with DFT simulation suggesting formation of
interface hybrid states
Jens Brede, PRL 105, 047204 (2010)
ć
Molecular Spin filter (SF) phenomena
Metallic SFs
Resistive SFs

2m 
 
T  T0  exp   2d
2



Higher MR effect
App. Phys. Rev. 1, 031101 (2014)
Magnetic hardening of interface magnetic layer
T =250 K
~22%
~50%
Field cooled response
Or + field
Field cooled
in -500 Oe
Magnetic anisotropy calculation of metal-org. supra-molecule
Surf. Co
MAE (Ksur) – 180ueV/Co atom (bulk 19ueV/atom)
Inter-planar Exchange coupling strength (J┴)
DFT calculations
Crane-pulley
Bulk (f.c.c Co) J ~ 8.54 meV/atom
effect
Clean surface Co, J┴ ~ 4.5 meV/atom
Hybridized surface Co, J┴ ~ 1.34meV/atom (86% reduction)
Surface
Bulk
Confirmation using spin-polarized STM studies
By Wiesendanger group, Germany
Spin polarized STM study of TbPc2 on Co/Ir(111)
ΔE
~ 200meV
Key Results:
Spin
DOWN
Interface
interaction
of
the
Molecular
orbitals
with
the
extended d-orbitals of surface Co
creates spin-split LUMO level in the
molecule with potential for spinfiltering and IMR effect.
Spin-UP
Spin-split LUMO state of the molecule
Spin-polarized STM study on Graphene/monolayer Co/Ir(111)
J. Schwöbel et. al., Nature Comm. 3, 953 (2012)
Key Results:
Decker et al., PRB 87, 041403(R) 2013
䇺0䇻 state
Graphene
䇺1䇻 state
FM
䇺1䇻
Observation of Moiŕe pattern due to
difference in the interlayer spacing
between
the
Co-monolayer
and
graphene sheet
Buckling of graphene sheet with three
distinct sites: 䇺top䇻, 䇺fcc䇻 and 䇺hcp䇻.
AFM Switching field (Coercivity) of the
䇺0䇻 Intercalated Co (Graphene/Co/Ir(111))
increases substantially than that of
Intercalated Co
Co/Ir(111) showing the strong effect of
Fragments of Graphene has induced moment with 䇺top䇻 site FM coupled interface hybridization on the magnetic
(positive moment) with the Co atoms, while 䇺fcc䇻 and 䇺hcp䇻 sites are AFM properties of the Co monolayer.
coupled (negative moment) with Co atoms
Potential for scalability and technology
Nano-molecular spintronic device
Raman et. al. Nature Lett., Jan 2013
Spin filter
Magnetic
supra-molecule
Spin analyzer
22% IMR
Co electrode
Spin polarizer
Interface magnetoresistance (IMR) effect
Exchange biased Near Room
temperature IMR effect
Relative switching in magnetization of spin polarizer
and analyzer leads to IMR
Engineer interface chemistry, magnetism, magnetic exchange coupling & surface anisotropy
Key Results of our work:
•
Induce magnetic moment to the molecule
A molecule on ferromagnet surface can have induced magnetic moment which can be stabilized
even at high temperature.
•
Tuning the exchange coupling between the molecule and the FM surface
This is possible by engineering the molecular chemistry to tune the interface chemistry and
magnetism.
JFM-molecule
•
Enhancement in surface magnetic anisotropy energy
Interface hybridization of a ferromagnet surface with a molecule can significantly enhance spinorbit coupling on the surface with a possibility to sufficiently enhance the coercivity of the
surface ferromagnet layer.
•
Interface magnetoresistance (IMR) effect by spin-filter action
Interface chemistry develops a spin dependent interface resistance due to spin-filter injection.
This arises due to difference in the barrier height for the two spin channels for injection.
Spin-filter
action
FM
New research horizons . . .
Challenges of current hard disk technology
50 Tb/in2
Lost momentum
Single molecular magnets: Quantum phenomena
Fe8,Mn12 systems
Resonant magnetization
tunneling
Single-ion anisotropy
(SO coupling)
Quantum
hysteresis
E. M. Chudnovsky, Science 1996
Quantum computing in SMM䇻s
M. N. Leuenberger & D. Loss, Nature 410 (2001)
Overcoming the SPM limit: How to stabilize the magnetic state above
room temperature
Single particle SPM limit
1.
FM particle on AFM or FM substrate can
show surface magnetic exchange
induced enhancement in the activation
barrier for reversal of magnetization.
2.
FM particle acting as a magnetic bit must
show
independent
magnetization
reversal for potential in data storage for
room temperature operation.
Or
FM
Reports on such attempts
Single molecule magnets
1K
Science, 2011
Nature Mater. 2009
Molecular magnets on
gold surface has shown
hysteretic response in
magnetization switching
of the molecule.
12 atom bit stabilized by
engineering
the
exchange
coupling between the Fe atoms
0.5 K
Low temperature operation, T<
20 K
Observed at very low
temperature < 0.5 K.
Gold surface
Fe atoms on Cu2N surface
Nature Mater. 2007
Fe center
300 K
Ni surface
Molecular magnet on Ni
substrate
Strong coupling of Fe
center
following
the
magnetization of the Ni
substrate even at room
temperature.
Electrical probing using magnetic
STM tip
Open challenges:
Stabilize the magnetization state near room temperature
Engineer interface magnetic exchange coupling strength to observe
independent magnetization hysteresis of the magnetic bit near room
temperature.
Electrical reading/writing the magnetic state using a simple
procedure. Magnetic tips are difficult due to oxidation of the tip.
Open questions
•
Induced molecular magnetism
•
New perspectives in engineering pseudo
molecular magnets
•
Engineer interface chemistry, magnetic
exchange coupling and surface anisotropy
JC-C
JCo-C
APR. 1, 031101 (2014)
•
•
APR. 1, 031101 (2014)
JC-C
J䇻C-Co
JCo-C
JC-C
JC-Co
Understanding crane-pulley effect
Designing molecular spin-filters
Molecular adsorption on non-magnetic
transition metal surfaces
Magnetization (emu)
Some recent works . . .
Univ. of Leeds
Induced magnetism in thin Cu films due to
adsorption of buckyball molecules
Magnetic field
Nano Lett. 15, 7921 (2015)
Nature 524, 69 (2015)
Ferro to antiferromagnetic coupling
via Cu spacer layers
Understanding magnetism in molecule-TM interface
Stoner Criteria for Ferromagnetism
Electron energy band
E
D(EF)
Fermi Level (EF)
I . D(EF) > 1
I : exchange integral
Asymmetry in density of states for the two spin
bands
D(E)
Raman & Moodera, Nature 524, 42 (2015)
Levitin, R. Z. & Markosyan, A. S. Sov. Phys.
Usp. 31, 730 (1988).
Conclusion
•
•
•
Exciting opportunity to uncover many fundamental phenomena at
the molecule-transition metal interface
Possibility to self-assemble molecules on surfaces
Interface response to external stimuli e.g. light
Collaborators
Dr. Jagadeesh S. Moodera (MIT)
Alexander Kamerbeek (MIT)
Nicolae Atodiresei (FZ, Juelich)
Predrag Lazić F), Juelich
V. Caciuc (FZ, Juelich)
Stefan Blügel (FZ, Juelich)
Daniel Buergler(FZ, Juelich)
Daniel Burgler (FZ, Juelich)
Frank Matthias (FZ, Juelich)
Volkmar Hess (FZ, Juelich)
Dietmar Stalk (Göttingen)
Reent Michel (Göttingen)
Markus Münzenberg (Gottingen)
Arup Mukherjee (IISER-Kolkata)
Swadhin Mandal (IISER-Kolkata)