Karthik V. Raman

Realizing Spintronic devices using

organic molecules

Molecular orbital theory Determining molecular electronic structure by treating electrons as moving under the influence

of the nuclei in the whole molecule

MO levels

Organic semiconductors

Molecular property

π-conjugated systems

pz

Optical bandgaps: 1.5 to 3 eV

Molecular magnets Manriquez et. al. Science 1991

Organic radical Spin-filters

C. Herrmann et. al. JACS, 2010

Molecular switches IBM Zurich

Spin-crossover molecules Nature Commun. 2012

Molecular Spin battery Y. Morita et. al. Nature Mater.2011

Organic conductors

/ superconductors

Functional

Molecules

Low Bandwidth ~ < 1 eV LUMO

HOMO

Transport in organic molecules

Courtesy: Bulovic

Courtesy: Bulovic

j

hfBi

hfB

k

hfB

i

hfB

-

Why organic molecules in spintronics ?

Light elements, low Z

Weak spin-orbit interaction Hyperfine interaction

De-coherence

α Z4

Long spin lifetimes ~10 - 100 s : Si ~ 10 - 100 ns

Magnetic pens on

OLED screens

Francis et. al. New J. Phys. (2004)

Bobbert et. al. Phy. Rev. Lett. (2008)

Hopping transport !

Mobility < 0.1cm2/V-s

s < 30 nm

Hyperfine fields

Semiconductor

Spintronics

Organic

Spintronics Metal

Spintronics

Multiferroics

Opto

Spintronics

Spintronics

Oxide

Spintronics

Molecular

Spintronics

Spin

caloritronics

Interface-assisted

molecular spintronics

K. V. Raman, App. Phys.

Rev. 1, 031101 (2014)

FM1 FM2 Spacer Layer

Tunnel barrier

Jullière’s Model

MTJ Spin Valve

Semiconductor

or Metal

21

21

1

2

PP

PPTMR

Modified Jullière’s Model

so

so

dd

dd

ePP

ePPGMR

/)(

21

/)(

21

1

2

P1 P2

FM1 FM2

Organic

semiconductor

Organic magnetic junctions

Organic tunnel junctions -100 -50 0 50 1000

2

4

6

8

300K

77K

4.2K

TM

R (

%)

H (Oe)

tOS < 10nm

Alq3

P1 P2

FM1 FM2

tOS

PRL 98, 016601 (2007) APL 90, 072506 (2007)

PRL 100, 226603 (2008)

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)

OMTJ devices

0.5 nm

2.2 nm

Hybrid junction

2.2 nm

Rubrene junction

(4)

Al 2O 3

~ 55 Å

~ 110 Å

~ 18 Å

~ 89 Å

~ 23 Å

Fe

Co

SiO

Rubrene

RT

x

(2)RT

2x

Layer formation observed by X-TEM

OS layer depends on seed layer, growth temperature

Growth related interface effects

0 2 4 6 8 10-20

-10

0

10

20

30

40

Rubrene

~21nm

~27nm

~16nm

~23nm

Real Rubrene Thickness

~ 9 nm Al

Hybrid

20nm15nm

Nominal Rubrene Thickness

Ste

p H

eig

ht

(nm

)

Scan Length (m)

Hybrid

Rubrene

~23nm

~16nm

~27nm

~21nm

Real Rub.

Thickness

9nm Al

On the oxidized surface

On the metal Cu surface

Surface electronic properties influence the growth of OS

Thayer et al. PRL (2005)

Profilometer studies Pentacene growth

Inelastic tunneling spectroscopy

To study vibrational modes of the molecule

Deduce structural, chemical and electronic modifications

Orientation dependence !

Selection rules

Strong electron phonon coupling in OS !

Tunneling Spectroscopy, Plenum Press, P.K. Hansma

Inelastic tunneling spectroscopy Principle of operation

Phonon

Excitation

ħωph

OS

M

M Inelastic path (~1%)

d2I/dV2

V ħωph

-ħωph Using

Lock in

Set-up

M-I-M structure

M

M Elastic path

Vbias>0 Vbias>=ħωph

2nd Harmonic signal

T = 4.2 K

Control Jn (CJ)

Inelastic tunneling spectroscopy measurements

Al-O stretching

mode

Hybrid Jn (HJ)

Inelastic tunneling spectroscopy measurements

Rubrene Jn (RJ)

Resonant states Defects

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

Inelastic tunneling spectroscopy measurements

New Research avenues

o Important role of molecule/FM interfaces in spin injection

o Interface morphology and Interface chemistry is strongly correlated

Can we tune molecule morphology to control interface

chemistry and magnetism ?

Sanvito, Nature 2010 FM

• Planar molecule like a Graphene

fragment

• Delocalized electron cloud

Unpaired spin

Magnetic moment

Stronger interaction with FM surface: Charge transfer and hybridization

Zinc Methyl Phenalenyl

Working with planar phenalenyl derivatives

FM1

FM2

4.2 K

50 mV

AMR

Raman et. al. Nature 493, 509 (2013)

F1

F2

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)

Magneto-resistance measurements with Co and Permalloy (Ni:Fe) electrodes

Only one FM

electrode ! 12%

Interface magnetoresistance (IMR) effect

Absence of IMR effect from

bottom interface

Decoupling

layer

Confirmation of Interface response

ZMP

Co

ZMP

Co

R

H

o MR is an interface effect

o A magnetically hard layer exist at the interface

Zn-C

Co

Py

Pinned layer

IMR model

Energy

DOS

Isolated

molecule

• Introduces spin dependency in the electronic interface states

• Emergence of new magnetic properties by molecular genome techniques

+ EF

Molecular

p-states

Surface

d-states

Un-

hybridized

molecular

states

π-d anti-

bonding

interface

states

π-d

bonding

interface

states

Spin-UP Spin-DOWN

Energy

DOS

dz2 dyz dzx

pz

K. V. Raman, App. Phys. Rev. 1, 031101 (2014)

Physisorption Chemisorption

Understanding molecular adsorption on transition metal surfaces

ZMP molecule develops a net magnetic moment aligned anti-

parallel to the bottom magnetic surface.

DFT: Main Results

→ µ

→ µ

→ µ

Å

→ µ

ćü

(University of Hamburg, Germany)

2 ML Fe

CoPc

Jens Brede, PRL 105, 047204 (2010)

Ha

Probing interface hybrid states using SP-STMs

• Observation of molecular states around the Fermi

level

• Matches with DFT simulation suggesting formation of

interface hybrid states

Spin contrast spectroscopy measurements

Molecular Spin filter (SF) phenomena

Metallic SFs

Higher MR effect

Resistive SFs

20

22exp

mdTT

App. Phys. Rev. 1, 031101 (2014)

T =250 K

~22% ~50%

Field cooled response

Field cooled

in -500 Oe

Magnetic hardening of interface magnetic layer

Or + field

MAE (Ksur) – 180ueV/Co atom (bulk 19ueV/atom)

Surf. Co

Inter-planar Exchange coupling strength (J┴)

DFT calculations

Bulk (f.c.c Co) J ~ 8.54 meV/atom

Clean surface Co, J┴ ~ 4.5 meV/atom

Hybridized surface Co, J┴ ~ 1.34meV/atom (86% reduction)

Magnetic anisotropy calculation of metal-org. supra-molecule

Crane-pulley effect

Surface

Bulk

Confirmation using spin-polarized STM studies By Wiesendanger group, Germany

Spin-split LUMO state of the molecule

Spin polarized STM study of TbPc2 on Co/Ir(111)

Key Results: 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 spin-

filtering and IMR effect.

Spin-polarized STM study on Graphene/monolayer Co/Ir(111) Key Results:

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: top , fcc and hcp .

Switching field (Coercivity) of the

Intercalated Co (Graphene/Co/Ir(111))

increases substantially than that of

Co/Ir(111) showing the strong effect of

interface hybridization on the magnetic

properties of the Co monolayer.

Fragments of Graphene has induced moment with top site FM coupled

(positive moment) with the Co atoms, while fcc and hcp sites are AFM

coupled (negative moment) with Co atoms

FM

1

AFM

0

Graphene

Intercalated Co

Potential for scalability and technology

J. Schwöbel et. al., Nature Comm. 3, 953 (2012)

Decker et al., PRB 87, 041403(R) 2013

ΔE

~ 200meV

Spin-UP

Spin

DOWN

0 state

1 state

Engineer interface chemistry, magnetism, magnetic exchange coupling & surface anisotropy

Raman et. al. Nature Lett., Jan 2013

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.

• Enhancement in surface magnetic anisotropy energy Interface hybridization of a ferromagnet surface with a molecule can significantly enhance spin-

orbit 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.

Interface magnetoresistance (IMR) effect Exchange biased Near Room

temperature IMR effect Relative switching in magnetization of spin polarizer

and analyzer leads to IMR

JFM-molecule

Spin-filter

action

FM

Co electrode

Spin polarizer

Spin filter Magnetic

supra-molecule

Spin analyzer

22% IMR

Nano-molecular spintronic device

New research horizons . . .

Challenges of current hard disk technology

50 Tb/in2

Lost momentum

M. N. Leuenberger & D. Loss, Nature 410 (2001)

Single-ion anisotropy

(SO coupling)

Single molecular magnets: Quantum phenomena

Resonant magnetization

tunneling

E. M. Chudnovsky, Science 1996

Quantum

hysteresis

Quantum computing in SMM s

Fe8,Mn12 systems

Or

FM

Reports on such attempts

Overcoming the SPM limit: How to stabilize the magnetic state above

room temperature

Single particle SPM limit

1 K

0.5 K

Gold surface

Single molecule magnets Nature Mater. 2009

Molecular magnets on

gold surface has shown

hysteretic response in

magnetization switching

of the molecule.

Observed at very low

temperature < 0.5 K.

Ni surface

Molecular magnet on Ni

substrate

Strong coupling of Fe

center following the

magnetization of the Ni

substrate even at room

temperature.

300 K

Fe center Nature Mater. 2007

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.

Fe atoms on Cu2N surface

Science, 2011

12 atom bit stabilized by

engineering the exchange

coupling between the Fe atoms

Low temperature operation, T<

20 K

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.

• Engineer interface chemistry, magnetic

exchange coupling and surface anisotropy

APR. 1, 031101 (2014)

• Induced molecular magnetism

JCo-C

JC-C

JC-C

JC-Co

J C-Co

JC-C

JCo-C

Open questions

• Designing molecular spin-filters

• Molecular adsorption on non-magnetic

transition metal surfaces

APR. 1, 031101 (2014)

Understanding crane-pulley effect

• New perspectives in engineering pseudo

molecular magnets

Some recent works . . .

Induced magnetism in thin Cu films due to

adsorption of buckyball molecules Nature 524, 69 (2015)

Univ. of Leeds

Magnetic field

Ma

gn

eti

zati

on

(e

mu

)

Nano Lett. 15, 7921 (2015)

Ferro to anti-

ferromagnetic coupling

via Cu spacer layers

Fermi Level (EF)

E

D(E)

I : exchange integral I . D(EF) > 1

Asymmetry in density of states for the two spin

bands

Stoner Criteria for Ferromagnetism Electron energy band

D(EF)

Understanding magnetism in molecule-TM interface

Raman & Moodera, Nature 524, 42 (2015)

Levitin, R. Z. & Markosyan, A. S. Sov. Phys.

Usp. 31, 730 (1988).

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)

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