Cosa è la Spintronica?

Valentin Alek Dediu

Consiglio Nazionale Delle Ricerche ISMN, via Gobetti 101, Bologna, ITALY

People from ISMN involved

G. Ruani M. Murgia P. Nozar

Nanotechnology Lab Spectroscopy Lab

F. Biscarini M. Cavallini

Spintronic Dev. Lab

V. Alek Dediu

I. Bergenti

L. Hueso

E. Lunedei

A. Riminucci

P. Graziosi (Ph. D)

C. Newby

F. Bona

S. Rose (tecn)

Strong support from

Prof. Carlo TALIANI

ISMN­CNR

Nobel prize 2007 in Physics

L’elettrone: la carica e lo spin

What is Spintronics?

H Rmin Rmax

d < L S d < L S

ST ST SI SA SI SA

Spin valve

Spintronics is a new branch of electronics based on purely quantum effects, where the information is stored, transmitted and read via electrical carrier spin orientation. It requires an artificial manipulation of the spins orientation

SI = spin polarized injector ST = spin polarized transporter SA = spin analyzer Ls = spin diffusion length

Is Spin Polarized injection POSSIBLE? Normal FM Half­metallic FM

E F

SP ~ 1­ 40% SP ~ 100%

FM S/c

????????

CURRENT interface

Anysotropic Magnetoresistance (AMR): William Thomson, 1856

Anysotropic Magnetoresistance (AMR): the nature

Non è basato sulla polarizzazione degli spin

Comincia l’era dello spin polarizzato

Comincia l’era dello spin polarizzato

•Original GMR effects were not directly usable : –The MR effect is spread over a too large field (low sensitivity) –Need for a biasing field

M. Baibich et al., PRL (1988) FNVD et al., J. Phys. (1988)

Giant Magnetoresistance metallic multilayers

Evolution of read/write sensor technology

Impact of the introduction of GMR heads

MR

GMR

L’effetto Spin Valve

M

H

H

R

Rmin

Rmax

La resistenza del dispositivo cambia drasticamente tra gli stati di magnetizzazione parallela e antiparallela

Magnetic tunnel junctions

J.S. Moodera et al., Phys. Rev. Lett. 74, 3273 (1995)

From CIP to CPP sensors : TMR vs GMR

Commercial product with TMR read­head S. Mao et al., IEEE Trans. Magn., 42, 97 (2006)

Navigation : use of an attitude detector

è 1 triaxial magnetometer → H E è 1 triaxial accelerometer → G è G x H E = Magnetic East

Ref. : www.ssec.honeywell.com

The simple sailing boat solution

= ? +

Organic spintronics

Most organic semiconductors are characterized by very weak spin scattering strength:

­ low Spin­Orbit Coupling (low Z values) and ­ low Hyperfine Interaction (π­conjugation)

possibility to transport the spin polarized signal to long distances (10 2 nm) even at room temperatures

Technological advantages: ­ easy to grow, low sensitivity to impurities ­ stable and easily controlable interfaces with many inorganic materials – interface tuning by Self Assembled Monolayers

OS in Spintronics ­ MOTIVATION ISMN­CNR

OUR MAIN APROACH ISMN­CNR

Direct (current) spin injection in long channels of Organic Semiconductors (>10 2 nm) by using both

conventional ­ Co, Ni, Fe, … non­conventional ­ manganite (La 0.7 Sr 0.3 MnO 3 )

for spin injection and detection

Why manganites –> so far show the best efficiency in applications to Organic Spintronics

easiest way for the investigation of the basic spin physics – still to be understood

Manganites: properties

3t 2g localized S = 3/2 t 2g

e g JT

R 1­x A x MnO 3

Mixed valence: Mn 3+ (1-x ), Mn 4+ (x)

Metallic and FM below Tc

Mn 4+ Mn 3+

R = La, Pr, Sm.. A = Ca, Sr, Ba…

Mn

Mother system (x=0)

Insulating, AFM

JT Mn 3+

JT Mn 3+

3t 2g localized S = 3/2 t 2g

FM ­ ferromagnetic metal, FI ­ ferromagnetic insulator, PM ­ paramagnetic metal, PI ­ paramagnetic insulator, AFM ­ antiferromagnetic, CO ­ charge ordering CAFI – canted antiferromagnetic

Phase diagrams of manganites

La 1­x Sr x MnO 3 Pr 1­x Ca x MnO 3

Large bandwidth Low bandwidth Colossal Magnetoresistance 100% spin polarization

Charge­orbital ordering

FM by external stimulation (light, H, E, pressure, X­ray)

our case

4 µm 4 µm

FM ≈ 50% dI(x,y)/dV maps

FM ≈ 99%

Magnetic homogeneity in La 0.7 Sr 0.3 MnO 3 films by SP STM

T<<T C T≈T C

High magnetic homogeneity at T<<T C results from highly homogeneous oxygen distribution

Film growth: 1. HIGS (Hybrid Integrated Growth System) combining:

organic growth chamber (5 + 3 Knudsen cells); Oxide electron beam ablation chamber (4 targets – manganite, magnetite, …); FM metals (3 e­ guns); analysing chamber (MR, EL, PL, MOKE)

2. 2 independent PPD (electron beam ablation) for various oxides

Characterizations:

­ Magnetoresistance at low and high magnetic fields (7 T) ­ MOKE: 632 nm, temperature 4,2­400 K, up to 1 Tesla ­ Time resolved Magneto ­ Optical spectroscopy: EL and PL as function of

Field, 100 ps – 1 ms ­ Micro­Raman ­ STM, AFM + STM spectroscopy with SP tips

­ Spin Polarized STM – UHV, variable T (100 –1000K), 0­1000 Oe

Facilities involved at ISMN

AFM: 50nm LSMO film

Supercon. Sci. Technol. 8, 160 (1995) Phys. Stat. Sol. 215, 1443 (1999)

La 0.7 Sr 0.3 MnO 3 films by electron beam ablation

6T monolayers on LSMO

A) grow a 6T film, desorb multilayers by annealing

B) gradual deposition

for chemisorption

heat

T 6

LSMO w=70­500 nm

V. Dediu, C. Taliani et al. Sol. St. Comm.122 (2002),181 Patent US6325914

Large negative magnetoresistance measured

Advantage: NO short circuits!

Problem: not possible (at least not at all easy) to reach AP configuration.

First Organic MR device ISMN­CNR Alek Dediu

w=200 nm w=400 nm

0,0 0,1 0,2 0,3 0,00

0,05

0,10

w=140 nm

Magnetoresistance across LSMO­T 6 ­LSMO devices

H=0 H=3.5 kOe

Spin valve effect (negative Magnetoresistance):

a) Spin polarized injection at interface

b) Spin polarized transport between electrodes

V.Dediu,C.Taliani et al. Sol.St.Comm.122,(2002),181

curren

t [nA

]

voltage [V]

LSMO work function (4.5­4.7 eV) close to T 6 HOMO

100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

Magnetoresistance vs channel length

Channel length (nm)

R(0)­R

(H) (G

Ω) Total LSMO/T6/LSMO resistance scales with w

Resistivity ρ = 10 6 Ωcm

d(T 6 )=100 nm

Spin relaxation length L S ∼ 70 nm

Spin relaxation time τ ∼ 10 ­6 s

Z. H. Xiong, V. Vardeny et al. Nature 427, 821 (2004)

La 0.7 Sr 0.3 MnO 3 /Alq3(130nm)/Co

Inverse spin valve effect: The inverse magnetoresistance was explained by the opposite spin polarizations of the LSMO and of the Co d­bands at the Fermi level

Demonstration of the Spin Valve effect ISMN­CNR Alek Dediu

While this explanation looks qualitatively convincing, the detailed mechanism of the inverse spin valve effect is still debating: see the presentation of L. Hueso (Tuesday)

becomes most used OS in organic spintronics

2002 2003 2004 2005 2006 2007 0 4 8 12 16 20 24 28 32 36

Num

ber o

f Articles (IS

I­Wok

)

Year

Organic Spintronics ­ publications ISMN­CNR

1st E

xperim

ental R

eport

1st S

pin Va

lve

­3 ­2 ­1 0 1 2 3

­2

­1

0

1

2 10 nm

Φ K (m

deg)

H (mT)

10 nm thick LSMO/NGO film at room temp.

MOKE characterization

MOKE setup built at ISMN by T. Mertelj

4 nm thick LSMO film at room temp.

Film thickness: 4nm

Coercive field H C = 5 Oe

NdGaO 3

­0.8 ­0.4 0.0 0.4 0.8

0

Φ K (m

deg)

H (mT)

INTE

NSITY (arb. units)

670 660 650 640 630 PHOTON ENERGY (eV)

60º

M

X­ray magnetic dichroism curve

Non­epitaxial LSMO, but clearly ferromagnetic at RT

Element specific, "surface sensitive" magnetic moments

XMCD of LSMO on Si

SrTiO 3

La 0.7 Sr 0.3 MnO 3

Alq3, T6 100 ­ 300 nm

Co 20 nm LiF, AlOx 2 nm

Vertical Spin Valve geometry

Tris(8­hydroxyquinoline)aluminium (Alq3) ISMN­CNR

Material widely used in organic LEDs – UHV Molecular Beam Dep.

Forms ordered polycrystalline films at 120­150°C substrate T – rough surface

Forms amorphous films at room substrate T – smooth surface

Van der Waals interaction between molecules

Alek Dediu

­1.0 ­0.5 0.0 0.5 1.0

­150

0

150

300 I (

µA)

Voltage (V)

LSMO/Alq3(80 nm)/Al 2 O

3 /Co

RT 100 K

Insulating like behavior is typical for most spin valve devices

LSMO/Alq3/TB/Co Spin Valve: IV curves

300 K

40 K

ISMN­CNR

­80 ­40 0 40 80

­0.15

­0.10

­0.05

0.00

­500 ­250 0 250 500 ­12

­8

­4

0

0 100 200 300 0

­4

­8

­12

Magnetic Field (Oe)

MR(%

)

MR(%

)

Magnetic Field (Oe) Temperature (K)

MR (%

)

100 200 300 0 5 10 15

MR (%

)

Alq 3 t (nm)

­80 ­40 0 40 80 ­2

­1

0

1

2

0 5 10 15 20

MOKE (a.u.)

Magnetic Field (Oe)

Reflectivity

(a.u.)

Angle (deg)

Immagazzinamento di informazioni e sensoristica nanomagnetica a base di dispisitivi spintronici ibridi

Dediu et al. Nature Mat. submitted

Raggiunto il funzionamento stabile a temperatura ambiente

Sviluppati nuovi approcci di crescita dei metalli (magnetici) sopra i semiconduttori organici: raggiunte interfacce molto “sharp” e parametri magnetici di altissimo livello

Refflettometria raggi­X, MOKE

a)

b)

LSMO Alq 3 TB Co

PLANAR DEVICES

TWO “PERFECT” INTERFACES

VERTICAL DEVICES

ONE “PERFECT” AND ONE “VERY BAD ”INTERFACE

Dimostrazione di memoria elettrica non­volatile in dispositivi spintronici ibridi: la possibilità di cambiare tra due stati di resistenza “scrivendo” con voltaggi positivi (1) e negativi (0)

up to room T Hueso, Dediu et al. Adv. Mat. 2007

Resistanc

e (Ohm

)

Field (Oe)

Principi della sensoristica magnetica a base dispositivi spintronici: nano­imaging

M

H

H

R

Rmin

Rmax

MNP

Il passaggio della nanoparticella magnetica inverte la magnetizzazione del elettrodo di sopra

La resistenza del dispositivo cambia drasticamente tra gli stati di magnetizzazione parallela e antiparallela

New EBL geometry: interdigitated strips

Au pads

Alq3/LSMO LSMO contacts

ISMN­CNR

SPECTROMETER

SrTiO 3 (0.5 mm)

La 0.7 Sr 0.3 MnO 3

TPD (70 nm) Alq3 (70 nm)

I

LiF (2 nm)

Manganite based OLEDs

Al

OLED off

OLED on

ANTOINE KAHN, NORBERT KOCH, WEIYING GAO

400 600 800

0.2

0.4

0.6

0.8

1.0

30 nm

90 nm 40 nm

12 nm

Transm

ittance

wavelenght (nm)

Optical transmission of La 0.7 Sr 0.3 MnO 3 ferromagnetic films at 300 K

460

480

500

520

540

560

580

LSMO/Al

ITO/Al

Peak Position (n

m)

Single Spin Polarized Electrode AlQ 3 OLED (LSMO and Iron)

EL threshold at V T = 6 V

Electroluminescence

400 500 600 700 800 0

2

4

6

8

10

12

Intensity (a

.u.)

λ (nm)

ITO/Al LSMO/Al

­10 ­5 0 5 10 15 20

0.000

0.005

0.010

0.015

0.020

0.025

Integrated intensity [a.u.]

Current density [A

/cm 2 ]

Voltage [V]

400 500 600 700 0

2

4

6

8

10

12 TPD/Alq3 NPB/Alq3 LSMO/Al ITO/Fe

ITO/Co LSMO/Al ITO/Al

Intensity (a.u.)

λ (nm)

Tris(8­hydroxyquinoline)aluminium (Alq3) ISMN­CNR

Material widely used in organic LEDs – UHV Molecular Beam Dep.

Forms ordered polycrystalline films at 120­150°C substrate T – rough surface

Forms amorphous films at room substrate T – smooth surface

Van der Waals interaction between molecules

Alek Dediu

Bottom FM

Org. Semic. Co clusters

Co electrode

Is the insulating/tunnel barrier needed? ISMN­CNR

OUR OPINION!!! ??? The success of organic spintronics, at least for vertical devices,

will be determined by the art of growing HIGH QUALITY TUNNEL BARRIERS

on top of organic layers

??? Difficult or almost impossible to control and reproduce the quality of the top interface for direct deposition of metal on top of soft organic layer

BARRIER

Double role

S U B S T R A T E S U B S T R A T E

Alek Dediu

PLANAR DEVICES

TWO “PERFECT” INTERFACES

VERTICAL DEVICES

ONE “PERFECT” AND ONE “VERY BAD ”INTERFACE

ISMN­CNR Alek Dediu

Exotic electrode materials. Highly spin polarized manganese oxides

Transformation of spin information into electrical signals

Spintronics with carbon nanotubes

L.E. Hueso, A. Fert et al., Nature 445, 410 (2007)

Long distance magnetoresistance as a proof of spin coherence

ISMN­CNR Alek Dediu

Graphene, CNT

OS

S/c: Si, GaAs, … Metals: Al, Au, …

Will we, and how, combine various materials accepting spin injection?

SP injectors

INTERFACES in Organic Spintronics – First EC project Poster P22

OFSPIN STRP 033370 2006­2009 Organic­Ferromagnetic Hybrid

Interfaces for Spintronic Applications

ISMN Bologna

Univ. Linkoeping

Univ. Wuerzburg

Univ. Parma

Univ. Fudan

Shanghai

CNRS Strasbourg

CNRS Palaiseau

THALES

Knowledge of structural, electronic and magnetic properties of the new interfaces between organic and spin polarized magnetic materials

Improved charge and spin injection at such interfaces

and REPRODUCIBILITY!!!

www.spinos.org